48 results on '"Valentina Mameli"'
Search Results
2. The Boundary Between Volume and Surface-Driven Magnetic Properties in Spinel Iron Oxide Nanoparticles
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Giuseppe Muscas, Francesco Congiu, Giorgio Concas, Carla Cannas, Valentina Mameli, Nader Yaacoub, Rodaina Sayed Hassan, Dino Fiorani, Sawssen Slimani, and Davide Peddis
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General Materials Science ,Condensed Matter Physics - Abstract
Despite modern preparation techniques offer the opportunity to tailor the composition, size, and shape of magnetic nanoparticles, understanding and hence controlling the magnetic properties of such entities remains a challenging task, due to the complex interplay between the volume-related properties and the phenomena occurring at the particle’s surface. The present work investigates spinel iron oxide nanoparticles as a model system to quantitatively analyze the crossover between the bulk and the surface-dominated magnetic regimes. The magnetic properties of ensembles of nanoparticles with an average size in the range of 5–13 nm are compared. The role of surface anisotropy and the effect of oleic acid, one of the most common and versatile organic coatings, are discussed. The structural and morphological properties are investigated by X-ray diffraction and transmission electron microscopy. The size dependence of the surface contribution to the effective particle anisotropy and the magnetic structure are analyzed by magnetization measurements and in-field Mössbauer spectrometry. The structural data combined with magnetometry and Mössbauer spectrometry analysis are used to shed light on this complex scenario revealing a crossover between volume and surface-driven properties in the range of 5–7 nm. Graphical Abstract
- Published
- 2022
3. On the role of the nature and density of acid sites on mesostructured aluminosilicates dehydration catalysts for dimethyl ether production from CO2
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Fausto Secci, Valentina Mameli, Elisabetta Rombi, Sarah Lai, Marco Sanna Angotzi, Patrícia A. Russo, Nicola Pinna, Mauro Mureddu, and Carla Cannas
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Process Chemistry and Technology ,Chemical Engineering (miscellaneous) ,Pollution ,Waste Management and Disposal - Published
- 2023
4. On the thermal and hydrothermal stability of spinel iron oxide nanoparticles as single and core-shell hard-soft phases
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Marco Sanna Angotzi, Valentina Mameli, Dominika Zákutná, Nicoletta Rusta, and Carla Cannas
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Mechanics of Materials ,Mechanical Engineering ,Materials Chemistry ,Metals and Alloys - Published
- 2023
5. Germination and early seedling development of
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Maria Enrica, Boi, Marco, Sanna Angotzi, Marco, Porceddu, Elodia, Musu, Valentina, Mameli, Gianluigi, Bacchetta, and Carla, Cannas
- Abstract
Arsenate, As(V), and arsenite, As(III), are the most available arsenicals present in the soil solutions, in particular in mine polluted substrates, and cause several symptoms of toxicity in plants (like inhibition of seed germination and reduction of seedling development). For these reasons, seeds germination studies are essential for the design of phytoremediation activities of mine sites. Seed germination and seedling development of
- Published
- 2022
6. Hexafluorosilicic Acid (FSA): from Hazardous Waste to Precious Resource in Obtaining High Value-Added Mesostructured Silica
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Carla Cannas, Valentina Mameli, Anna Maria Giovanna Musinu, Claudio Cara, Luca Pala, Maria Giorgia Cutrufello, Mirko Antonio Vacca, Mariano Andrea Scorciapino, Vaclav Tyrpekl, and Marco Sanna Angotzi
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Waste management ,Renewable Energy, Sustainability and the Environment ,General Chemical Engineering ,02 engineering and technology ,General Chemistry ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,Hexafluorosilicic acid ,Industrial waste ,0104 chemical sciences ,chemistry.chemical_compound ,MCM-41 ,chemistry ,Hazardous waste ,Environmental Chemistry ,Environmental science ,High surface area ,0210 nano-technology - Abstract
In this work, industrial waste hexafluorosilicic acid (H2SiF6 or FSA) has been proven to be a low-cost alternative to silicate esters for the synthesis of high-quality MCM-41 (high surface area, hi...
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- 2020
7. Arsenic 'Phyto-Speciation' in Germination and Early Seedling Development of Helichrysum Microphyllum Cambess. Subsp. Tyrrhenicum Bacch., Brullo & Giusso
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Maria Enrica Boi, Marco Sanna Angotzi, Marco Porceddu, Elodia Musu, Valentina Mameli, Gianluigi Bacchetta, and Carla Cannas
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History ,Polymers and Plastics ,Business and International Management ,Industrial and Manufacturing Engineering - Published
- 2022
8. On the Thermal and Hydrothermal Stability of Spinel Iron Oxide Nanoparticles as Single and Core-Shell Hard-Soft Phases
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Marco Sanna Angotzi, Valentina Mameli, Dominika Zákutná, and Carla Cannas
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- 2022
9. Intra-Source Provenance Study on Monte Arci (Sardinia) Obsidian by pXRF: Role of the Data Acquisition and Analysis Tools
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Valentina Mameli, Marco Sanna Angotzi, Emanuele Farinini, Riccardo Leardi, Carlo Lugliè, and Carla Cannas
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History ,Multidisciplinary ,Polymers and Plastics ,Business and International Management ,Industrial and Manufacturing Engineering - Published
- 2022
10. As
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Marco, Sanna Angotzi, Valentina, Mameli, Alessandra, Fantasia, Claudio, Cara, Fausto, Secci, Stefano, Enzo, Marianna, Gerina, and Carla, Cannas
- Abstract
Iron oxides/oxyhydroxides, namely maghemite, iron oxide-silica composite, akaganeite, and ferrihydrite, are studied for As
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- 2021
11. Germination and early seedling development of Helichrysum microphyllum Cambess. subsp. tyrrhenicum Bacch., Brullo & Giusso in the presence of arsenates and arsenites
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Maria Enrica Boi, Marco Sanna Angotzi, Marco Porceddu, Elodia Musu, Valentina Mameli, Gianluigi Bacchetta, and Carla Cannas
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Multidisciplinary - Published
- 2022
12. On the design of mesostructured acidic catalysts for the one-pot dimethyl ether production from CO2
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Claudio Cara, Fausto Secci, Sarah Lai, Valentina Mameli, Kai Skrodczky, Patricia A. Russo, Francesca Ferrara, Elisabetta Rombi, Nicola Pinna, Mauro Mureddu, and Carla Cannas
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Process Chemistry and Technology ,Chemical Engineering (miscellaneous) ,Waste Management and Disposal - Published
- 2022
13. Meso- and macroporous silica-based arsenic adsorbents: effect of pore size, nature of the active phase, and silicon release
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Konstantin B. L. Borchert, Dana Schwarz, Valentina Mameli, Claudio Cara, Regine Boldt, Marco Sanna Angotzi, Christine Steinbach, and Carla Cannas
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Silicon ,General Engineering ,chemistry.chemical_element ,Bioengineering ,Sorption ,General Chemistry ,Atomic and Molecular Physics, and Optics ,Adsorption ,chemistry ,Chemical engineering ,Surface modification ,General Materials Science ,Water treatment ,Porosity ,Dissolution ,Arsenic - Abstract
Arsenic pollution in ground and drinking water is a major problem worldwide due to the natural abundance of arsenic by dissolution from ground sediment or mining activities from anthropogenic activities. To overcome this issue, iron oxides as low-cost and non-toxic materials, have been widely studied as efficient adsorbents for arsenic removal, including when dispersed within porous silica supports. In this study, two head-to-head comparisons were developed to highlight the As(V)-adsorptive ability of meso- and macrostructured silica-based adsorbents. First, the role of the textural properties of a meso-(SBA15) and macrostructured (MOSF) silica support in affecting the structural-morphological features and the adsorption capacity of the active phase (Fe2O3) have been studied. Secondly, a comparison of the arsenic removal ability of inorganic (Fe2O3) and organic (amino groups) active phases was carried out on SBA15. Finally, since silica supports are commonly proposed for both environmental and biomedical applications as active phase carriers, we have investigated secondary silicon and iron pollution. The batch tests at different pH revealed better performance from both Fe2O3-composites at pH 3. The values of qm of 7.9 mg g−1 (53 mg gact−1) and 5.5 mg g−1 (37 mg gact−1) were obtained for SBA15 and MOSF, respectively (gact stands for mass of the active phase). The results suggest that mesostructured materials are more suitable for dispersing active phases as adsorbents for water treatment, due to the obtainment of very small Fe2O3 NPs (about 5 nm). Besides studying the influence of the pore size of SBA15 and MOSF on the adsorption process, the impact of the functionalization was analyzed on SBA15 as the most promising sample for As(V)-removal. The amino-functionalized SBA15 adsorbent (3-aminopropyltriethoxysilane, APTES) exhibited a qm of 12.4 mg g−1 and faster kinetics. Furthermore, issues associated with the release of iron and silicon during the sorption process, causing secondary pollution, were evaluated and critically discussed.
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- 2021
14. Liquid Phase Synthesis of Nanostructured Spinel Ferrites—A Review
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Valentina Mameli, Claudio Cara, Carla Cannas, and Marco Sanna Angotzi
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Aqueous solution ,Materials science ,Spinel ,Biomedical Engineering ,Liquid phase ,Maghemite ,Bioengineering ,02 engineering and technology ,General Chemistry ,engineering.material ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,Chemical synthesis ,chemistry.chemical_compound ,Chemical engineering ,chemistry ,engineering ,Magnetic nanoparticles ,General Materials Science ,0210 nano-technology ,Chemical composition ,Magnetite - Abstract
Nanostructured spinel ferrites with formula M2+Fe2O4 (where M2+ ═ Fe, Co, Mn, Ni, Zn, etc.) represent a class of magnetic materials widely employed in different research fields, due to the possibility of finely tuning the magnetic properties by changing the chemical composition, size, shape, capping agents, etc. Herein, a review of the most common chemical synthesis in liquid media, distinguishing among aqueous and non-aqueous routes, is presented. The experimental procedure together with the mechanism, advantages and disadvantages, and examples are illustrated for each synthetic strategy.
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- 2019
15. 57Fe Mössbauer Spectroscopy for the Study of Nanostructured Mixed Mn–Co Spinel Ferrites
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Valentina Mameli, Anna Maria Giovanna Musinu, Marco Sanna Angotzi, and Daniel Ni Ňnanský
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Materials science ,Spinel ,Biomedical Engineering ,Analytical chemistry ,chemistry.chemical_element ,Nanoparticle ,Bioengineering ,02 engineering and technology ,General Chemistry ,Manganese ,engineering.material ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,chemistry ,Mössbauer spectroscopy ,X-ray crystallography ,engineering ,Ferrite (magnet) ,General Materials Science ,Crystallite ,0210 nano-technology ,Cobalt - Abstract
An oleate-based solvothermal approach has been employed to produce pure CoFe₂O₄ and MnFe₂O₄ and mixed Co-Mn ferrites having manganese content in the range 0.13-0.65 and crystallite size of about 8-9 nm. The structural and magnetic properties, studied by powder X-ray diffraction and room temperature 57Fe Mossbauer spectroscopy, have allowed to ascertain the formation of a unique spinel phase in which cobalt and manganese are present and to get insights on the cation distribution and the correlated magnetic properties.
- Published
- 2019
16. Raman Spectroscopy as a Probe for Monitoring the Zinc Presence in Zn-Substituted Cobalt Ferrites
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Carla Cannas, Valentina Mameli, Noemi Monni, Maria Laura Mercuri, Suchithra Ashoka Sahadevan, and Stefano Gai
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Materials science ,Thermal decomposition ,Biomedical Engineering ,Analytical chemistry ,chemistry.chemical_element ,Bioengineering ,General Chemistry ,Zinc ,Condensed Matter Physics ,Metal ,symbols.namesake ,chemistry ,visual_art ,Molecular vibration ,visual_art.visual_art_medium ,symbols ,General Materials Science ,Particle size ,Crystallite ,Raman spectroscopy ,Cobalt - Abstract
A Raman study on Zinc-substituted Cobalt ferrites, with different Zn(II) contents in each sample of formula: CoFe₂O₄ (1), Zn0.30Co0.70Fe2.00O₄ (2), Zn0.46Co0.54Fe2.02O₄ (3), Zn0.53Co0.47Fe2.02O₄ (4) is reported. These samples show the same crystallite size (∼6 nm), particle size (∼7 nm) and particle size distribution (∼20%) and they have been synthesized through heating up surfactant-assisted thermal decomposition of metalorganic precursors. The effect of Zn(II) substitution in the cationic distribution is investigated by using the known metal-oxygen vibrational modes in tetrahedral and octahedral sites. The presence of Zn(II) metal ion is determined through the band at 150 cm-1 (T2g(1) phonon mode), which is not present in the pure Co-ferrite, a blue-shift of the Eg vibrational mode depending on Zn(II)/Co(II) cationic distribution and a shoulder at ∼250 cm-1, which appears when zinc enters in the structure, and a broadening and a red-shift in the A1g phonon mode is observed in Raman spectra of 2-4 samples. Interestingly the latter represents a potential key probe to monitor the Zn(II) presence in Zn-substituted Co-ferrites.
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- 2019
17. Magnetic Interactions Versus Magnetic Anisotropy in Spinel Ferrite Nanoparticles
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Carla Cannas, M. Cobianchi, Alessandro Lascialfari, Anna Maria Giovanna Musinu, Davide Peddis, Giuseppe Muscas, Valeria Rodionova, Valentina Mameli, Dino Fiorani, and Alexander Omelyanchik
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Spinel ferrite ,Magnetic anisotropy ,chemistry.chemical_compound ,Materials science ,Chemical engineering ,chemistry ,Cobalt ferrite ,Magnetic nanoparticles ,Nanoparticle ,Electronic, Optical and Magnetic Materials ,Magnetite - Published
- 2019
18. Nanoscaled Metal-Organic Frameworks: Challenges Towards Biomedical Applications
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Suchithra Ashoka Sahadevan, Maria Laura Mercuri, Marco Sanna Angotzi, Carla Cannas, Noemi Monni, Mariangela Oggianu, and Valentina Mameli
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Materials science ,Biomedical Engineering ,Bioengineering ,Nanotechnology ,02 engineering and technology ,General Chemistry ,010402 general chemistry ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,Biocompatible material ,01 natural sciences ,0104 chemical sciences ,Nanoparticles ,General Materials Science ,Metal-organic framework ,0210 nano-technology ,Porosity ,Metal-Organic Frameworks - Abstract
Achieving metal-organic frameworks (MOFs) in the form of nanoparticles (NanoMOFs) represents a recent challenge due to the possibility to combine the intrinsic porosity of these materials with the nanometric dimension, a fundamental requirement for strategic biomedical applications. In this outlook we envision the current/future opportunities of the NanoMOFs in the field of biomedicine, with particular emphasis on (i) biocompatible MOFs composition; (ii) MOFs miniaturization and (iii) nanoMOFs applications.
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- 2021
19. Spinel ferrite nanoparticles in core–shell architecture for heat release
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Marco Sanna Angotzi, Valentina Mameli, Claudio Cara, Anna Musinu, Claudio Sangregorio, Daniel Niznansky, Huolin L. Xin, Jana Vejpravova, and Carla Cannas
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Inorganic Chemistry ,Structural Biology ,General Materials Science ,Physical and Theoretical Chemistry ,Condensed Matter Physics ,Biochemistry - Published
- 2021
20. The Hyvärinen scoring rule in Gaussian linear time series models
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A. Philip Dawid, Valentina Mameli, Silvia Columbu, and Monica Musio
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Statistics and Probability ,Series (mathematics) ,Autoregressive model ,Moving average ,Applied Mathematics ,Scoring rule ,Applied mathematics ,Estimator ,White noise ,Statistics, Probability and Uncertainty ,Sufficient statistic ,Autoregressive fractionally integrated moving average ,Mathematics - Abstract
In this work we study stationary linear time-series models, and construct and analyse “score-matching” estimators based on the Hyvarinen scoring rule. We consider two scenarios: a single series of increasing length, and an increasing number of independent series of fixed length. In the latter case there are two variants, one based on the full data, and another based on a sufficient statistic. We study the empirical performance of these estimators in three special cases, autoregressive (AR), moving average (MA) and fractionally differenced white noise (ARFIMA) models, and make comparisons with full and pairwise likelihood estimators. The results are somewhat model-dependent, with the new estimators doing well for MA and ARFIMA models, but less so for AR models.
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- 2021
21. A comparative study on high-dimensional bayesian regression with binary predictors
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Valentina Mameli, Debora Slanzi, and Philip J. Brown
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Statistics and Probability ,Lead optimization ,021103 operations research ,p≫n ,0211 other engineering and technologies ,Binary number ,02 engineering and technology ,High dimensional ,Bayesian regression ,Binary predictors ,Multiple regression ,p n ,01 natural sciences ,Multiple regression, Bayesian regression, p≫n, Binary predictors, Lead optimization ,010104 statistics & probability ,Modeling and Simulation ,Statistics ,Prior probability ,Linear regression ,0101 mathematics ,Bayesian linear regression ,Settore SECS-S/01 - Statistica ,Mathematics - Abstract
Bayesian regression models have been widely studied and adopted in the statistical literature. Many studies consider the development of reliable priors to select the relevant variables and derive accurate posterior predictive distributions. Moreover in the context of small high-dimensional data, where the number of observations is very small with respect to the number of predictors, sparsity is assumed and many parameters can be set to values close to zero without affecting the fit of the model. Aim of this work is to develop a comparative analysis to empirically evaluate the performances of several Bayesian regression approaches in these contexts. In this study we assume that the predictors can be expressed only as binary variables coding the presence or the absence of a particular characteristic of the system. This binary structure is often present in many real studies, in particular in laboratory experimentation and in very high-dimension genome wide association studies.
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- 2021
- Full Text
- View/download PDF
22. Technological insights on the Early-Middle Bronze Age pottery of Monte Meana cave (Sardinia, Italy)
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Giacomo Paglietti, Giuseppa Tanda, Rita Teresa Melis, Anna Musinu, Gabriele Cruciani, Marcello Franceschelli, Carla Cannas, Valentina Mameli, and Mariano Casu
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History ,Multidisciplinary ,Polymers and Plastics ,Business and International Management ,Industrial and Manufacturing Engineering - Abstract
An important Bronze Age settlement was discovered during an archaeological excavation in the Monte Meana karst cave in south-western Sardinia (Italy) between 2007 and 2012. In this region, the caves were used since the Neolithic for different purposes, such as burials or other rituals. The dig highlighted a rare example of domestic use of a cave and showed a case study of household space of the Early -Middle Bronze Age, at the beginning of the Nuragic civilization. This provided the opportunity to investigate through a multidisciplinary approach, the empirical knowledge of ancient potters and technological characters of local pottery production especially in relation to domestic use, in a context at that time devoid of external cultural interferences. For this purpose, a selection of 24 pottery sherds related to vessel forms for cooking, storage, and eating were studied through macroscopic surveys and archaeometric analysis by petrography, scanning electron microscopy, X-ray powder diffraction, and Fourier transform infrared spectroscopy. The results revealed some discriminant variables (shape, wall thickness, features of the paste, surface smoothing, presence of diagnostic mineralogical phases, and tempers), within the ceramic products of this Sardinian Bronze Age site, showing skillful management of firing temperatures.
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- 2022
23. Defect-assisted synthesis of magneto-plasmonic silver-spinel ferrite heterostructures in a flower-like architecture
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Anna Maria Giovanna Musinu, Stefano Enzo, Vincenzo Grillo, Claudio Cara, Carla Cannas, Valentina Mameli, and Marco Sanna Angotzi
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Diffraction ,Materials science ,Iron oxide ,Stacking ,lcsh:Medicine ,02 engineering and technology ,engineering.material ,010402 general chemistry ,01 natural sciences ,Silver nanoparticle ,Article ,chemistry.chemical_compound ,Nanoscience and technology ,Chemical synthesis ,lcsh:Science ,Multidisciplinary ,Rietveld refinement ,Spinel ,lcsh:R ,Heterojunction ,021001 nanoscience & nanotechnology ,0104 chemical sciences ,Chemistry ,chemistry ,Chemical engineering ,Physical chemistry ,engineering ,Ferrite (magnet) ,lcsh:Q ,Materials chemistry ,0210 nano-technology - Abstract
Artificial nano-heterostructures (NHs) with controlled morphology, obtained by combining two or more components in several possible architectures, make them suitable for a wide range of applications. Here, we propose an oleate-based solvothermal approach to design silver-spinel ferrite flower-like NHs. Small oleate-coated silver nanoparticles were used as seeds for the growth of magnetic spinel ferrite (cobalt ferrite and spinel iron oxide) nanodomains on their surface. With the aim of producing homogeneous flower-like heterostructures, a careful study of the effect of the concentration of precursors, the reaction temperature, the presence of water, and the chemical nature of the spinel ferrite was carried out. The magnetic and optical properties of the NHs were also investigated. A heterogeneous growth of the spinel ferrite phase on the silver nanoparticles, through a possible defect-assisted mechanism, was suggested in the light of the high concentration of stacking faults (intrinsic and twins) in the silver seeds, revealed by Rietveld refinement of powder X-ray diffraction patterns and High-Resolution electron microscopy.
- Published
- 2020
24. Identifying spatial patterns with the Bootstrap ClustGeo technique
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Valentina Mameli, Irene Poli, and Veronica Distefano
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Statistics and Probability ,business.industry ,Computer science ,Agglomerative hierarchical clustering, Bootstrap Technique, ClustGeo approach, Internal Validation Measures, Hamming distance, spatial data ,Agglomerative hierarchical clustering ,Pattern recognition ,Management, Monitoring, Policy and Law ,Hamming distance ,Pattern recognition (psychology) ,spatial data ,Spatial ecology ,Spatial clustering ,Artificial intelligence ,Computers in Earth Sciences ,Bootstrap Technique ,ClustGeo approach ,Internal Validation Measures ,Cluster analysis ,business ,Relevant information - Abstract
Building clusters for pattern recognition and analysis of geographical areas can be a useful way to provide relevant information for economic and social decisions. In this paper, we introduce a novel spatial clustering technique, called Bootstrap ClustGeo (BCG), which is a hierarchical approach, based on bootstrap techniques with spatial constraints. We evaluate the performance of the proposed approach BCG through some real case studies and simulations studies with different complexity, by computing Clustering Validation Measures (CVM) and then we compare the approach with the recently proposed ClustGeo (CG). These analyses exhibit the accuracy of BCG, also with respect to CG, in the presented applications, and highlight the great potentiality of this new clustering technique to provide meaningful information for spatial analysis.
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- 2020
25. γ-Fe2O3-M41S Sorbents for H2S Removal: Effect of Different Porous Structures and Silica Wall Thickness
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Anna Maria Giovanna Musinu, Elisabetta Rombi, Claudio Cara, Carla Cannas, Andrea Ardu, Valentina Mameli, Daniel Niznansky, and Marco Sanna Angotzi
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Materials science ,Iron oxide ,Sulfidation ,Maghemite ,Nanoparticle ,02 engineering and technology ,engineering.material ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,0104 chemical sciences ,Surfaces, Coatings and Films ,Electronic, Optical and Magnetic Materials ,chemistry.chemical_compound ,General Energy ,chemistry ,MCM-41 ,Chemical engineering ,engineering ,Physical and Theoretical Chemistry ,0210 nano-technology ,Porosity ,Spectroscopy ,Iron oxide nanoparticles - Abstract
In this work, the effect of the M41S support pore structure (hexagonal or cubic) and of the wall thickness of the silica mesochannels has been evaluated aimed at achieving more and more efficient and regenerable iron oxide-based sorbents for H2S removal at midtemperature. With this purpose, we set up a simple Pluronic-free synthetic strategy capable of producing silica supports with hexagonal (MCM-41) or cubic (MCM-48) pore structure with different wall thicknesses that have been used to fabricate the corresponding sorbents made up of iron oxide nanoparticles homogeneously dispersed into the mesochannels. The combined use of 57Fe-Mossbauer Spectroscopy and DC magnetometry has allowed for ascertaining the presence of maghemite in the form of ultrasmall nanoparticles in both composites and gives new insights on the influence of the different silica matrices on the active phase features. The performances of the sorbents have been evaluated at midtemperature (300 °C) through three repeated sulfidation and reg...
- Published
- 2018
26. Geo-material provenance and technological properties investigation in Copper Age menhirs production at Allai (central-western Sardinia, Italy)
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Carla Cannas, Valentina Mameli, and Marco Serra
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provenance study ,Archeology ,Provenance ,Geography ,2nd millennium BC ,lcsh:Archaeology ,lcsh:CC1-960 ,Chalcolithic ,technological properties ,Sardinia ,Copper Age ,Archaeology ,menhir - Abstract
During the 2nd millennium BC anthropomorphic menhirs belonging to a 3rd millennium BC sanctuary were reused as building material in the Arasseda Nuraghe (Sardinia, Italy). To analyse the Arasseda menhirs and the local Monte Ironi geological samples (presenting similar visual features), chemical (pXRF, ICP-OES, ICP-MS), mineralogical-chemical (PXRD) and physical (Mohs hardness) measurements were performed. Through the experimental data, the menhirs source provenance and the technological properties (workability, durability) of the raw material chosen for sculptural purposes during Copper Age were investigated. To the authors’ knowledge this is the first archaeometric study on the Arasseda menhirs (the third on Sardinian menhirs) and one between the few recently developed on European megaliths.
- Published
- 2017
27. Spinel Ferrite Core–Shell Nanostructures by a Versatile Solvothermal Seed-Mediated Growth Approach and Study of Their Nanointerfaces
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Carla Cannas, Anna Maria Giovanna Musinu, Valentina Mameli, Huolin L. Xin, Claudio Cara, Andrea Ardu, Marco Sanna Angotzi, and Daniel Niznansky
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Nanostructure ,Materials science ,Electron energy loss spectroscopy ,Spinel ,General Engineering ,General Physics and Astronomy ,Mineralogy ,Nanoparticle ,Maghemite ,02 engineering and technology ,engineering.material ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,0104 chemical sciences ,Electron tomography ,Chemical engineering ,Scanning transmission electron microscopy ,engineering ,Ferrite (magnet) ,General Materials Science ,0210 nano-technology - Abstract
An easy, low-cost, repeatable seed-mediated growth approach in solvothermal condition has been proposed to synthesize bimagnetic spinel ferrite core-shell heterostructures in the 10-20 nm particle size range. Cobalt ferrite and manganese ferrite nanoparticles (CoFe2O4 and MnFe2O4) have been coated with isostructural spinel ferrites like maghemite/magnetite, MnFe2O4, and CoFe2O4 with similar cell parameters to create different heterostructures. The conventional study of the structure, morphology, and composition has been combined with advanced techniques in order to achieve details on the interface at the nanoscale level. Clear evidence of the heterostructure formation have been obtained (i) indirectly by comparing the 57Fe Mossbauer spectra of the core-shell samples and an ad hoc mechanical mixture and (ii) directly by mapping the nanoparticles' chemical composition by electron energy loss spectroscopy (EELS) and energy-dispersive X-ray spectroscopy (EDX) in the scanning transmission electron microscopy mode (STEM). In addition, chemical-sensitive electron tomography in STEM-EDX mode has been applied in order to obtain detailed 3D images with a sub-nanometer spatial resolution.
- Published
- 2017
28. MCM-41 support for ultrasmall γ-Fe2O3 nanoparticles for H2S removal
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Luciano Atzori, Valentina Mameli, Daniel Niznansky, Claudio Cara, Andrea Ardu, Huolin L. Xin, Elisabetta Rombi, M. Sanna Angotzi, Anna Maria Giovanna Musinu, and Carla Cannas
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Sorbent ,Materials science ,Nanocomposite ,Renewable Energy, Sustainability and the Environment ,Maghemite ,Nanoparticle ,Nanotechnology ,02 engineering and technology ,General Chemistry ,Hydrogen sulphide ,engineering.material ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,0104 chemical sciences ,Chemical engineering ,MCM-41 ,Integrated gasification combined cycle ,engineering ,General Materials Science ,Reactivity (chemistry) ,0210 nano-technology - Abstract
MCM-41 is proposed to build mesostructured Fe2O3-based sorbents as an alternative to other silica or alumina supports for mid-temperature H2S removal. MCM-41 was synthesized as micrometric (MCM41_M) and nanometric (MCM41_N) particles and impregnated through an efficient two-solvent (hexane–water) procedure to obtain the corresponding γ-Fe2O3@MCM-41 composites. The active phase is homogeneously dispersed within the 2 nm channels in the form of ultrasmall maghemite nanoparticles assuring a high active phase reactivity. The final micrometric (Fe_MCM41_M) and nanometric (Fe_MCM41_N) composites were tested as sorbents for hydrogen sulphide removal at 300 °C and the results were compared with a reference sorbent (commercial unsupported ZnO) and an analogous silica-based sorbent (Fe_SBA15). MCM-41 based sorbents, having the highest surface areas, showed superior performances that were retained after the first sulphidation cycle. Specifically, the micrometric sorbent (Fe_MCM41_M) showed a higher SRC value than the nanometric one (Fe_MCM41_N), due to the low stability of the nanosized particles over time caused by their high reactivity. Furthermore, the low regeneration temperature (300–350 °C), besides the high removal capacity, renders MCM41-based systems an alternative class of regenerable sorbents for thermally efficient cleaning up processes in Integrated Gasification Combined Cycles (IGCC) systems.
- Published
- 2017
29. Much More Than a Glass: The Complex Magnetic and Microstructural Properties of Obsidian
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Davide Peddis, Anna Maria Giovanna Musinu, Valentina Mameli, Guido Ennas, Carla Cannas, Andrea Ardu, Dl Niznansky, and Carlo Lugliè
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Materials science ,Magnetism ,Mineralogy ,02 engineering and technology ,010502 geochemistry & geophysics ,021001 nanoscience & nanotechnology ,01 natural sciences ,obsidian ,Surfaces, Coatings and Films ,Electronic, Optical and Magnetic Materials ,Volcanic glass ,Condensed Matter::Materials Science ,Paramagnetism ,General Energy ,Ferromagnetism ,Chemical physics ,Antiferromagnetism ,Physical and Theoretical Chemistry ,0210 nano-technology ,High-resolution transmission electron microscopy ,Powder diffraction ,0105 earth and related environmental sciences ,Superparamagnetism - Abstract
Obsidian is a natural volcanic glass in which nanocrystalline and microcrystalline phases can coexist with the glassy one. In this paper, magnetic properties of Monte Arci obsidians are investigated by an experimental approach commonly applied to synthetic nanostructured materials and rarely to natural ones, and correlated with the mineralogical composition and microstructure. Among the different crystalline phases, the iron-containing components are found to be responsible for a great variety of magnetic behaviors, including paramagnetism, antiferromagnetism, ferromagnetism, and superparamagnetism. The combined use of powder X-ray diffraction (PXRD), Fe-57 Mossbauer spectroscopy, DC magnetometry, and transmission electron microscopy (TEM/HRTEM) provides new insights in the Monte Arci obsidian: (i) the presence of magnetite nanoparticles spread into the glassy matrix; (ii) the presence of an antiferromagnetic phase responsible for a discontinuity at about 45 K; (iii) exchange bias phenomena, for the first time revealed in obsidians, due to the coupling between the nanostructured ferrimagnetic phase and the antiferromagnetic one; (iv) Goldanskii-Karyagin effect (GKE) associated with biotite.
- Published
- 2016
30. Oleate-Based Solvothermal Approach for Size Control of M
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Marco Sanna, Angotzi, Valentina, Mameli, Claudio, Cara, Andrea, Ardu, Daniel, Ni Ňnanský, and Anna, Musinu
- Abstract
The versatility of a promising and repeatable oleate-based solvothermal approach has been explored through the synthesis of MnFe₂O₄ and
- Published
- 2019
31. Estimating High-Dimensional Regression Models with Bootstrap group Penalties
- Author
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Valentina Mameli, Irene Poli, and Debora Slanzi
- Subjects
Group (mathematics) ,Computer science ,media_common.quotation_subject ,Bi-level selection ,Structure (category theory) ,Regression analysis ,Feature selection ,Regression models ,computer.software_genre ,Variable selections ,Bootstrap ,Lasso (statistics) ,High-dimensionality ,Quality (business) ,Data mining ,High dimensionality ,Settore SECS-S/01 - Statistica ,High dimensional regression ,computer ,media_common - Abstract
Currently many research problems are addressed by analysing datasets characterized by a huge number of variables, with a relatively limited number of observations, especially when data are generated by experimentation. Most of the classical statistical procedures for regression analysis are often inadequate to deal with such datasets as they have been developed assuming that the number of observations is larger than the number of the variables. In this work, we propose a new penalization procedure for variable selection in regression models based on Bootstrap group Penalties (BgP). This new family of penalization methods extends the bootstrap version of the LASSO approach by taking into account the grouping structure that may be present or introduced in the model. We develop a simulation study to compare the performance of this new approach with respect several existing group penalization methods in terms of both prediction accuracy and variable selection quality. The results achieved in this study show that the new procedure outperforms the other penalties procedures considered.
- Published
- 2019
32. Eneolithic menhirs of Laconi (central Sardinia, Italy): from provenance to technological properties
- Author
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Marco Serra, Valentina Mameli, and Carla Cannas
- Subjects
010506 paleontology ,Archeology ,Provenance ,060102 archaeology ,Visual examination ,06 humanities and the arts ,Chalcolithic ,01 natural sciences ,Archaeology ,Archaeological science ,Megalith ,Prehistory ,0601 history and archaeology ,Geological materials ,Optical emission spectrometry ,Geology ,0105 earth and related environmental sciences - Abstract
A multi-technique approach was carried out to investigate the provenance and technological properties of 34 anthropomorphic menhirs from Laconi (central Sardinia, Italy) dated back to the local Copper Age (III millennium BC). After visual inspection, geological materials coming from local ignimbritic outcrops were characterized by elemental and mineralogical analyses: portable X-Ray Fluorescence (pXRF), Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES), Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) and Powder X-Ray Diffraction (PXRD). Conversely, the menhirs were studied only by visual examination and non-destructive pXRF measurements. By means of a comparison between pXRF data collected on the menhirs and the lithological outcrops, it was possible to assign the prehistoric artifacts of Laconi to local geo-sources, taking into account geochemical intra- and inter-source variabilities. pXRF was demonstrated to be a useful tool for provenance studies while mineralogical analysis resulted fundamental for the technological aspects. The latter technique pointed out the main use of hardstones, probably owing to the durability they give to the menhirs. To the best of our knowledge, this is the first archaeometric study on Sardinian menhirs and one of the few recently developed on European megaliths.
- Published
- 2016
33. Designing Magnetic NanoMOFs for Biomedicine: Current Trends and Applications
- Author
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Mariangela Oggianu, Suchithra Ashoka Sahadevan, Maria Laura Mercuri, Valentina Mameli, Carla Cannas, and Noemi Monni
- Subjects
biomedical applications ,theranostics ,Theranostic Nanomedicine ,Computer science ,business.industry ,Magnetism ,nanoMOFs ,Context (language use) ,Nanotechnology ,Electronic, Optical and Magnetic Materials ,lcsh:Chemistry ,metal–organic frameworks ,biocompatibility ,lcsh:QD1-999 ,Chemistry (miscellaneous) ,magnetism ,Materials Chemistry ,Miniaturization ,Nanocarriers ,business ,Biomedicine - Abstract
Metal–organic frameworks (MOFs) have shown a great potential in biomedicine due to their promising applications in different fields, including drug delivery, thermometry, theranostics etc. In this context, the development of magnetic sub-micrometric or nanometric MOFs through miniaturization approaches of magnetic MOFs up to the nanoscale still represents a crucial step to fabricate biomedical probes, especially in the field of theranostic nanomedicine. Miniaturization processes have to be properly designed to tailor the size and shape of particles and to retain magnetic properties and high porosity in the same material, fundamental prerequisites to develop smart nanocarriers integrating simultaneously therapeutic and contrast agents for targeted chemotherapy or other specific clinical use. An overview of current trends on the design of magnetic nanoMOFs in the field of biomedicine, with particular emphasis on theranostics and bioimaging, is herein envisioned.
- Published
- 2020
34. Anchoring ultrasmall FeIII-based nanoparticles on silica and titania mesostructures for syngas H2S purification
- Author
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Claudio Cara, Anna Maria Giovanna Musinu, Carla Cannas, Elisabetta Rombi, Marco Sanna Angotzi, Daniel Nižňanský, Valentina Mameli, and Nicola Pinna
- Subjects
Anatase ,Materials science ,Sorbent ,Sulfidation ,Nanoparticle ,Maghemite ,Sintering ,Sorption ,02 engineering and technology ,General Chemistry ,engineering.material ,010402 general chemistry ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,01 natural sciences ,0104 chemical sciences ,Chemical engineering ,Mechanics of Materials ,engineering ,General Materials Science ,Reactivity (chemistry) ,0210 nano-technology - Abstract
Mesostructured titania (Anatase) and silica (MCM-41) were proposed as supports to design highly active, selective, and regenerable FeIII-based nanostructured sorbents for mid-temperature H2S removal in a model sour syngas. The resulting sorbents (Fe–SiO2 and Fe–TiO2) were tested as H2S removers at 300 °C and exhibited high reactivity and regenerability over repeated sulfidation cycles, with the best sorption performances achieved by the silica-based sorbent. Specifically, Fe–SiO2 showed a constant sorption capacity of 19 ± 1 mgS gsorbent−1 after the first sulfidation cycle. Meanwhile, a lower sorption capacity of 10 ± 1 mgS gsorbent−1 was found for the Fe–TiO2 composite. As evidenced by combining 57Fe Mossbauer spectroscopy with DC magnetometry, the nature (amorphous or crystalline) and composition (SiO2 or TiO2) of the inorganic mesostructures played a crucial role in the formation of the ultrasmall FeIII-active phase: maghemite (γ-Fe2O3) and pseudobrookite (Fe2TiO5) in the case of silica and titania, respectively. Therefore, the performance can be mainly justified in the light of the different reactivity of the active phases (Fe2O3 vs Fe2TiO5). FeIII-active phase in the form of ultrasmall Fe2O3 nanoparticles (about 2 nm) homogeneously dispersed in a highly stable mesostructured silica support assured high reactivity (85%–100% of the active phase involvement) and regenerability in the mid-temperature range as for the sulfidation run during the repeated sulfidation cycles (steady performances) avoiding the common drawbacks of unsupported sorbents (unreacted core and sintering phenomena causing loss of activity).
- Published
- 2020
35. Likelihood Asymptotics in Nonregular Settings. A Review with Emphasis on the Likelihood Ratio
- Author
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Brazzale , Alessandra R. and Valentina , Mameli
- Subjects
SECS-S/01 Statistica - Abstract
We review the most common situations where one or some of the regularity conditions which underlie likelihood based parametric inference fail. Three main classes of problems will be treated: boundary problems, indeterminate parameters and singular information matrices, and change-point problems. The review is focused on the large- and small-sample properties of the likelihood ratio, though other approaches to hypothesis testing and connections to estimation will be mentioned in passing.
- Published
- 2018
36. Multi-objective Optimization in High-Dimensional Molecular Systems
- Author
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Marina Khoroshiltseva, Irene Poli, Debora Slanzi, and Valentina Mameli
- Subjects
0301 basic medicine ,Computer science ,Statistical model ,High dimensional ,Molecular systems ,Evolutionary strategies ,computer.software_genre ,Multi-objective optimization ,Statistical models ,03 medical and health sciences ,030104 developmental biology ,Statistical inference ,Data mining ,Settore SECS-S/01 - Statistica ,computer - Abstract
The paper proposes a methodological approach to design complex experiments for multi-objective optimization. The strategy is based on evolutionary statistical inference to search for the optimal values in high-dimensional experimental spaces. We developed this approach to study a particular molecular system and discover the best molecules to be proposed as candidate drugs.
- Published
- 2018
37. Bootstrap adjustments of signed scoring rule root statistics
- Author
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Monica Musio, Valentina Mameli, and Laura Ventura
- Subjects
Statistics and Probability ,Scoring rule ,Tsallis scoring rule ,Inference ,01 natural sciences ,Parametric Bootstrap ,Asymptotic expansions, Higher-order inference, Parametric Bootstrap, Regression models, Robustness, Tsallis scoring rule ,010104 statistics & probability ,Robustness (computer science) ,0502 economics and business ,Statistics ,Statistical inference ,Higher-order inference ,0101 mathematics ,Robustness ,Statistic ,050205 econometrics ,Parametric statistics ,Mathematics ,Asymptotic expansions ,Regression models ,05 social sciences ,Estimator ,Regression analysis ,Modeling and Simulation ,Settore SECS-S/01 - Statistica - Abstract
Scoring rules give rise to methods for statistical inference and are useful tools to achieve robustness or reduce computations. Scoring rule inference is generally performed through first-order approximations to the distribution of the scoring rule estimator or of the ratio-type statistic. In order to improve the accuracy of first-order methods even in simple models, we propose bootstrap adjustments of signed scoring rule root statistics for a scalar parameter of interest in presence of nuisance parameters. The method relies on the parametric bootstrap approach that avoids onerous calculations specific of analytical adjustments. Numerical examples illustrate the accuracy of the proposed method.
- Published
- 2018
38. Higher-order asymptotics for scoring rules
- Author
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Laura Ventura and Valentina Mameli
- Subjects
Statistics and Probability ,Third-order inference ,Tsallis scoring rule ,Applied Mathematics ,Scoring rule ,Ratio test ,Estimator ,Likelihood asymptotics ,Hyvärinen scoring rule ,Information identity ,Asymptotic expansions Hyvärinen scoring rule Information identity Likelihood asymptotics Robustness Third-order inference Tsallis scoring rule ,Asymptotic expansions ,Robustness (computer science) ,Scoring algorithm ,Robustness ,Statistics ,Applied mathematics ,Statistics, Probability and Uncertainty ,Settore SECS-S/01 - Statistica ,Statistic ,Mathematics - Abstract
In this paper we discuss higher-order asymptotic expansions for proper scoring rules generalizing results for likelihood quantities, but meanwhile bring in the difficulty caused by the failure of the information identity. In particular, we derive higher-order approximations to the distribution of the scoring rule estimator, of the scoring rule ratio test statistic and, for a scalar parameter of interest, of the signed scoring rule root statistic. From these expansions, a modified signed scoring rule root statistic is proposed. Examples are given illustrating the accuracy of the modified signed scoring rule root statistic with respect to first-order methods.
- Published
- 2015
39. The Kumaraswamy skew-normal distribution
- Author
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Valentina Mameli
- Subjects
Statistics and Probability ,Mathematical optimization ,Distribution (number theory) ,Beta skew-normal distribution ,Skew normal distribution ,Generalization ,Kumaraswamy distribution ,Skew-normal distribution ,Combinatorics ,Kumaraswamy skew-normal distribution ,Statistics, Probability and Uncertainty ,Settore SECS-S/01 - Statistica ,Generalized normal distribution ,Mathematics - Abstract
We propose a new generalization of the skew-normal distribution (Azzalini, 1985) referred to as the Kumaraswamy skew-normal. The new distribution is computationally more tractable than the Beta skew-normal distribution (Mameli and Musio, 2013) with which it shares some properties.
- Published
- 2015
40. Dialkylamide as Both Capping Agent and Surfactant in a Direct Solvothermal Synthesis of Magnetite and Titania Nanoparticles
- Author
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Claudio Cara, Mariano Andrea Scorciapino, Anna Maria Giovanna Musinu, Carla Cannas, Daniel Niznansky, Valentina Mameli, Andrea Ardu, Josef Buršík, and Giorgia Manzo
- Subjects
Materials science ,Solvothermal synthesis ,Dispersity ,Inorganic chemistry ,Iron oxide ,Infrared spectroscopy ,General Chemistry ,Condensed Matter Physics ,Nanomaterials ,chemistry.chemical_compound ,chemistry ,Chemical engineering ,Oleylamine ,General Materials Science ,Particle size ,Magnetite - Abstract
An ecofriendly, low-cost, one-pot solvothermal approach has been developed to prepare spherical magnetite nanoparticles with sizes in the 7–12 nm range capped with a dialkylamine. Iron isopropoxide, water vapor, absolute ethanol, oleic acid, and oleylamine were used as iron oxide precursor, hydrolysis agent, solvent and surfactants, respectively. The surfactants’ role was investigated and an accurate correlation among the synthetic parameters, the crystallographic phases, and both crystallite and particle size was found. The amounts of oleylamine and oleic acid and the temperature have been revealed to be the key parameters in order to tune particle size and their polydispersity. An in-depth study on the role of each surfactant has pointed out the fundamental role of the amine as a reduction promoter as demonstrated by using different amines and confirmed by Mossbauer measurements. A dual 1H NMR-Fourier transform infrared spectroscopy approach on selected experiments for the investigation of the capping a...
- Published
- 2015
41. Reducing dimensionality of molecular systems: a Bayesian non-parametric approach
- Author
-
Debora Slanzi, Marina Khoroshiltseva, Irene Poli, Valentina Mameli, and Nicola Lunardon
- Subjects
0301 basic medicine ,Penalized regression procedures ,Computer science ,Evolutionary algorithms for optimization ,Bayesian probability ,Complex system ,Binary number ,Feature selection ,Statistical model ,Lead molecule optimization ,computer.software_genre ,01 natural sciences ,Bayesian non-parametric clustering ,High-dimensional models ,0104 chemical sciences ,010404 medicinal & biomolecular chemistry ,03 medical and health sciences ,030104 developmental biology ,Binary data ,Data mining ,Cluster analysis ,Settore SECS-S/01 - Statistica ,computer ,Curse of dimensionality - Abstract
In this paper we present a methodology that can be used to design experiments of complex systems characterized by a huge number of variables. The strategy combines the evolutionary principles with the information provided by statistical models tailored to the problem under consideration. Here, we are concerned with the process of design molecules, which is a quite challenging problem due to the presence of a high number of variables with a binary structure. Recent works on clustering of binary data and variable selection in the high-dimensional setting allow to develop an approach capable of recovering useful information derived from the incorporation of a grouping structure into the model.
- Published
- 2017
42. Studying the effect of Zn-substitution on the magnetic and hyperthermic properties of cobalt ferrite nanoparticles
- Author
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Anna Maria Giovanna Musinu, Daniel Niznansky, Nguyen T. K. Thanh, Valentina Mameli, Guido Ennas, Claudia Innocenti, Davide Peddis, Claudio Sangregorio, Andrea Ardu, and Carla Cannas
- Subjects
Materials science ,Magnetometer ,Dispersity ,Analytical chemistry ,chemistry.chemical_element ,Nanotechnology ,02 engineering and technology ,Zinc ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,0104 chemical sciences ,law.invention ,Magnetization ,Surface coating ,chemistry ,law ,Mössbauer spectroscopy ,hyperthermic properties ,General Materials Science ,0210 nano-technology ,Saturation (magnetic) ,Chemical composition - Abstract
The possibility to finely control nanostructured cubic ferrites ((MFe2O4)-Fe-II) paves the way to design materials with the desired magnetic properties for specific applications. However, the strict and complex interrelation among the chemical composition, size, polydispersity, shape and surface coating renders their correlation with the magnetic properties not trivial to predict. In this context, this work aims to discuss the magnetic properties and the heating abilities of Zn-substituted cobalt ferrite nanoparticles with different zinc contents (ZnxCo1-xFe2O4 with 0 < x < 0.6), specifically prepared with similar particle sizes (similar to 7 nm) and size distributions having the crystallite size (similar to 6 nm) and capping agent amount of 15%. All samples have high saturation magnetisation (M-s) values at 5 K (>100 emu g(-1)). The increase in the zinc content up to x = 0.46 in the structure has resulted in an increase of the saturation magnetisation (Ms) at 5 K. High Ms values have also been revealed at room temperature (similar to 90 emu g(-1)) for both CoFe2O4 and Zn0.30Co0.70Fe2O4 samples and their heating ability has been tested. Despite a similar saturation magnetisation, the specific absorption rate value for the cobalt ferrite is three times higher than the Zn-substituted one. DC magnetometry results were not sufficient to justify these data, the experimental conditions of SAR and static measurements being quite different. The synergic combination of DC with AC magnetometry and Fe-57 Mossbauer spectroscopy represents a powerful tool to get new insights into the design of suitable heat mediators for magnetic fluid hyperthermia.
- Published
- 2016
43. Some New Results on the Beta Skew-Normal Distribution
- Author
-
Monica Musio and Valentina Mameli
- Subjects
Work (thermodynamics) ,Distribution (number theory) ,Skew normal distribution ,Generalized beta distribution ,Applied mathematics ,Beta (velocity) ,Variance (accounting) ,Settore SECS-S/01 - Statistica ,Mathematics - Abstract
In this paper we study the Beta skew-normal distribution introduced by Mameli and Musio (2013). Some new properties of this distribution are derived including formulae for moments in particular cases and bi-modality properties. Furthermore, we provide expansions for its distribution and density functions. Bounds for the moments and the variance of the Beta skew-normal are derived. Some of the results presented in this work can be extended to the entire family of the Beta-generated distribution introduced by Jones (Test 13(1):1–43, 2004).
- Published
- 2016
44. Modern likelihood inference for the maximum/minimum of a bivariate normal vector
- Author
-
Alessandra Rosalba Brazzale and Valentina Mameli
- Subjects
Statistics and Probability ,Skew normal distribution ,Multivariate random variable ,Coverage probability ,Multivariate normal distribution ,modified likelihood root ,01 natural sciences ,010104 statistics & probability ,0502 economics and business ,Statistics ,Bivariate normal distribution ,higher order likelihood inference ,skew-normal distribution ,0101 mathematics ,Generalized normal distribution ,050205 econometrics ,Mathematics ,Applied Mathematics ,Modeling and Simulation ,Statistics, Probability and Uncertainty ,05 social sciences ,Confidence interval ,Confidence distribution ,Probability and Uncertainty ,Likelihood function ,Settore SECS-S/01 - Statistica - Abstract
We consider the use of modern likelihood asymptotics in the construction of confidence intervals for the parameter which determines the skewness of the distribution of the maximum/minimum of an exchangeable bivariate normal random vector. Simulation studies were conducted to investigate the accuracy of the proposed methods and to compare them to available alternatives. Accuracy is evaluated in terms of both coverage probability and expected length of the interval. We furthermore illustrate the suitability of our proposals by means of two data sets, consisting of, respectively, measurements taken on the brains of 10 mono-zygotic twins and measurements of mineral content of bones in the dominant and non-dominant arms for 25 elderly women.
- Published
- 2015
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45. A Generalization of the Skew-Normal Distribution: The Beta Skew-Normal
- Author
-
Valentina Mameli and Monica Musio
- Subjects
FOS: Computer and information sciences ,Statistics and Probability ,Beta negative binomial distribution ,Skew normal distribution ,Asymptotic distribution ,Methodology (stat.ME) ,Ratio distribution ,Combinatorics ,symbols.namesake ,Generalized beta distribution ,FOS: Mathematics ,Statistics - Methodology ,Mathematics ,Probability (math.PR) ,Balakrishnan skew-normal ,Skew-normal distribution ,Beta prime distribution ,Pearson distribution ,Order statistics ,Beta skew-normal ,symbols ,Matrix normal distribution ,62H10 ,Settore SECS-S/01 - Statistica ,Mathematics - Probability - Abstract
The aim of this article is to introduce a new family of distributions, which generalizes the skew normal distribution (SN). This new family, called Beta skew-normal (BSN), arises naturally when we consider the distributions of order statistics of the SN. The BSN can also be obtained as a special case of the Beta generated distribution (Jones (2004)). In this work we pay attention to three other generalizations of the SN distribution: the Balakrishnan skew-normal (SNB) (Balakrishnan (2002), as a discussant of Arnold and Beaver (2002), Gupta and Gupta (2004), Sharafi and Behboodian (2008)), the generalized Balakrishnan skew-normal (GBSN) (Yadegari et al. (2008)) and a two parameter generalization of the Balakrishnan skew-normal (TBSN) (Bahrami et al. (2009)). The above three extensions are related to the Beta skew-normal distribution for particular values of the parameters. The paper is organized as follows: after describing briefly, in Section 2, the skew-normal distribution, its generalizations and listing their most important properties, in Section 3 we present some generalizations of the Beta distribution. In the last Section we define the Beta skew-normal distribution, we present its properties and some special cases. In particular the BSN contains the Beta half-normal distribution (Pescim et al. (2010)) as limiting case. Besides, we investigate its shape properties. We derive its moment generating function and we also compute numerically the first moment, the variance, the skewness and the kurtosis. We present two different methods which allow to simulate a BSN distribution. We explore its relationships with the other generalizations of the SN. We give some results concerning the SNB distribution. In particular we derive the exact distributions of the largest order statistic from SNB with parameters m and 1 and the shortest order statistic SNB with parameters m and -1., 21 pages, 3 figures and 1 table
- Published
- 2013
46. Using Integrated Nested Laplace Approximations for Modelling Spatial Healthcare Utilization
- Author
-
Erik-A. Sauleau, Valentina Mameli, and Monica Musio
- Subjects
Hyperparameter ,education.field_of_study ,Computer science ,Bayesian probability ,Population ,Markov chain Monte Carlo ,Bayesian inference ,Data science ,Statistics::Computation ,Deviance information criterion ,Binomial distribution ,symbols.namesake ,Covariate ,Statistics ,symbols ,Settore SECS-S/01 - Statistica ,education - Abstract
In this work we propose different spatial models to study hospital recruitment, including some potentially explanatory variables, using data from the hospital of Mulhouse a town located in the north-east of France. Interest is on the distribution over geographical units of the number of patients living in this geographical unit. Models considered are within the framework of Bayesian latent Gaussian models. Our response variable is assumed to follow a binomial distribution, with logit link, whose parameters are the population in the geographical unit and the corresponding risk. The structured additive predictor accounts for effects of various covariates in an additive way, including smoothing functions of the covariates (for example a spatial effect). To approximate posterior marginals, which are not available in closed form, we use integrated nested Laplace approximations (INLA), recently proposed for approximate Bayesian inference in latent Gaussian models. INLA has the advantage of giving very accurate approximations and being faster than MCMC methods when the number of hyperparameters does not exceed 6 (as in our case). Model comparison is performed using the Deviance Information Criterion.
- Published
- 2012
47. Large sample confidence intervals for the skewness parameter of the skew-normal distribution based on Fisher's transformation
- Author
-
Valentina Mameli, Annibale Biggeri, Monica Musio, and Erik Sauleau
- Subjects
Statistics and Probability ,Skew normal distribution ,Pearson distribution ,Nonparametric skew ,Shape parameter ,symbols.namesake ,Student's t-distribution ,Skewness ,Statistics ,symbols ,Kurtosis ,Confidence distribution ,Statistics, Probability and Uncertainty ,Settore SECS-S/01 - Statistica ,Mathematics - Abstract
The skew-normal model is a class of distributions that extends the Gaussian family by including a skewness parameter. This model presents some inferential problems linked to the estimation of the skewness parameter. In particular its maximum likelihood estimator can be infinite especially for moderate sample sizes and is not clear how to calculate confidence intervals for this parameter. In this work, we show how these inferential problems can be solved if we are interested in the distribution of extreme statistics of two random variables with joint normal distribution. Such situations are not uncommon in applications, especially in medical and environmental contexts, where it can be relevant to estimate the distribution of extreme statistics. A theoretical result, found by Loperfido [7], proves that such extreme statistics have a skew-normal distribution with skewness parameter that can be expressed as a function of the correlation coefficient between the two initial variables. It is then possible, using...
- Published
- 2012
48. Bayesian Inference for directional data through ABC and homogeneous proper scoring rules
- Author
-
Monica, Musio, Valentina, Mameli, erlis ruli, and LAURA VENTURA
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