Search

Poly-L-lactide (PLLA) offers a unique possibility for processing into biocompatible, biodegradable, and implantable piezoelectric structures. With such properties, PLLA has potential to be used as an advanced tool for mimicking biophysical processes that naturally occur during the self-repair of wounds and damaged tissues, including electrostimulated regeneration. The piezoelectricity of PLLA strongly depends on the possibility of controlling its crystallinity and molecular orientation. Here, it is shown that modifying PLLA with a small amount (1 wt%) of crystalline filler particles with a high aspect ratio, which act as nucleating agents during drawing-induced crystallization, promotes the formation of highly crystalline and oriented PLLA structures. This increases their piezoelectricity, and the filler-modified PLLA films provide a 20-fold larger voltage output than nonmodified PLLA during ultrasound (US)-assisted activation. With 99% PLLA content, the ability of the films to produce reactive oxygen species (ROS) and increase the local temperature during interactions with US is shown to be very low. US-assisted piezostimulation of adherent cells directly attach to their surface (such as skin keratinocytes), stimulate cytoskeleton formation, and as a result cells elongate and orient themselves in a specific direction that align with the direction of PLLA film drawing and PLLA dipole orientation., Small, ISSN:1613-6810, ISSN:1613-6829

Understanding the interplay between lattice mismatch and supersaturation in the hydrothermal topochemical transformation from Bi4Ti3O12 to SrTiO3 nanoplatelets in view of nucleation–crystallization theory and atomic scale interfacial analysis.

Low-temperature hydrothermal epitaxial growth and topochemical conversion (TC) reactions offer unexploited possibilities for the morphological engineering of heterostructural and non-equilibrium shape (photo)catalyst particles. The hydrothermal epitaxial growth of SrTiO3 on Bi4Ti3O12 platelets is studied as a new route for the formation of novel nanoheterostructural SrTiO3/Bi4Ti3O12 platelets at an intermediate stage or (100)-oriented mesocrystalline SrTiO3 nanoplatelets at the completed stage of the TC reaction. The Bi4Ti3O12 platelets act as a source of Ti(OH)62– species and, at the same time, as a substrate for the epitaxial growth of SrTiO3. The dissolution of the Bi4Ti3O12 platelets proceeds faster from the lateral direction, whereas the epitaxial growth of SrTiO3 occurs on both bismuth-oxide-terminated basal surface planes of the Bi4Ti3O12 platelets. In the progress of the TC reaction, the Bi4Ti3O12 platelet is replaced from the lateral ends toward the interior by SrTiO3, while Bi4Ti3O12 is preserved in the core of the heterostructural platelet. Without any support from noble-metal doping or cocatalysts, the SrTiO3/Bi4Ti3O12 platelets show stable and 15 times higher photocatalytic H2 production (1265 μmol·g–1·h–1; solar-to-hydrogen (STH) efficiency = 0.19%) than commercial SrTiO3 nanopowders (81 μmol·g–1·h–1; STH = 0.012%) in pH-neutral water/methanol solutions. A plausible Z scheme is proposed to describe the charge-transfer mechanism during the photocatalysis.

ABO 3 -type perovskite crystallites with symmetrical crystal structures do not show a tendency to grow in the shape of rods and plates, which are of great scientific interest due to their anisotropic physical properties. A topochemical transformation is one of the most convenient approaches for the preparation of such defined-shape perovskite particles, the functional properties of which can be additionally tailored by the formation of solid solutions. In this study a one-step topochemical conversion reaction from a Bi 4 Ti 3 O 12 template to A-site-substituted complex Ba 1-x Sr x TiO 3 perovskite microplates was investigated in order to understand the mechanism and to determine the compositional, structural and morphological correlations between the template and the final plate-like particles. Based on our finding that Sr incorporates in a 2.7-times larger amount than expected from the initial Ba/Sr ratio (0 1-x Sr x TiO 3 solid-solution composition could be prepared using this topochemical transformation. The tetragonal distortion of the Ba 1-x Sr x TiO 3 plates, expressed as the c/a ratio, was found to decrease linearly from 1.0092 (x = 0) to 1.0037 (x = 0.23), while the compositions with higher x were cubic (c/a=1). According to a piezo-response force microscope (PFM) examination, Ba 1-x Sr x TiO 3 plates with 0 ≤x ≤ 0.175 showed ferroelectric and piezoelectric characteristics. The temperature of the ferroelectric-to-paraelectric phase transition, as determined by differential scanning calorimetry (DSC), linearly decreased from 124 °C (x = 0) to 88 °C (x = 0.11). XRD examinations revealed a strong (001) preferential orientation for the plates (0 ≤ x ≤ 0.11) and the typical (001)/(100) peak splitting indicated the presence of c - and a -domains. The successful preparation and design of Ba 1-x Sr x TiO 3 plates increase the potential of this topochemical transformation for the preparation of plates with a variety of solid-solution compositions and controlled characteristics.

Iron and copper are immiscible elements, but can be combined in the presence of a third element such as aluminium or cerium. We varied the additions of immiscible Fe and Cu (ratio y) in the stoichiometric Al20.5Ce41.5(FeyCu1 − y)38 alloys in order to trace their impact on the structure, microstructure, thermal properties and magnetism. By powder X-ray diffraction (PXRD), we found that Fe-rich alloys (0.87 ≤ y ≤ 1) do not completely hinder crystallisation and yield nanocrystalline precipitates. Cu-rich alloys completely vitrify for 0 ≤ y ≤ 0.74 and exhibit a characteristic broad hump in PXRD patterns. DSC shows that Cu-rich alloys exhibit a noticeably lower crystallisation temperature Tx of about 510 K, liquidus temperature Tl at approximately 900 K and a more pronounced drop Δ of the heat flux at the glass transition temperature Tg. These criteria indicate that Cu-rich alloys are better glass formers. Transmission electron microscopy (TEM) shows that the specimens with y = 0 and y = 0.34 are composed of an amorphous matrix with minor impurities of CeO2 nanocrystals, which can only be observed via transmission electron microscopy. However, these nanocrystals do not interfere with the formation of the glassy phase and are therefore inert. Magnetic measurements show that Fe-rich alloys possess higher magnetisation at maximum field (Mmax H), peaking at 3.3 and 3 emu/g for y = 1 and y = 0.87, respectively. From ferromagnetism in the latter two alloys, the behaviour turns to paramagnetism for 0 ≤ y ≤ 0.74. Cryogenic measurements on SQUID show that the sample with y = 0 displays two different temperature regions at 2–50 K and 100–300 K. The first one exhibits nearly pure paramagnetic behaviour with Θ = − 0.7, while the second one shows a highly negative Weiss constant of Θ = − 80.6.

The main goal of this research was to improve a carbonate-type foaming agent for the production of Al foams. Various systematic treatments, i.e. mechanical, thermal and chemical, were applied to naturally occurring dolomite, in order to affect its thermal decomposition. Structural modifications after the treatments as well as after the thermal decomposition were monitored by X-ray diffraction, while thermal gravimetry and differential scanning calorimetry coupled with mass spectrometry were employed to monitor the processes during the heating experiments. The as-received dolomite, without any pre-treatments, decomposes at a relatively high temperature, which prevents its wider application as a foaming agent. However, by using various treatments the decomposition of the dolomite could be shifted towards lower temperatures, making it suitable for the production of aluminium foams.

The conditions for the topochemical transformation of variously sized Bi4Ti3O12 template plates into BaTiO3 particles were studied in order to control their morphology, crystal structure, and preferential orientation. In the transformation from sub-200 nm- and μm-sized template plates in the presence of surplus BaCO3 (Bi4Ti3O12/BaCO3 = 1:10), the final BaTiO3 particles retained a memory of the precursor size when the conversion reaction in the molten salt (NaCl/KCl) occurred at 660 and 900 °C, respectively. In both cases the side length of the template was well preserved, while the thicknesses of the final BaTiO3 plates were larger compared to those of the templates. The morphology of the BaTiO3 particles formed from micrometer-sized Bi4Ti3O12 plates at 660 °C did not closely resemble the template shape because of the exfoliation and disintegration processes. Through the transformation of sub-200 nm Bi4Ti3O12 plates at 900 °C the formed BaTiO3 particles grew by Ostwald ripening, and thus also the shape of...

Nearly stoichiometric Ti1.005S2 was prepared by a solid-state synthesis of the constituent elements and consolidated under a variety of processing conditions applied using pulsed-electric-current sintering. The non-stoichiometry in the Ti1 + xS2 induced during the sintering was quantitatively determined with a Rietveld structural refinement and a thermogravimetric analysis combined with inductively coupled plasma atomic emission spectroscopy. Significant cation off-stoichiometry and sulphur losses were already observed at a sintering temperature of 700 °C, resulting in heavily self-intercalated Ti1.076S2, which significantly altered the thermoelectric properties in comparison with the nearly stoichiometric compound. By applying a high sintering pressure of 500 MPa the Ti1.005S2 could be consolidated at 500 °C with no detectable sulphur loses and with a high bulk density of 97.6%. The sulphur volatilization in the TiS2 was also effectively suppressed up to 800 °C by using a gas-tight sintering mould, which resulted in the composition Ti1.017S2, prepared at 800 °C and 200 MPa, with an optimal thermoelectric figure of merit ZT equal to 0.32 at 375 °C. Keywords: Titanium disulphide, Thermoelectric materials, Pulsed electric current sintering, Structure-property relationship, Transport properties

The present work describes the formation of amorphous alloys in the (Al1−xCex)62Cu25Fe13 quaternary system (0 ≤ x ≤ 1). When the amount of Ce falls in the range 0.67 ≤ x ≤ 0.83, the alloys obtained exhibit a completely amorphous structure confirmed by powder X-ray diffraction. Otherwise, at compositions x = 0.5, 0.58, 0.92 and 1, a primary crystalline phase forms together with an amorphous matrix. The crystallisation temperature (Tx) decreases with increasing Ce content, varying from 593 K for x = 0.5–383 K for x = 1. Composition x = 0.75 is considered as the best glass former, exhibiting a large supercooled liquid region of 40 K width that precedes crystallisation. In order to form bulk amorphous alloys, ribbons with this later composition were consolidated into few millimetre thick discs using pulsed electric current sintering at different temperatures, yet preserving the amorphous structure. Meanwhile, increasing temperature above 483 K triggers crystallisation of a primary phase isostructural ...

The goal of this study was to justify the reprocessing of recycled HDDR Nd-Fe-B powders with spark plasma sintering (SPS) and to investigate the dependence of the final magnetic properties on SPS and thermal treatment. The initial recycled HDDR powder of the composition Nd13.4Dy0.6Fe78.6B6.1Nb0.4Al0.7 with 4760 ppm O2 content, coercivity (HCi) = 830 kA/m, and large particle size distribution

This study was supported by the ERA-NET HarvEnPiez project. The authors would like to thank their national funding agencies (Latvian State Education Development Agency, Slovenian Ministry of Higher Education, Science and Technology, Romanian National Authority for Scientific Research and Innovation, CCCDI-UEFISCDI, project number 49/2016 within PNCDI III – M-ERA NET Program)., The results of experimental and theoretical ab initio study of structural and piezoelectric properties of (Ba,Sr)TiO3 perovskite solid solutions are discussed and compared. Experimentally, plate-like (Ba,Sr)TiO3 particles were synthesized by the topochemical conversion in the molten salt from Bi4Ti3O12 template plates. All dimensions (side length ≈ 1 µm, thickness ≈ 200–400 nm) were well above the critical size necessary for observation of piezo- and ferroelectricity. The first-principles computations of the structural and electromechanical properties of solid solutions were performed with CRYSTAL14 computer code within the linear combination of atomic orbitals (LCAO) approximation, using three advanced hybrid functionals of the density-functional-theory (DFT). Different chemical compositions are considered for the ferroelectric and paraelectric phases. Calculated structural properties of solid solutions in tetragonal and cubic phases are in a very good agreement with experimental data. Experimentally obtained and calculated band gaps are compared for cubic SrTiO3 and tetragonal BaTiO3. BaTiO3/SrTiO3 heterostructures were considered theoretically for different chemical compositions. The calculated piezoelectric properties of solid solutions and heterostructures in ferroelectric phase are compared. It is predicted that both solid solutions and heterostructures improve the piezoelectric properties of the bulk BaTiO3, but solid solutions are more preferable for equal Sr concentrations., ERA-NET HarvEnPiez project; Latvian State Education Development Agency, Slovenian Ministry of Higher Education, Science and Technology, Romanian National Authority for Scientific Research and Innovation, CCCDI-UEFISCDI, project number 49/2016 within PNCDI III – M-ERA NET Program; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART²

The formation of tetragonal BaTiO3 from K2Ti6O13 (K2T6) nanowires was examined under hydrothermal conditions at 100–240 °C. The morphology and the crystal structure of the BaTiO3 particles that grew via a dissolution–precipitation mechanism were controlled by the temperature and the concentrations of the NaOH and the K2T6. The degree of tetragonality of the formed BaTiO3 powders was quantitatively evaluated on the basis of a Rietveld crystal-structure refinement and compared with the results of Raman and DSC analyses. The powders consisted mainly of star-like BaTiO3 particles that formed at 100 °C and contained cubic (22%) and tetragonal (78%) phases with a low c/a value of 1.0042. The BaTiO3 particles changed their shape to dendritic and seaweed-like with an increase of temperature to 150 °C and 200–240 °C, respectively. According to the Rietveld analysis this increase of the temperature led to the formation of a tetragonal phase with a high c/a ratio (1.008–1.010), which dominated over the cubic and tetragonal phases with a low c/a (1.004–1.007). An increase in the K2Ti6O13 and NaOH concentrations was found to decrease the tetragonality of the formed BaTiO3 particles. The electron-diffraction patterns suggested a multidomain structure for the seaweed-like, mainly tetragonal, BaTiO3 particles.

The experimental conditions for the growth of shape-controlled BaTiO3 particles in NaOH and Ba(NO3)2 aqueous and water/ethanol solutions using various TiO2-containing precursors were studied at 80°C–100°C. The different chemistries and physical characteristics of the precursors resulted in different BaTiO3 formation rates and morphologies. Nanocrystalline anatase led to irregularly shaped BaTiO3 particles, whereas star-like, single-crystalline BaTiO3 particles grew from aerogel TiO2 and sodium titanate (NT) belts in alkaline aqueous solutions. With the addition of ethanol, the star-like BaTiO3 particles changed to square-like, the size of which decreased with an increase in the ethanol content. The electron microscopy observations supported a dissolution–precipitation mechanism as the primary reaction mechanism for the formation of BaTiO3 nanocrystals, which further aggregated into single-crystalline star- or square-like particles by oriented attachment. The modification in the water solution with ethanol is believed to influence both the nucleation and aggregation process and consequently influence the particle shape and size.

Microstructure development in ZnO ceramics with Bi 4 Ti 3 O 12 (BIT) additions was studied in dependence of sintering temperature, inversion boundary (IBs) nucleation, heating rate and doping with transition metal oxides (NiO, MnO 2 and Co 3 O 4 ). We demonstrated that one of the essential conditions for homogeneous microstructure development in this system is rapid release and efficient distribution of TiO 2 , necessary for the formation of Ti-rich (tail-to-tail) IBs in ZnO grains. This can be achieved via the so-called shock-sintering procedure described in this article. Immediate decomposition of BIT to TiO 2 -rich Bi 2 O 3 liquid phase above 1200 °C leads to nucleation of ZnO grains with IBs. Exploiting the growth of ZnO grains with IBs, microstructure development can be easily controlled via the IB-induced grain growth mechanism, previously described in SnO 2 -doped and Sb 2 O 3 -doped ZnO. In contrast to conventional sintering, where erratic nucleation of IBs leads to bimodal grain size distribution, shock-sintering sintering regime produces microstructures with uniform coarse-grain sizes, required for low-voltage varistor ceramics.

The nucleation and crystallization of MgO–B2O3–SiO2 (MBS) glasses with various TiO2 additions (0–15 wt%) were studied by means of a nonisothermal, thermal analysis technique, X-ray diffraction, and electron microscopy. The effect of the TiO2 on the nucleation rate was determined from the dependence of the inverse temperature at the differential scanning calorimetry (DSC) peak (1/Tp; Tp→crystallization peak temperature) on the nucleation temperature (Tn). A significant increase in the nucleation rate was observed for the MBS glasses with a TiO2 content greater than 3 wt%. The TiO2 was also found to influence the crystallite growth mechanism. In the pure MBS glass the crystallization of the Mg2B2O5 was 3-dimensional (3D) bulk with a diffusion-controlled crystal growth rate. The value of both the Avrami parameter (n) and the morphology index (m) was 1.5 and the crystallization activation energy (E) was in the range 410–440 kJ/mol. The change of the nucleation rate (a) from zero (a = 0) to a constant nucleation rate (a = 1) was observed at 3 wt% TiO2. The change from a diffusion- to an interface-controlled growth rate occurred at 5 wt% TiO2, where n and m became 4 and 3, respectively. According to the analysis of the DSC data using the modified Kissinger method, a small TiO2 concentrations (1 wt%) lowered E, whereas E increased with a further increase in the TiO2 concentration, exceeding at 5 wt% TiO2 the E of pure MBS glass.

Vanadium oxide nanocrystalline films were prepared by spin-coating from vanadium (V) oxoisopropoxide sol and thermally treated at 150 °C, in order to deposit them on PET–ITO substrates. The sols were made from either isopropanol or its mixture with n-butanol, both exhibiting similar rheological behaviour and a viscosity of the order of magnitude of 10−3 Pa s. A cross-hatch test revealed the excellent adhesion of these films on the ITO surface and delamination occurred preferentially at the ITO–plastic interface when the films were subjected to deliberate bending. The electrochemical and optical properties of the vanadium oxide films were analysed using in-situ UV–vis absorbance spectroelectrochemistry. The response was correspondingly lower than with films deposited on FTO-glass. Ex-situ IR reflection–absorption spectroscopy showed that vanadyl V–OA and superimposed bridging V–OB–V and 3V–OC bands appear at lower frequencies compared to crystalline V2O5, due to the hydrated nanocrystalline structure. An intercalation induced red-shift of the vanadyl mode and the disappearance of the superimposed V–OB–V and 3V–OC bands, but the spectrum was restored to the initial situation after deintercalation. The modulation of polaron absorption that extends from the near to mid-IR spectral region up to 2000 cm−1 was not visible due to transfer of the films from electrochemical cell to the IR spectrometer. The electrochromic properties of the vanadium oxide films deposited on PET–ITO substrate were tested in a liquid electrolyte-based cell with an electrochromically active PEDOT film and 1 M LiClO4/propylene carbonate electrolyte.

The densification and crystallization behaviors of MgO–B 2 O 3 –SiO 2 (MBS) glass with various amounts of TiO 2 additions (0–10 wt.%) were investigated by means of thermal analysis, X-ray powder diffraction and scanning electron microscopy. A microwave dielectric characterization was performed in order to evaluate the suitability of MBS glass–ceramics as a low-permittivity dielectric substrate. The densification of the MBS glass started below 700 °C. The main crystalline phases of Mg 2 B 2 O 5 and MgSiO 3 appeared at 800 and 950 °C, respectively. The Mg 3 TiB 2 O 8 and TiB 0.024 O 2 phases additionally crystallized in TiO 2 -added MBS glass–ceramics at 1000 °C. The permittivity increased from 6.1 in pure MBS glass to 6.9 in MBS glass with 10 wt.% of TiO 2 . The addition of TiO 2 enhanced the crystallization and consequently increased the Qxf-values of the MBS glass (11 300 GHz) up to 16 500 GHz. The improvement of the Qxf-values became the most evident at 1050 °C. Dense MBS glass–ceramics sintered at 850 ≤ T ≤ 950 °C exhibited Qxf-values of 5000–8000 GHz (at ∼12 GHz), which are comparable with the values of CaO–B 2 O 3 –SiO 2 -based glass–ceramics.

The nucleation and crystallization of MgO‐B2O3‐SiO2 (MBS) glass were studied by means of a non-isothermal, thermal analysis technique, X-ray diffraction and scanning electron microscopy. The temperature range of the nucleation and the temperature of the maximum nucleation rate for MBS glass were determined from the dependences of the inverse temperature at the DSC peak (1/Tp) and the maximum intensity of the exothermic DSC crystallization peak ((δT)p) on the nucleation temperature (Tn). For MBS glass the nucleation occurred at 600‐750 ◦ C, with the maximum nucleation rate at 700 ◦ C, whereas the nucleation and crystal growth processes overlapped at 700 ◦ C < T ≤ 750 ◦ C. The analyses of the non-isothermal data for the bulk MBS glass using the most common models (Ozawa, Kissinger, modified Kissinger, Ozawa‐Chen, etc.) revealed that the crystallization of Mg2B2O5 was three-dimensional bulk with a diffusion-controlled crystal growth rate, that n = m = 1.5 and that the activation energy for the crystallization was 410‐440 kJ/mol. © 2011 Elsevier Ltd. All rights reserved.

Nanocrystalline TiO 2 powders with a high specific surface area (up to 335 m 2 /g) were synthesized by a combination of sol–gel and solvothermal syntheses. The influence of the TiO 2 framework template, the triblock copolymer P123, and the framework stabilizer, phosphorous (P) in the form of phosphoric acid (H 3 PO 4 ), on the photocatalytic oxidation of isopropanol was studied. As-prepared samples were characterized by means of X-ray powder diffraction (XRD), thermogravimetric analysis (TG-DTA), measurements of the specific surface area ( s BET ), field-emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). The photocatalytic activity under UV (ultraviolet) irradiation of the as-prepared samples was measured by monitoring the degradation of the isopropanol in the gaseous medium. The application of P123 increased the specific surface area of the samples; furthermore, the addition of phosphorous inhibited the particle growth during the thermal treatment and stabilized the anatase crystal phase at higher annealing temperatures (up to 900 °C), thus leading to a higher photocatalytic activity. However, when P123 and phosphorous were used together the specific surface area was increased and an improvement in the photocatalytic oxidation was observed.

MWO4 (M=Ca, Sr and Ba) scheelite ceramics were studied in terms of their syntheses, sintering, solubility in water, and dielectric response after storing them in dry and moist atmospheres. Of the studied scheelites, the CaWO4 possessed the most promising dielectric properties (ɛ=10.9, Q × f=105 600 GHz), which were stable under the influence of humidity. BaWO4 and SrWO4 exhibited ɛ=9.0 and Q × f values of 32 200 and 62 600 GHz, respectively. The most detrimental effect of the moisture was observed for SrWO4. A sodium impurity present in the SrCO3 reagent (0.35 wt%), which was used for the synthesis of the SrWO4, was found to lower the sintering temperature, enhance the grain growth, and change the other properties of the ceramics, such as humidity susceptibility and solubility in water. The evident tendency of the ceramics to attract water and the increased dissolution of tungstate were observed for all MWO4 scheelite ceramics, which were sintered with the help of Na2CO3 or Li2CO3 (0.5 wt%) sintering aids. The results of the present study suggest that the physical and chemical properties of the ceramics should be carefully considered in the case of using of alkaline-containing sintering aids.

The microwave dielectric properties and microstructure of Mg 3 B 2 O 6 ceramics with the addition of 0-5 wt% Mg 2 B 2 O 5 were investigated. Scanning electron microscopy of the Mg 3 B 2 O 6 ceramics sintered at 1310°C revealed the presence of anisotropic, exaggeratedly grown grains with a crystallographic fault that extends along the direction of the anisotropy. The most pronounced, exaggerated grain growth was observed for Mg 3 B 2 O 6 with 5 wt% of Mg 2 B 2 O 5 . The anisotropic grains were examined by electron backscatter diffraction. The analysis of the Kikuchi pattern showed that the crystallographic fault is a (011) twinning plane. The duration of annealing was found to have no significant effect on the dielectric permittivity, while the Q x f values increased two times with a prolongation of the annealing from 20 to 160 h. The highest Q × f value of 240 000 GHz was measured for Mg 3 B 2 O 6 ceramics with 5 wt% of Mg 2 B 2 O 5 . These ceramics consisted of 10 times larger grains than the Mg 3 B 2 0 6 ceramics without the added Mg 2 B 2 O 5 , which also showed high Q × f values of 200 000 GHz. The decrease of the amorphous phase during the annealing could be one of the reasons for this improvement of the Q × f values.

The KxBa1−xGa2−xGe2+xO8 (x=0.6−1.0) solid solutions undergo a structural phase transition that has a significant effect on their sintering behavior and their microwave dielectric properties. The crystal structures of both phases within the solid-solution region were determined by the Rietveld method using powder X-ray diffraction data. We found that the low-temperature-stable phase is isostructural with the pseudo-orthorhombic KGaGe3O8 (space group P21/a), while the high-temperature-stable phase has a typical monoclinic feldspar structure (space group C2/m). Due to the topological differences between the two structures, the T–O bonds within the tetrahedra must be partially recombined to make a new framework, which causes an endothermic effect during the P21/a to C2/m phase transition. The correlation between the crystal structures, the microwave dielectric properties and the phase-transition behaviors were discussed in terms of the crystallographic features, the lattice parameters, and the strain-induced anisotropic peak-broadening.

KxBa1−xGa2−xGe2+xO8 (0.6≤x≤1) polycrystalline ceramics are potential materials for glass-free low-temperature cofired ceramics (LTCC) substrates. We have made a comprehensive study of the kinetics of the monoclinic-to-monoclinic P21/a⇔C2/m phase transition. The low-temperature-stable P21/a phase with a high Q×f value was synthesized using a subsolidus method and was well sintered at the LTCC temperature with a H3BO3 additive. A good combination of low sintering temperature (910°–920°C), high Q×f values (96 700–104 500 GHz), low permittivities (5.6–6.0), and a small temperature coefficient of resonant frequency (∼−20 ppm/°C) was obtained for ceramics with x=0.67 and 0.9 and with 0.1 wt% of H3BO3.

We report unusual sintering behavior of Ca-deficient hydroxyapatite processed with different heating rates; particularly, faster heating resulted in higher densification. We hypothesize that faster heating delays formation of β-tricalcium phosphate which yields to unobstructed densification in intermediate sintering stage, allowing further tailoring of both microstructure and phase composition (hydroxyapatite/tricalcium phosphate).

X-ray powder-diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), electron diffraction (ED), infrared spectroscopy (IR), thermogravimetry (TG) and mass spectroscopy (MS) were performed to investigate the composition and the crystal structure of tetra-barium di-niobate (V) Ba4Nb2O9. The TG, MS and IR studies revealed that the compound is a hydrated oxycarbonate. Assuming that the carbonate stoichiometrically replaces oxygen, the composition of the low-temperature α-modification, obtained by slow cooling from 1100 °C, corresponds to Ba4Nb2O8.8(CO3)0.2·0.1H2O, while the quenched high-temperature γ-modification has the Ba4Nb2O8.42(CO3)0.58·0.38H2O composition. The α-phase has a composite incommensurately modulated structure consisting of two mutually interacting [Ba]∞ and the [(Nb,□)O3]∞ subsystems. The composite modulated crystal structure of the α-phase can be described with the lattice parameters a=10.2688(1) A, c=2.82426(8) A, q=0.66774(2)c* and a superspace group R 3 ¯ m(00γ)0s. The HRTEM analysis demonstrates the nanoscale twinning of the trigonal domains parallel to the {1 0 0} crystallographic planes. The twinning introduces a one-dimensional disorder into the [(Nb,□)O3]∞ subsystem, which results in an average P 6 ¯ 2c crystal structure of the α-phase. Possible places for the carbonate group in the structure are discussed using a comparison with other hexagonal perovskite-based oxycarbonates.

Tungsten (W) and various composites are being considered as the primary plasma-facing materials for fusion reactors. Like all engineering materials, they contain certain levels of impurities, which can have an important impact on mechanical properties. In the present work, oxygen was identified as a major impurity in our starting tungsten powder. At elevated temperatures, the presence of interstitial elements such as oxygen leads to the formation of an oxide-rich tungsten phase at the tungsten grain boundaries. In this study, we determined the capacity of tungsten carbide (WC) nanoparticles to remove the oxide impurities from a tungsten body. Tungsten composites with 0.05, 0.25 and 0.51 wt. % carbon (C) in the form of WC were sintered using a field-assisted sintering technique (FAST) at 1900 °C for 5 min. The sintered samples were characterized using field-emission scanning and transmission electron microscopy. Thermodynamic and kinetic considerations allowed us to determine the optimum theoretical amount of WC to prevent the in-situ formation of WO2.