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Ferroelectric HfO2-based materials hold great potential for widespread integration of ferroelectricity into modern electronics due to their robust ferroelectric properties at the nanoscale and compatibility with the existing Si technology. Earlier work indicated that the nanometer crystal grain size was crucial for stabilization of the ferroelectric phase of hafnia. This constraint caused high density of unavoidable structural defects of the HfO2-based ferroelectrics, obscuring the intrinsic ferroelectricity inherited from the crystal space group of bulk HfO2. Here, we demonstrate the intrinsic ferroelectricity in Y-doped HfO2 films of high crystallinity. Contrary to the common expectation, we show that in the 5% Y-doped HfO2 epitaxial thin films, high crystallinity enhances the spontaneous polarization up to a record-high 50 µC/cm2 value at room temperature. The high spontaneous polarization persists at reduced temperature, with polarization values consistent with our theoretical predictions, indicating the dominant contribution from the intrinsic ferroelectricity. The crystal structure of these films reveals the Pca21 orthorhombic phase with a small rhombohedral distortion, underlining the role of the anisotropic stress and strain. These results open a pathway to controlling the intrinsic ferroelectricity in the HfO2-based materials and optimizing their performance in applications.

The van der Waals magnets CrX

The van der Waals magnets CrX$_3$ (X = I, Br, and Cl) exhibit highly tunable magnetic properties and are promising candidates for developing novel two-dimensional (2D) magnetic devices such as magnetic tunnel junctions and spin tunneling transistors. Previous studies of CrCl$_3$ have mainly focused on mechanically exfoliated samples. Controlled synthesis of high quality atomically thin flakes is critical for their technological implementation but has not been achieved to date. Here, we report the growth of large CrCl$_3$ flakes with well-defined facets down to monolayer thickness (~0.6 nm) via the physical vapor transport technique. Both isolated flakes with well-defined facets and long stripe samples with the trilayer portion exceeding 60 $\mu$m have been obtained. High-resolution transmission electron microscopy studies show that the CrCl$_3$ flakes are single crystalline in the monoclinic structure, consistent with the Raman results. The room temperature stability of the CrCl$_3$ flakes decreases with decreasing thickness. The tunneling magnetoresistance of graphite/CrCl$_3$/graphite tunnel junctions confirms that few-layer CrCl$_3$ possesses in-plane magnetic anisotropy and N\'eel temperature of 17 K. Our study paves the path for developing CrCl$_3$-based scalable 2D spintronic applications., Comment: 14 pages, 5 figures, with Supporting Information

Application of conducting ferroelectric domain walls (DW) as functional elements may facilitate development of conceptually new resistive switching devices. In a conventional approach, several orders of magnitude change in resistance can be achieved by controlling the DWs density using super-coercive voltage. However, a deleterious characteristic of this approach is high-energy cost of polarization reversal due to high leakage current. Here, we demonstrate a new approach based on tuning the conductivity of DWs themselves rather than on domain rearrangement. Using LiNbO3 capacitors with graphene, we show that resistance of a device set to a polydomain state can be continuously tuned by application of sub-coercive voltage. The tuning mechanism is based on the reversible transition between the conducting and insulating states of DWs. The developed approach allows an energy-efficient control of resistance without the need for domain structure modification. The developed memristive devices are promising for multi-level memories and neuromorphic computing applications.

A TiNxOy (TiNO) material system has been synthesized in thin film form for the first time using a pulsed laser deposition process. X-ray diffraction and X-ray photoelectron spectroscopy measurements have been carried out to show partial oxidation of TiN to TiNO. The current (I)-voltage (V) characteristics recorded from TiNO films sandwiched between indium tin oxide (ITO) and gold (Au) layers and/or copper (Cu) electrodes have shown that the I–V curves lie in the first and third quadrants (i.e., both I and V are either positive or negative) in the dark conditions, while the I–V curves lie in the second and fourth quadrants (i.e., I and V with opposite sign) in the illuminated conditions. The positive sign of power (I × V = Positive) under dark conditions indicates dissipation of power in the TiNO system, while the negative sign of the power (I × V = Negative) under optical illumination indicates the power generation capability of TiNO system. The bandgap of the TiNO thin film samples, measured using ultra violet (UV)–visible (400–800 nm) spectroscopy, was found to be ~ 1.6 eV. As the number of photocatalysts/semiconductors that are active under the visible light irradiation is very limited, our approach to develop a unique visible-light-driven TiNO photoactive material system can open a new avenue for the realization of novel optical devices.

A magnetocaloric effect (MCE) with sizable isothermal entropy change (ΔS) maintained over a broad range of temperatures above the blocking temperature is reported for a rare earth-free superparamagnetic nanoparticle system comprising of Fe–TiN heterostructure. Superparamagnetic iron (Fe) particles were embedded in a titanium nitride (TiN) thin film matrix in a TiN/Fe/TiN multilayered pattern using a pulsed laser deposition method. High angle annular dark-field images in conjunction with dispersive energy analysis, recorded using scanning transmission electron microscopy, show a clear presence of alternating layers of Fe and TiN with a distinct atomic number contrast between Fe particles and TiN. Quantitative information about the isothermal entropy change (ΔS) and the magnetocaloric effect in the multilayer Fe–TiN system has been obtained by applying Maxwell relation to the magnetization vs temperature data at various fields. With the absence of a dynamic magnetic hysteresis above the blocking temperature, the negative ΔS as high as 4.18 × 103 J/Km3 (normal or forward MCE) is obtained at 3 T at 300 K.

In this work, we exploit the ferroelectric-dielectric layer thickness ratio r as an effective tuning parameter to control the ferroelectric polarization and transient negative capacitance (NC) state in epitaxial ${\mathrm{Pb}\mathrm{Zr}}_{0.2}{\mathrm{Ti}}_{0.8}{\mathrm{O}}_{3}/{\mathrm{Sr}\mathrm{Ti}\mathrm{O}}_{3}$ bilayer heterostructures. The remnant polarization decreases monotonically with decreasing r, with the system exhibiting an abrupt transition from ferroelectric to dielectric dominated behavior at a critical ratio ${r}_{c}$ of 8 to 7, which is consistent with the evolution of the free-energy profile modeled via Landau theory. For samples with large r, the polarization switching dynamics during the transient NC regime can be well described by the nucleation and growth model, with the narrow distribution of the characteristic switching time (${t}_{0}$) pointing to a domain-wall-motion-limited behavior. Right below ${r}_{c}$, we observe a significantly broadened distribution of ${t}_{0}$, which can be attributed to the emergence of a multidomain state. The transient NC mode is quenched in samples with r 6, confirming that the ferroelectric order is suppressed. Our study provides critical information for optimizing the materials design for complex oxide-based NC transistors, paving the path for their application in low-power nanoelectronics.

Ferroelectric HfO

Laser processing of metal surfaces by ultrafast pulse lasers is a developing technology with many potential uses including applications in heat transfer, medical implants, and tribology. Laser processed silver surfaces has several potential applications such as biomedical devices, antibacterial surfaces, and chemical sensors. However, there is a lack of previous research on laser processing of silver is more difficult to process compared with other metals. A newly investigated dual-pulse femtosecond laser surface processing technique was capable of producing self-organized, micro/nanoscale surface features on silver where single-pulse techniques had previously failed. Three-dimensional (3D) scanning electron microscopy (SEM) was used to examine mound-like structures produced by this new method to determine their composition and formation processes. The interior microstructure revealed that the mounds were comprised mostly of resolidified Ag grains with approximately 1% porosity. Hydrodynamically-driven fluid flow was the primary process that forms these surface structures without significant oxidation.

The flexible, transparent, and low-weight nature of ferroelectric polymers makes them promising for wearable electronic and optical applications. To reach the full potential of the polarization-enabled device functionalities, large-scale fabrication of polymer thin films with well-controlled polar directions is called for, which remains a central challenge. The widely exploited Langmuir-Blodgett, spin-coating, and electrospinning methods only yield polymorphous or polycrystalline films, where the net polarization is compromised. Here, an easily scalable approach is reported to achieve poly(vinylidene fluoride-trifluoroethylene) P(VDF-TrFE) thin films composed of close-packed crystalline nanowires via interface-epitaxy with 1T'-ReS

Femtosecond laser surface processing (FLSP) is a material processing technique used to produce self-organized micro/nanostructures on metals. The hierarchal structures can improve the surface properties of materials when applied to specific applications such as enhancing heat transfer. In this paper, we demonstrate a recently developed technique termed multi-material, multi-layer FLSP (3ML-FLSP). With 3ML-FLSP, micro/nanoscale features can be produced that are composed of multiple materials by processing surfaces using traditional FLSP techniques that are layered with thin foils of different materials. We demonstrate results with three layers of different metals (304 stainless steel, copper, and aluminum) clamped together during laser processing to create structures composed of all three metals. Ion beam milling is used to cross-section structures for subsurface analysis of the microstructure. The three metals did not mix within the bulk of the microstructures indicating that the microstructures were produced primarily through preferential removal of material around the structures. However, there was mixing of all three materials within the nanoparticle layer that covers the microstructures.

Flexible nanocomposite films, with cobalt ferrite nanoparticles (CFN) as the ferromagnetic component and polyvinylidene fluoride–trifluoroethylene (PVDF-TrFE) copolymer as the ferroelectric matrix, were fabricated using a blade coating technique. Nanocomposite films were prepared using a two-step process; the first process involves the synthesis of cobalt ferrite (CoFe2O4) nanoparticles using a sonochemical method, and then incorporation of various weight percentages (0, 2.5, 5, and 10%) of cobalt ferrite nanoparticles into the PVDF-TrFE to form nanocomposites. The ferroelectric polar β phase of PVDF-TrFE was confirmed by x-ray diffraction (XRD). Thermal studies of films showed notable improvement in the thermal properties of the nanocomposite films with the incorporation of nanoparticles. The ferroelectric properties of the pure polymer/composite films were studied, showing a significant improvement of maximum polarization upon 5wt% CFN loading in PVDF-TrFE composite films compared to the PVDF-TrFE film. The magnetic properties of as-synthesized CFN and the polymer nanocomposites were studied, showing a magnetic saturation of 53.7 emu g−1 at room temperature, while 10% cobalt ferrite-(PVDF-TrFE) nanocomposite shows 27.6 emu/g. We also describe a process for fabricating high optical quality pure PVDF-TrFE and pinhole-free nanocomposite films. Finally, the mechanical studies revealed that the mechanical strength of the films increases up to 5 wt% loading of the nanoparticles in the copolymer matrix and then decreases. This signifies that the obtained films could be suited for flexible electronics.

It is very challenging to achieve near perfect absorption or emission that is both broadband and omnidirectional while utilizing a scalable fabrication process. Femtosecond laser surface processing is an emerging low-cost and large-scale manufacturing technique used to directly and permanently modify the surface properties of a material. The versatility of this technique to produce tailored surface properties has resulted in a rapidly growing number of applications. Here, we demonstrate near perfect, broadband, omnidirectional emissivity from aluminum surfaces by tuning the laser surface processing parameters including fluence, pulse count, and the ambient gas. Full-wave simulations and experimental results prove that the obtained increase in emissivity is mainly a result of two distinct features produced by femtosecond laser surface processing: the introduction of microscale surface features and the thick oxide layer. This technique leads to functionalized metallic surfaces that are ideal for emerging applications, such as passive radiative cooling and thermal management of spacecraft. Surfaces with wide-angle high emissivity in the infrared spectrum are important for applications in radiative cooling, thermophotovoltaics, and spacecraft thermal management. Here, near perfect, broadband, omnidirectional emissivity is realized via scalable laser surface processing of aluminum.

In this paper, experimental results of pool boiling inversion and hysteresis on functionalized 304 stainless steel using distilled water at atmospheric pressure are presented. The surface was modified using femtosecond laser surface processing to create hierarchical micro/nanostructures. Boiling curves were generated once steady-state was reached, with steady-state defined as having a temperature slope less than 0.25 degrees Celsius per minute. The steady-state criterion was examined by allowing the surface to have extended boiling times at specified heat fluxes. In separate runs, the surface boiled for four hours at heat fluxes of 25, 40, 60, and 70 watts per square centimeter once the steady-state criterion was reached. No change in superheat was seen for the 25 and 40 watts per square centimeter heat fluxes over the four hour period. Drastic reductions in superheat of 6.4 and 4.9 degrees Celsius were seen for the 60 and 70 watts per square centimeter heat fluxes, respectively, resulting in higher heat transfer coefficients when compared to a run without extended boiling. It was found that at heat fluxes below the pool boiling inversion point, the superheat did not change with extended boiling times and the steady-state condition was adequate. The opposite was true for heat fluxes above the pool boiling inversion point. The decrease in superheat at higher heat fluxes was attributed to the change in nucleation dynamics, which are the cause of the pool boiling inversion phenomenon. These changes in the nucleation dynamics are suspected to cause a delay in the activation of nucleation sites, requiring longer times to achieve true steady-state conditions. To illustrate the difference in times, lower heat fluxes reached true steady-state in 15 to 20 minutes whereas higher heat fluxes took nearly three hours. Hysteresis of the boiling curve was examined by decreasing the heat flux once it reached about 105 watts per square centimeter. Hysteresis occurred during each run and resulted in a more efficient boiling curve when compared to the increasing heat flux portion. The hysteresis curves for each run were nearly identical regardless of the extended boiling at varying heat fluxes. These results highlight the need for careful consideration and understanding of the steady-state criterion, especially with surfaces that result in pool boiling inversion. These results further confirm that combining pool boiling inversion with hysteresis can result in stable and repeatable enhancement in the boiling curve, regardless of the thermal history of the surface.

Author(s): Yin, Y; Yin, Y; Jiang, X; Koten, MA; Shield, JE; Chen, X; Yun, Y; N'Diaye, AT; Hong, X; Xu, X | Abstract: We report a spin-rectification effect in a spin-valve structure consisting of ferroelectric croconic acid (C5H2O5) sandwiched between ferromagnetic electrodes La0.7Sr0.3MnO3 and Co, which can be switched between a high-resistance (off) and a low-resistance (on) state by a poling voltage. In the off state, the magnetoresistance (MR) sign reverses with the measurement voltage with a 0.1-V offset, suggesting a spin-rectification behavior, while in the on state the MR remains negative. These observations can be understood in terms of electrically controlled interfacial energy-band alignment either from the electrostatic effect or from the interfacial redox process. The observed spin-rectification effect suggests the possibility of diodelike devices for spin-polarized current.

Cluster beam deposition provides the opportunity to create novel, highly uniform nanostructures, and the far-from-equilibrium nature of the processing offers pathways to new materials and phases. Because cluster-based processing produces uniform sub-10 nm nanoparticles, nanostructures can be constructed at the convergence of important magnetic length scales, notably to produce exchange-coupled permanent magnet materials with superior energy densities. Further, the nonequilibrium nature of the condensation process promotes extension of the solid solubility of normally immiscible components, providing opportunities to explore new materials. Here, we report efforts at developing new nanostructures and materials for use in permanent magnet applications.

Improving the heat transfer characteristics of materials can be accomplished by increasing the surface area. This surface roughening can be accomplished efficiently and directly using pulsed-laser processing. Here, laser processing of copper using a picosecond laser pulse technique produced mound-like structures, with surface morphologies and subsurface microstructures dependent on laser pulse count, with higher pulse counts producing a unique layering of Cu and Cu2O in the mounds. Processing in ambient air resulted in the formation of surface oxides. The presence of oxides deleteriously influenced the heat transfer characteristics, with a lower heat transfer coefficient compared to unprocessed Cu. To remove the oxidation, the laser-processed copper was subjected to different acid treatments. It was found that treatment using citric acid resulted in efficient and effective removal of oxides both surface and subsurface. For low pulse count samples, an improvement in the heat transfer characteristics was observed after oxide removal, and outperformed the polished reference sample at heat fluxes above approximately 90 W/cm2. However, the removal of oxides using citric acid was found to actually decrease heat transfer characteristics for high pulse count samples with the onion-like subsurface layers, as oxide removal left behind voids in the structure which were detrimental to heat transfer. Thus, optimized laser surface processing which avoids subsurface onion-like layer formation enhances heat transfer performance.

Self-organized mound-like micro/nanoscale structures are reported for the first time on silver using a dual-pulse femtosecond laser surface processing technique. The dual-pulse laser processing technique reported in this paper uses femtosecond laser pulse pairs with a controlled temporal delay between the leading and trailing pulses. Using dual pulses at higher fluence values, mound-like micro/nanostructures have been created on silver samples for the first time. Formation of the self-organized microstructures is shown to be dependent on the time delay between the leading and trailing pulses. Mound-like microstructures do not develop on silver for overlapped pulses or using single-pulse femtosecond laser surface processing for the parameter space studied. Subsurface microstructure characterization of a single mound-like surface structure is analyzed by cross-sectional analysis using focused ion beam milling followed by scanning electron microscopy and energy dispersive X-ray spectroscopy.

The effect of adding a low melting point Pr-Cu-Al alloy during spark plasma sintering of melt-spun Nd-Fe-B ribbons is investigated. Regions of coarse grains were reduced and overall grain refinement was observed after the addition of Pr68Cu25Al7, leading to an enhancement of coercivity from 12.7 kOe to 20.4 kOe. Hot deformation of the samples in the spark plasma sintering system resulted in the formation of platelet-like grains, producing crystallographic alignment and magnetic anisotropy. The hot deformation process improved the remanence and energy product but reduced the coercivity. The decrease of coercivity resulted from grain growth and aggregation of Pr and Nd elements at triple-junction phases.

We investigated the ambient temperature structural properties (thickness, width, microstructure, and lattice parameter), and the ambient and high temperature (up to 900 K) direct current (DC) magnetic properties—saturation magnetization (MS) and intrinsic coercivity (HCI)—of rapidly-solidified (melt-spun) Fe-x wt.% Si (x = 3, 5, & 8) alloys. The wheel surface speeds selected for the study were 30 m/s and 40 m/s. The ribbons produced at the lower wheel surface speed (30 m/s) were continuous having relatively uniform edges compared to the ribbons produced at the higher wheel surface speed. The thickness and the width of the melt-spun ribbons ranged between ∼15 and 60 μm and 500–800 μm, respectively. The x-ray diffraction spectra of the melt-spun ribbons indicated the presence of disordered α-phase, irrespective of the composition, and the wheel surface speed. The lattice parameter decreased gradually as a function of increasing silicon content from ∼0.2862 nm (Fe-3 wt.% Si) to ∼0.2847 nm (Fe-8 wt.% Si). The wheel surface speed showed an insignificant effect on MS while increased silicon content resulted in a decreasing trend in MS. Elevated temperature evaluation of the magnetization (M-T curves at ∼7.96 kA/m) in the case of Fe-3 & 5 wt.% Si alloy ribbons was distinctly different from that of the Fe-8 wt.% Si alloy ribbons. The curves of the as-prepared Fe-3 wt.% Si and Fe-5 wt.% Si alloy ribbons were irreversible while that of Fe-8 wt.% Si was reversible. The MS for any of the combinations of wheel surface speed and composition decreased monotonically with the increase in temperature (from 300 to 900 K). While HCI increased with the increase in temperature for all the wheel surface speed and composition combination, its nature of increase is distinct for Fe-8 wt.% Si alloy ribbons compared to Fe-3 & 5 wt.% Si alloys ribbons. It appears that rapidly-solidified Fe-3 wt.% Si and Fe-5 wt.% Si alloys ribbons are primarily comprised of the α-phase (disordered phase) while the Fe-8 wt.% Si alloy ribbons are comprised primarily of disordered phase along with minor constituents of an ordered phase.

Fe-Si alloys ranging from 3 to 8 wt% Si were rapidly solidified using melt spinning. Wheel speeds of 30 m/s and 40 m/s were employed to vary cooling rates. Mossbauer spectroscopic studies indicated the Si content significantly influenced the number of Fe sites, relative abundance of various Fe species, and internal magnetic fields/structural environments. Wheel speed altered Fe speciation only in the 3 wt% sample. Scanning electron microscopy confirmed that increasing the wheel speed refined both the ribbon thickness and grain size. Electron backscatter diffraction results suggest tailoring melt spinning process parameters and alloy chemistry may offer the ability to manipulate {001} texture development. Electrical resistivity measurements were observed to increase in response to elevated Si content. Increased hardness was correlated to elevated Si content and wheel speed.

Femtosecond laser surface processing (FLSP) is a technique that can be used to modify surfaces imparting many desirable properties, such as an improved wettability, heat transfer, and drag reduction. The morphology of the FLSP surface along with surface chemistry is critical factors that strongly influences the wetting behavior of surfaces produced. Reproducing the FLSP original surface over a large area in an economic manner is a challenge. In this paper, imprinting the FLSP surface on oxygen-free copper is investigated. Based on the topology of an FLSP titanium surface, a model has been established to investigate the topology of the mounds, which is defined by the roughness and period between the self-organized structures. This topology affects the imprint quality indicated by the plastic deformation in the blank and is defined as the target material specimen. Both oxygen-free copper and stainless steel 304 were used as blank target material in this study and represent both soft and hard material specimens. The stress–displacement relationship was used to determine the sensitivity of imprint parameters. The imprint model was verified by comparing an equivalent imprint model with nanoindentation experimental results. The model results showed that the die’s morphology, in particular the die roughness-to-period ratio (referring to the peak-to-valley height and the peak-to-peak distance), had greater influence than the blank material itself.

Domain wall nanoelectronics is a rapidly evolving field, which explores the diverse electronic properties of the ferroelectric domain walls for application in low-dimensional electronic systems. One of the most prominent features of the ferroelectric domain walls is their electrical conductivity. Here, using a combination of scanning probe and scanning transmission electron microscopy, the mechanism of the tunable conducting behavior of the domain walls in the sub-micrometer thick films of the technologically important ferroelectric LiNbO

Pool boiling heat transfer with the use of femtosecond laser surface processing (FLSP) on copper surfaces has been studied. FLSP creates a self-organized micro/nanostructured surface. In the previous pool boiling heat transfer studies with stainless steel FLSP surfaces, enhancements in critical heat flux (CHF) and heat transfer coefficients (HTCs) were observed compared to the polished reference surface. However, this study shows that copper FLSP surfaces exhibit reductions in both CHF and HTCs consistently. This reduction in heat transfer performance is a result of an oxide layer that covers the surface of the microstructures and acts as an insulator due to its low thermal conductivity. The oxide layer was observed and measured with the use of a focused ion beam milling process and found to have thickness of a few microns. The thickness of this oxide layer was found to be related to the laser fluence parameter. As the fluence increased, the oxide layer thickness increased and the heat transfer performance decreased. For a specific test surface, the oxide layer was selectively removed by a chemical etching process. The removal of the oxide layer resulted in an enhancement in the HTC compared to the polished reference surface. Although the original FLSP copper surfaces were unable to outperform the polished reference curve, this experiment illustrates how an oxide layer can significantly affect heat transfer results and dominate other surface characteristics (such as increased surface area and wicking) that typically lead to heat transfer enhancement.

Femtosecond laser surface processing (FLSP) is a unique material processing technique that can produce self-organized micro/nanostructures on most materials including metals, semiconductors, and dielectrics. These structures have demonstrated the enhancement of surface properties such as heat transfer and broadband light absorption. The chemical composition and morphology of FLSP structures is highly dependent on processing parameters including background gas composition, pressure, laser fluence, and number of laser pulses. When the laser processing is carried out in open atmosphere, a thick oxide layer forms on the FLSP surface structures due to the high reactivity of the surface with the environmental constituents immediately after laser processing. In this work, N2 and forming gas are used during laser processing in an effort to form a metal nitride on the surface of aluminum. Aluminum nitride is a promising material for enhancing the heat transfer performance of surfaces because of its thermal conductivity, which can be as high as 285 W/m-K, whereas aluminum oxide has a low thermal conductivity (30 W/m-K). Aluminum nitride incorporation into FLSP surfaces has the potential to act as a passivation layer to decrease the oxygen content and increase the thermal conductivity of the surface. Nitrogen incorporation is studied by applying FLSP in air, N2, and a 95% N2/5% H2 mixture. The chemical composition of the FLSP surfaces is determined by X-ray photoelectron spectroscopy (XPS) and energy-dispersive X-ray spectroscopy (EDS). Cross-sectional analysis of the FLSP microstructures is performed using ion beam milling.

The use of self-organized micro/nanostructured surfaces formed using femtosecond laser surface processing (FLSP) techniques has become a promising area of research for enhancing surface properties of metals, with many applications including enhancing heat transfer. In this work, we demonstrate advantages of the use of dual-pulse versus single-pulse FLSP techniques to produce self-organized micro/nanostructures on copper. With the dual-pulse technique, the femtosecond pulses out of the laser (spaced 1 ms apart) are split into pulse pairs spaced < 1 ns apart and are focused collinear on the sample surface. Single-pulse FLSP techniques have been widely used to produce self-organized “mound-like” structures on a wide range of metals including a number of stainless steel alloys, aluminum, nickel, titanium, and recently on copper. Due to its high thermal conductivity, copper is used in many critical heat transfer applications and micro/nanostructured copper surfaces are desired to further improve heat transfer characteristics. Using single-pulse (pulses spaced 1 ms apart) FLSP techniques, self-organized microstructure formation on copper requires much higher pulse fluence than is commonly used for producing microstructures on other metals, which results in instabilities during laser processing (non-uniform surfaces), low processing efficiency, and limitations on the control of the types of structures produced. In this paper, we report results that demonstrate that the dual-pulse FLSP technique can be used to produce microstructures on copper more efficiently than using single-pulse FLSP, with better control of the surface structures produced. Cross-sectional subsurface microstructure analysis is also presented for single-pulse versus dual-pulse FLSP functionalized copper surfaces.

Layered perovskites have been shown to improve the stability of perovskite solar cells while its operation mechanism remains unclear. Here we investigate the process for the conversion of light to electrical current in high performance layered perovskite solar cells by examining its real morphology. The layered perovskite films in this study are found to be a mixture of layered and three dimensional (3D)-like phases with phase separations at micrometer and nanometer scale in both vertical and lateral directions. This phase separation is explained by the surface initiated crystallization process and the competition of the crystallization between 3D-like and layered perovskites. We further propose that the working mechanisms of the layered perovskite solar cells involve energy transfer from layered to 3D-like perovskite network. The impact of morphology on efficiency and stability of the hot-cast layered perovskite solar cells are also discussed to provide guidelines for the future improvement., It is well-accepted that the two dimensional layered halide perovskite can improve the device stability of perovskite solar cells but the operation mechanism remains unclear. Here Lin et al. reveal the real morphology of the hot-cast layered perovskite solar cells and understand the working mechanism.

A comparison is made between the self-organized surface features that form on copper using femtosecond versus picosecond laser pulses. Modification of material surfaces (like metals and dielectrics) have shown to affect the properties for applications including enhancing heat transfer and producing anti-corrosive or antibacterial surfaces. Femtosecond laser surface processing is a precise and repeatable technique to produce uniform self-organized quasi-periodic structures on the surface for most metals. However, it is difficult to produce uniform quasi-periodic structures on noble metals (e.g. copper). Using femtosecond pulses in the processing of copper results in non-uniform structure formation with areas that have little to no mound development. Lengthening the pulses from femtoseconds to picoseconds is shown to result in uniform quasi-periodic structure formation with high repeatability. The subsurface microstructure formed using femtosecond versus picosecond pulses while keeping other processing parameters the same was investigated, providing insight into mound formation processes. Picosecond pulses resulted in finer-grained regions within the mounds, and after a critical pulse count threshold was reached, onion-like layers with alternating high-density copper and highly porous copper oxide regions formed near the surface and blanketed the mounds. Multiple formation mechanisms are proposed to help explain the microstructures that formed using picosecond pulses.

Owing to their low density and high strength-to-weight ratio, Mg alloys are attractive for automotive and aerospace applications. However, the current use of magnesium in the industry is limited due to the low strength and poor formability at ambient temperatures. Two-phase nanostructured materials are a promising solution to improve the mechanical properties of Mg alloys. Depending on processing parameters, either fibrous or lamellar morphologies can be formed via the eutectic transformation. In this work, the mechanical behavior of these two different morphologies were examined by in-situ SEM microcompression tests. The lamellar morphology exhibited a superior combination of strength and plasticity with respect to the fibrous morphology, and both showed superior strength relative to pure Mg, up to five-fold in the former and 12-fold in the latter. However, the higher strength in the fibrous eutectic was obtained at the expense of a low plasticity. According to the HR/TEM analyses of the deformed micropillars, the relatively high plasticity of the lamellar morphology was attributed to the two reasons; on the one hand, high dislocation activity was observed in both α and β phases and resulted in plastic co-deformation of the layers. On the other hand, the α layers played a significant role in restricting the shear instabilities that initiated in the β layers. Such an effect was not observed with the fibrous morphologies, and thus, they failed by cleavage along the closest-packed planes in the β crystal.

The structural and magnetic properties of Nd12Fe82B6 and Nd10M2Fe82B6 (M = Nb, Ti, Zr, Cr) alloys prepared using arc melting and melt spinning have been investigated. All the samples are found to crystallise with a tetragonal Nd2Fe14B phase without any alloy or elemental impurities. There is a small decrease in the unit cell volume of Nd2Fe14B due to transition metal (M) addition. The substitution of Nb and Ti refines and homogenises the nanostructure of the alloys, promoting intergrain exchange coupling leading to an increase in the remanence and energy product. For example, the remanence and energy product of Nd12Fe82B6 and Nd10Nb2Fe82B6 are 8.4 kG and 15 MGOe, and 9.9 kG and 20 MGOe, respectively. The J(T) curves are similar to those of a single phase ferromagnetic material suggesting no segregation of ferromagnetic impurities. The observed structural and magnetic properties are consistent with the fact that the substitutional transition metal atoms occupy the Nd site of the tetragonal Nd2Fe14B...

Intermetallic compounds having compositions from HfCo 4 to HfCo 8 were investigated by transmission electron microscopy, selected-area electron diffraction, and energy-dispersive x-ray spectroscopy. A major crystalline phase, closely related to the orthorhombic Zr 2 Co 11 phase in structure, has been observed in the samples with the composition ranges from HfCo 6 to HfCo 8 . The phase, referred to as either Hf 2 Co 11 or HfCo 7 phase in the literature, is actually one common phase, having a broad composition range and referred to as μ -phase in the present paper. In addition to the μ -phase, we have found coexisted fcc-Co phase and a minor Hf 2 Co 7 phase. The Hf 6 Co 23 phase has been identified in the HfCo 4 and HfCo 5 samples. Tilt-series electron diffraction revealed that the μ -phase is a one-dimensional incommensurate structure, which can be described approximately as B-centered orthorhombic with a = 0.47 nm, b = 0.82 nm, and c = 3.86 nm.

One grand challenge for long-lived perovskite solar cells is that the common electrode materials in solar cells, such as silver and aluminum or even gold, strongly react with hybrid perovskites. Here we report the evaluation of the potential of copper (Cu) as the electrode material in perovskite solar cells for long-term stability. In encapsulated devices which limit exposure to oxygen and moisture, Cu in direct contact with CH3NH3PbI3 showed no reaction at laboratory time scales, and is predicted to be stable for almost 170 years at room temperature and over 22 years at the nominal operating cell temperature of 40 °C. No diffusion of Cu into CH3NH3PbI3 has been observed after thermal annealing for over 100 hours at 80 °C, nor does Cu cause charge trap states in direct contact with CH3NH3PbI3 after long-term thermal annealing or illumination. High performance devices with efficiency above 20% with a Cu electrode retain 98% of the initial efficiency after 816 hours storage in an ambient environment without encapsulation. The results indicate Cu is a promising low-cost electrode material for perovskite solar cells for long-term operation.

We report the formation of complex core–shell and three-layer Mn–Bi nanoparticles in a single step inert-gas condensation process. These structures have been achieved by controlling the thermal environment of the nanoparticles. High resolution transmission electron microscopy, high-angle annular dark-field imaging in scanning transmission electron microscopy mode, and elemental mapping by energy dispersive spectroscopy have been used to determine the crystal structure and chemical composition of the nanoparticles. These particles exist in two forms: (1) a crystalline Bi core with an amorphous Mn-rich shell, and (2) a crystalline Bi annular shell between two amorphous layers with high Mn concentration. These particles show significant magnetic hysteresis possibly arising from the change in bond length between Mn atoms introduced by Bi atoms in the bonding environment of the Mn atoms.

The effect of Hf and B additions on the phase stability, microstructure and magnetic properties of the metastable SmCo7 (1:7H) ribbons has been investigated with a combined approach of experimental measurements and first principle DFT (density functional theory) calculations. A series of (Sm0.12Co0.88)95Hf5−xBx (x = 0, 1, 2, 3, 4 and 5) alloys were arc-melted in a TIG (tungsten inert gas) arc melting furnace, followed by melt-spinning onto a copper roller at a wheel velocity of 40 m/s. Characterization based on X-ray diffraction indicates that the major phase is SmCo7 having meta-stable (TbCu7-type) structure. From the total energy calculations using DFT, the phase stability of (Sm0.12Co0.88)95Hf5−xBx ribbons have been confirmed. Moreover, Hf and B addition results in an effective grain refinement; average grain size being as low as ~ 80 nm. The reduction in grain size leads to significant changes (increase or decrease) in magnetic properties depending on the Hf/B ratio. The coercivity value (Hc) varies between 7 kOe and 12 kOe as x (at.% B) increases from 0 to 5 at.%. The experimental coercivity values have been compared with the computed anisotropy energies. The saturation magnetization (Ms) increases from ~ 54 emu/g to 77 emu/g with increasing B concentration (x).

Mg-33.3 wt.%Al eutectic alloys were fabricated by melt spinning at wheel speeds from 5 to 50 ms−1. SEM and S/TEM analyses showed the formation of anomalous eutectic structures at all wheel speeds. Four distinctive eutectic microstructures were observed, depending on the wheel speed, each with a unique formation mechanism. At low wheel speeds, solute trapping and high interfacial energy resulted in the remelting of primarily grown regular eutectic microstructures and was responsible for the formation of the anomalous eutectic Type A. A coupled-to-decoupled-growth transition occurred at intermediate wheel speeds giving rise to the anomalous eutectic Type B. The decoupled nucleation and growth of the eutectic phases followed by a coupled dendritic growth lead to the formation of flower-like microstructures at anomalous eutectic Type C. At sufficiently high wheel speed, no sign of regular eutectic was found and a unique cellular microstructure of ϒ phase along with inter- or intragranular α phase grains was formed, anomalous eutectic Type D, as a result of the decoupled nucleation and growth of the eutectic phases. The observed microstructures were examined thoroughly, and the correlated formation mechanisms were discussed on the basis of the SEM images and S/TEM and EDS analysis.

Femtosecond laser surface processing (FLSP) is an emerging technique for creating functionalized surfaces with specialized properties, such as broadband optical absorption or superhydrophobicity/superhydrophilicity. It has been demonstrated in the past that FLSP can be used to form two distinct classes of mound-like, self-organized micro/nanostructures on the surfaces of various metals. Here, the formation mechanisms of below surface growth (BSG) and above surface growth (ASG) mounds on polycrystalline Ni60Nb40 are studied. Cross-sectional imaging of these mounds by focused ion beam milling and subsequent scanning electron microscopy revealed evidence of the unique formation processes for each class of microstructure. BSG-mound formation during FLSP did not alter the microstructure of the base material, indicating preferential valley ablation as the primary formation mechanism. For ASG-mounds, the microstructure at the peaks of the mounds was clearly different from the base material. Transmission electron microscopy revealed that hydrodynamic melting of the surface occurred during FLSP under ASG-mound forming conditions. Thus, there is a clear difference in the formation mechanisms of ASG- and BSG-mounds during FLSP.

An experimental investigation of the effects of layers of nanoparticles formed during femtosecond laser surface processing (FLSP) on pool boiling heat transfer performance has been conducted. Five different stainless steel 304 samples with slightly different surface features were fabricated through FLSP, and pool boiling heat transfer experiments were carried out to study the heat transfer characteristics of each surface. The experiments showed that the layer(s) of nanoparticles developed during the FLSP processes, which overlay FLSP self-organized microstructures, can either improve or degrade boiling heat transfer coefficients (HTC) depending on the overall thickness of the layer(s). This nanoparticle layer thickness is an indirect result of the type of microstructure created. The HTCs were found to decrease with increasing nanoparticle layer thickness. This trend has been attributed to added thermal resistance. Using a focused ion beam milling process and transmission electron microscopy (TEM), the physical and chemical properties of the nanoparticle layers were characterized and used to explain the observed heat transfer results. Results suggest that there is an optimal nanoparticle layer thickness and material composition such that both the HTCs and critical heat flux (CHF) are enhanced.

FeNi, with the chemically-ordered L10 tetragonal structure, is a promising material for next-generation rare-earth-free permanent magnets. Due to the extremely low atomic Fe and Ni mobility below the critical chemical order/disorder temperature of 320 °C, no conventional metallurgical methods are able to induce its formation. Diffusion rates could be enhanced with the creation of excessive vacancies. High-energy mechanical alloying was employed to produce a nanocrystalline Fe–50 at.%Ni alloy. The high energy mechanical milling is expected to change the defect characteristics that would be used to study the effect that mechanical alloying has on the defect concentrations. X-ray diffraction revealed that over the entirety of the milling period, the Fe and Ni powders formed an fcc solid solution. The Williamson–Hall equation and Scherrer equation revealed that reductions in grain sizes were caused during alloying. The initial increase in internal strain, and by extension dislocations, was followed by a decrease in strain with further alloying, which was due to the combination effect of creation of dislocation from mechanical deformation and annihilation of dislocations at increasing grain boundaries. Doppler broadening positron annihilation spectroscopy (DB-PAS) showed that the overall number of open-volume defects decreased with increased milling time. Finally, changes of relative permeability, for the mechanically alloyed FeNi phase, were explained by the pinning effect of dislocations.