Bulk Polycrystalline Materials Research Articles

Grain boundary mobilities in polycrystals

Most metals, ceramics, semiconductors and rocks are composed of small crystals known as grains. When annealed, this polycrystalline structure coarsens, thus allowing the properties of a material to be tailored for a particular application. The mobility of grain boundaries is thought to be determined by the crystallography of the adjacent crystals, but experimental validation in bulk polycrystalline materials is lacking. Here we developed a novel fitting methodology by direct comparison of a time-resolved three-dimensional experimental data to simulations of the evolution of 1501 grains in iron. The comparison allows reduced mobilities of 1619 grain boundaries to be determined simultaneously. We find that the reduced mobilities vary by three orders of magnitude and in general exhibit no correlation with the boundary’s five macroscopic degrees of freedom, implying that grain growth is governed by other factors.

Lattice strains and diffraction elastic constants of cubic polycrystals

In situ synchrotron X-ray and neutron diffraction experiments provide a powerful approach to measure lattice strains in bulk polycrystalline materials. They are being increasingly used for quantitative characterization of microscale deformation within and between grains and phases. Here we use a self-consistent micromechanics model to obtain a general analytic solution of the grain-level lattice strains and diffraction elastic constants for a broad class of elastically isotropic polycrystals with cubic crystal symmetry. This analytic solution reveals a direct linear relationship between the reciprocal of the elastic diffraction constant and the orientation index parameter along the direction of any diffraction vector, including tensile loading and transverse directions. The straightforward numerical implementation of this solution provides diffraction elastic constants for 26 representative cubic polycrystals. Analytic solutions of this kind can serve to benchmark in situ diffraction measurements of lattice strains and also facilitate high-throughput studies of microscale stresses and diffraction elastic constants in polycrystalline materials.

Y3Al5O12-α-Al2O3 composites with fine-grained microstructure by hot pressing of Al2O3-Y2O3 glass microspheres

Yttrium aluminate glass microspheres with the eutectic composition 76.8 mol. % Al2O3 and 23.2 mol. % Y2O3 were prepared by combining the sol-gel Pechini method with flame synthesis. The sol-gel method was applied to achieve the desired composition homogeneity of the prepared glass and hence, improve the microstructure homogeneity and mechanical properties of bulk polycrystalline materials. The latter were prepared by hot pressing, more specifically pressure assisted sintering, at 1050 °C, 1300 °C and 1600 °C using pressures of 30 MPa and 80 MPa and holding times between 0 and 30 min. This also led to the crystallization of the glass. A composite with the Vickers hardness 18.0 ± 0.7 GPa and an indentation fracture toughness 4.9 ± 0.3 MPa.m1/2 was obtained by sintering at 1600 °C, at the pressure of 80 MPa and with 30 min isothermal heating at the maximum temperature. Improved mechanical properties were observed when increasing the temperature of sintering and the holding time. This can be attributed to the formation of a unique microstructure consisting of α-Al2O3 grains in the μm-scale embedded in a YAG (yttrium-aluminium garnet) matrix in the hot-pressed samples.

Colossal Elastocaloric Effect in Ferroelastic Ni-Mn-Ti Alloys.

Energy-efficient and environment-friendly elastocaloric refrigeration, which is a promising replacement of the conventional vapor-compression refrigeration, requires extraordinary elastocaloric properties. Hitherto the largest elastocaloric effect is obtained in small-size films and wires of the prototype NiTi system. Here, we report a colossal elastocaloric effect, well exceeding that of NiTi alloys, in a class of bulk polycrystalline NiMn-based materials designed with the criterion of simultaneously having large volume change across phase transition and good mechanical properties. The reversible adiabatic temperature change reaches a strikingly high value of 31.5K and the isothermal entropy change is as large as 45 J kg^{-1} K^{-1}. The achievement of such a colossal elastocaloric effect in bulk polycrystalline materials should push a significant step forward towards large-scale elastocaloric refrigeration applications. Moreover, our design strategy may inspire the discovery of giant caloric effects in a broad range of ferroelastic materials.

Phase retrieval for Bragg coherent diffraction imaging at high x-ray energies

Coherent X-ray beams with energies $\geq 50$ keV can potentially enable three-dimensional imaging of atomic lattice distortion fields within individual crystallites in bulk polycrystalline materials through Bragg coherent diffraction imaging (BCDI). However, the undersampling of the diffraction signal due to Fourier space compression at high X-ray energies renders conventional phase retrieval algorithms unsuitable for three-dimensional reconstruction. To address this problem we utilize a phase retrieval method with a Fourier constraint specifically tailored for undersampled diffraction data measured with coarse-pitched detector pixels that bin the underlying signal. With our approach, we show that it is possible to reconstruct three-dimensional strained crystallites from an undersampled Bragg diffraction data set subject to pixel-area integration without having to physically upsample the diffraction signal. Using simulations and experimental results, we demonstrate that explicit modeling of Fourier space compression during phase retrieval provides a viable means by which to invert high-energy BCDI data, which is otherwise intractable.

Electrodeposition of Pt and Gd from the Same Ionic Liquid

Platinum rare earth alloys show several times higher electrocatalytic activity than pure platinum while still maintaining an excellent stability [1-3]. This has been demonstrated for polycrystalline bulk materials [1, 3], for nanoparticles prepared in a cluster source [2] and for thin sputtered films [4, 5]. Once exposed to aqueous solutions, dealloying leads to formation of a several layers thick Pt skin that is under compressive strain, causing the activity enhancement [1]. The controlled preparation of such alloys using scalable techniques is difficult due to the high reactivity of the rare earth elements [5]. Electrochemical deposition from ionic liquids should be a possibility [6]. The electrodeposition of a number of rare earth elements has been reported in the literature [7-10], even though not in all cases clear proof has been given that a pure metal has been obtained. Several studies showed that in TFSI based ionic liquids rare earth electrochemistry can be complicated, and that not always a metal can be easily achieved [11]. Our own experiments – usually applying the electrochemical quartz crystal microbalance technique - on the electrodeposition of yttrium from two ionic liquids did not lead to the desired metal deposition [6], while there was indication for deposition of lanthanum [6] and gadolinium from BMP TFSI using TFSI precursors. However, the deposition was very slow, and independent proof for actual metal deposition was not obtained. Motivated by a study reporting lanthanum electrodeposition from butylmethylimidazolium dicyanamide (BMIm DCA) at elevated temperatures [10], we decided to attempt the gadolinium deposition from this ionic liquid. While experiments at room temperature failed due to passivation of the counter electrode, the deposition worked well at 60°C using GdCl3 as precursor. Under these conditions a thick metal greyish layer was obtained that in contact with air rapidly reacted and became whitish. The electrodeposition of Pt has been accomplished in different ionic liquids in literature [12-14], and we also managed to deposit Pt from two ionic liquids using a –still water containing – H2PtCl6 precursor [6]. In this work we discuss the results obtained for Pt deposition using different water-free precursors in BMIm DCA, namely Pt(acac)2, PtCl2 and PtCl4, and in part using additives to control complex formation. From Pt(acac)2, electrodeposition of Pt was not achieved at all. Addition of Pt(acac)2 to a GdCl3 electrolyte did not result in alloy formation, but the electrodeposition of Gd was blocked as well. Using PtCl2 and PtCl4, indication for successful Pt deposition was obtained. The role of complex formation on Pt deposition and alloy deposition will be elaborated in this contribution. The project leading to this application has received funding from the Fuel Cells and Hydrogen 2 Joint Undertaking under grant agreement No 700127. This Joint Undertaking receives support from the European Union’s Horizon 2020 Research and Innovation Programme and Hydrogen Europe and N.ERGHY.

Performance of Functionally Graded Thermoelectric Materials and Devices: A Review

Direct energy conversion using thermoelectric generators (TEGs) is a research area of growing interest because of its potential for increasing energy efficiency. Bulk thermoelectric modules are used widely in industry as Peltier cooling devices. Currently, only bismuth telluride modules are commercially available for power generation. Significant efforts have been put into exploring promising materials and techniques to improve the figure of merit (zT) at laboratory scale (5–20 g). A variety of techniques have been investigated to improve the output and useful temperature range for common industrial TEGs made from bulk polycrystalline materials including segmentation, geometric pinning, and property gradients. However, the improvement in zT at device level (500–1000 g and up) is exceptionally limited. In addition, the thermal degradation of TEGs occurs when cracks form due to thermal stresses that arise from transient heat sources, which consequently lead to a decreased lifetime. Functionally graded material (FGM) thermoelectrics in bulk and polycrystalline form have been developed to mitigate some of these issues by improving the temperature bandwidth, current output range, and lifetime. The present work provides a review of functionally graded TEGs, including their manufacturing, usage and current techniques for improving their performance. This article also provides a pathway to additional research and approaches for improving the efficiency and temperature range, as well as reducing the property degradation of bulk polycrystalline TEGs.

Enhanced thermoelectric properties of p-type polycrystalline SnSe by regulating the anisotropic crystal growth and Sn vacancy**Project supported by the National Natural Science Foundation of China (Grant Nos. 51572049, 51562005, and 51772056), the Natural Science Foundation of Guangxi Zhuang Automomous Region, China (Grant Nos. 2015GXNSFFA139002 and 2016GXNSFBA380152), and the Open Fund of Key Laboratory of Cryogenics, Technical Institute of Physics and

Thermoelectric selenides have attracted more and more attentions recently. Herein, p-type SnSe polycrystalline bulk materials with good thermoelectric properties are presented. By using the SnSe2 nanostructures synthesized via a wet-chemistry route as the precursor, polycrystalline SnSe bulk materials were successfully obtained by a combined heat-treating process under reducing atmosphere and following spark plasma sintering procedure. As a reference, the SnSe nanostructures synthesized via a wet-chemistry route were also fabricated into polycrystalline bulk materials through the same process. The thermoelectric properties of the SnSe polycrystalline transformed from SnSe2 nanostructures indicate that the increasing of heattreating temperature could effectively decrease the electrical resistivity, whereas the decrease in Seebeck coefficient is nearly invisible. As a result, the maximum power factor is enhanced from to at 612 °C. On the other hand, the reference sample, which was obtained by using SnSe nanostructures as the precursor, displays very poor power factor of only at 537 °C. The x-ray diffraction (XRD), scanning electron microscope (SEM), x-ray fluorescence (XRF), and Hall effect characterizations suggest that the anisotropic crystal growth and existing Sn vacancy might be responsible for the enhanced electrical transport in the polycrystalline SnSe prepared by using SnSe2 precursor. On the other hand, the impact of heat-treating temperature on thermal conductivity is not obvious. Owing to the boosting of power factor, a high zT value of 1.07 at 612 °C is achieved. This study provides a new method to synthesize polycrystalline SnSe and pave a way to improve the thermoelectric properties of polycrystalline bulk materials with similar layered structure.

Bi12TiO20 crystallization in a Bi2O3-TiO2-SiO2-Nd2O3 system

Polycrystalline mono-phase bismuth titanate was produced by free cooling from melts heated to 1170 °C. The control over the initial amounts in the starting compositions in the system Bi2O3/TiO2/SiO2/Nd2O3 and over the thermal gradient of the heat process resulted in the formation of specific structures and microstructures of monophase sillenite ceramics. The main phase Bi12TiO20 belongs to the amorphous network groups based on oxides of silicon, bismuth and titanium. In this work, we demonstrated a way to control the crystalline and amorphous phase formation in bulk poly-crystalline materials in the selected system.

Low thermal conductivity CoSb3 materials prepared by rapid synthesis process

Nano-particles and defects have effective influence on reducing the lattice thermal conductivity. In this work, a serious of high concentration Te doping bulk polycrystalline materials Co4Sb11.2Te0.8 has been synthesized successfully at different pressures by the high pressure and high temperature (HPHT) method. All samples were characterized by X-ray diffraction. The Seebeck coefficient α, electrical resistivity ρ and thermal conductivity κ were all measured from 373K to 673K. It could be observed obviously that, as the synthesis pressure rised, the thermal conductivity of Co4Sb11.2Te0.8 decreased remarkably. The minimum thermal conductivity of 1.36Wm−1K−1 was obtained by Co4Sb11.2Te0.8 sample synthesized at 3.7GPa. The corresponding microstructures were also studied by SEM and HRTEM images.

Ultrasound measurement of texture in bulk polycrystalline materials

Manufacturing of metal components often results in significant texture, that is, to say, preferred orientations of their polycrystals. Since each crystal can be strongly anisotropic, this can give the component orientation-dependent material properties, affecting stiffness, thermal expansion, strength, and fatigue and creep resistance. So, it is important to be able to measure texture, especially for high-value safety-critical components. Typically, the Orientation Distribution Function (ODF) of the crystals can be measured on exposed surfaces using EBSD, or in thin samples using neutron diffraction. But both are expensive, and until recently there has not been a means to measure ODFs internally in bulk materials. However, the authors have recently developed a method to determine internal texture from measurements of wave speeds at selected angles through the volume of the material. This is based on a convolution of wave speeds in a single crystal with the ODF, giving the resultant polycrystal wave speed angular function. The principles of the method have been established and validated for single phase hexagonal and cubic materials. Ongoing work is investigating the use of multiple wave modes (shear and compressional) to identify further information; the extraction of volume fraction and texture of two-phase materials; and the pursuit of further refinement of the texture results by improved knowledge of the single crystal properties.

Crystallography of Functional Materials

The goal of this special issue is to obtain new insights into the roles of crystallography in functional materials. This special issue consists of eight papers illustrating the structure and property relationships, as well as applications of selected classes of materials that deal with various aspects of functional materials, ranging from battery, magnetic, photocatalysis, and waveguide materials, to luminescent metal-organic frameworks and borates, semiconductors, and inorganic electrides. This issue provides further evidence of the importance of crystallography in understanding and improving various properties of functional materials, whether they are single crystals, bulk polycrystalline materials, or thin films.

Self-Propagating High Temperature Synthesis of MgB2 Superconductor in High-Pressure of Argon Condition

<p>Magnesium diboride can be synthesized under argon ambient, elevated or high pressures. High-pressure syntheses are promising methods for manufacturing of the bulk MgB<sub>2</sub> superconductor material. We have been used high pressure of Ar gas in order to investigate its effect on properties of MgB<sub>2</sub> superconductor such as critical temperature and current density. Bulk MgB<sub>2</sub> superconductor was synthesized from elemental Mg–B powders in thermal explosion mode of self-propagating high-temperature synthesis (SHS) under argon pressure of 25 atm. XRD pattern of the as-synthesized product indicates an almost complete conversion of the reactants to the MgB<sub>2</sub> single phase. Most of the diffractions peaks are related to the MgB<sub>2</sub> polycrystalline bulk material. The impurity fraction is less than 24.3% in total sample and identified as MgO and MgB<sub>4</sub> secondary phases. The positive effect of pressure of Ar gas during synthesis of MgB2 on critical current density JC has been confirmed. The critical current density of the sample was achieved in high pressure reactor was 3.8×10<sup>6</sup> A/cm<sup>2</sup>. A superconducting volume fraction of 16% under a magnetic field of 10 Oe was obtained at 5 K, indicating that the superconductivity was bulk in nature. The succeeded level of superconductor parameters of the high-pressure synthesized MgB<sub>2</sub> and the possibility to produce a large bulk products make this technology very promising for practical applications.</p>

Structural Texture Induced in SnSe Thermoelectric Compound via Open Die Pressing.

Outstanding ZT values registered on single crystals recently renewed the interest of thermoelectric community for SeSn compound. Owing to the strong anisotropy of the phenomenon, so far only single crystals proved to be the suitable for its application. Here we present the production and the characterization of bulk polycrystalline materials processed by open die pressing, aimed at reducing the gap with single crystal materials by taking advantage from the highly texture degree derived by the processing and by the improved phonon scattering promoted by grain boundaries. The resulting bulks display good compaction, improved mechanical properties and strong texture of the phase. Structural and morphological analyses confirmed the successful orientation according to the (400) cleavage plane. The structural transition responsible for the ultra-low thermal conductivity has been investigated and possible irreversible effects on the starting phase due to thermal cycling have been evaluated. Preliminary measurements of thermal conductivity are reported.

Non-destructive identification of unknown minor phases in polycrystalline bulk alloys using three-dimensional X-ray diffraction

Minor phases make considerable contributions to the mechanical and physical properties of metals and alloys. Unfortunately, it is difficult to identify unknown minor phases in a bulk polycrystalline material using conventional metallographic methods. Here, a non-destructive method based on three-dimensional X-ray diffraction (3DXRD) is developed to solve this problem. Simulation results demonstrate that this method is simultaneously able to identify minor phase grains and reveal their positions, orientations and sizes within bulk alloys. According to systematic simulations, the 3DXRD method is practicable for an extensive sample set, including polycrystalline alloys with hexagonal, orthorhombic and cubic minor phases. Experiments were also conducted to confirm the simulation results. The results for a bulk sample of aluminum alloy AA6061 show that the crystal grains of an unexpected γ-Fe (austenite) phase can be identified, three-dimensionally and nondestructively. Therefore, we conclude that the 3DXRD method is a powerful tool for the identification of unknown minor phases in bulk alloys belonging to a variety of crystal systems. This method also has the potential to be used for in situ observations of the effects of minor phases on the crystallographic behaviors of alloys.

Porosity Effects on the Thermoelectric Properties of Nanostructured Bismuth

Bismuth nanocrystals were synthesized using a polyol process and densified using spark plasma sintering to form polycrystalline bulk materials with uniformly dispersed nanoscale grains. The Bi nanocrystals were consolidated at different pressures in order to investigate the magnitude and temperature dependence of transport properties as a function of porosity. We also compare these transport properties with those of single-crystal bismuth. This investigation expands our understanding of the role of porosity in the thermoelectric properties of nanostructured bismuth.

Evolution of thermoelectric properties of substituted Si-based clathrates prepared by HPHT

Polycrystalline bulk materials Ba8CuxGeySi46-x-y (x = 0, 6; y = 0, 20) were prepared by high-pressure and high-temperature (HPHT) method and characterized using X-ray diffraction and Rietveld refinement. Results of EDS analysis showed that the solubility limit of copper composition in this clathrate is about 5. Electrical properties measurements between 300 K and 720 K revealed the behavior of a degenerate semiconductor for all samples. As the framework elements increased, the Seebeck coefficient and electrical resistivity increased, but the thermal conductivity decreased greatly, which leads to significant enhancement on thermoelectric properties of the clathrates. The lattice defects of samples generated by high pressure efficiently reduced the lattice thermal conductivity.

Prospects for thermoelectricity in quantum dot hybrid arrays.

The electronic, chemical and mechanical properties of quantum dot structures may lead to thermoelectric devices with a range of advantages with respect to existing ones based on bulk polycrystalline materials.

Microstructure and thermoelectric properties of porous Bi2Te2.85Se0.15 bulk materials fabricated by semisolid powder processing

Abstract

Unusual behaviour of (Np,Pu)B2C

Two transuranium metal boron carbides, NpB2C and PuB2C have been synthesized by argon arc melting. The crystal structures of the {Np,Pu}B2C compounds were determined from single-crystal X-ray data to be isotypic with the ThB2C-type (space group , a = 0.6532(2) nm; c = 1.0769(3) nm for NpB2C and a = 0.6509(2) nm; c = 1.0818(3) nm for PuB2C; Z = 9). Physical properties have been derived from polycrystalline bulk material in the temperature range from 2 to 300 K and in magnetic fields up to 9 T. Magnetic susceptibility and heat capacity data indicate the occurrence of antiferromagnetic ordering for NpB2C with a Neel temperature TN = 68 K. PuB2C is a Pauli paramagnet most likely due to a strong hybridization of s(p,d) electrons with the Pu-5f states. A pseudo-gap, as concluded from the Sommerfeld value and the electronic transport, is thought to be a consequence of the hybridization. The magnetic behaviour of {Np,Pu}B2C is consistent with the criterion of Hill.