Hot-pressed Samples Research Articles

Enhanced thermoelectric properties of selenium-deficient layered TiSe(2-x): a charge-density-wave material.

In the present work, we report on the investigation of low-temperature (300-5 K) thermoelectric properties of hot-pressed TiSe2, a charge-density-wave (CDW) material. We demonstrate that, with increasing hot-pressing temperature, the density of TiSe2 increases and becomes nonstoichiometric owing to the loss of selenium. X-ray diffraction, scanning electron microscopy, and transimission electron microscopy results show that the material consists of a layered microstructure with several defects. Increasing the hot-press temperature in nonstoichiometric TiSe2 leads to a reduction of the resistivity and enhancement of the Seebeck coefficient in concomitent with suppression of CDW. Samples hot-pressed at 850 °C exhibited a minimum thermal conductivity (κ) of 1.5 W/m·K at 300 K that, in turn, resulted in a figure-of-merit (ZT) value of 0.14. This value is higher by 6 orders of magnitude compared to 1.49 × 10(-7) obtained for cold-pressed samples annealed at 850 °C. The enhancement of ZT in hot-pressed samples is attributed to (i) a reduced thermal conductivity owing to enhanced phonon scattering and (ii) improved power factor (α(2)σ).

A comparative study on hot dynamic compaction and quasi-static hot pressing of Al7075/SiCnp nanocomposite

Quasi-static and hot dynamic compactions of Al7075/SiC nanocomposite powder are studied in this work. Mechanically milled micron-sized Al7075 with different amounts of SiC nano particles (SiCnp), 0vol%, 5vol%, and 10vol%, are used to fabricate the samples. Dynamic and quasi-static hot compaction is conducted at strain rates of 103s−1 and 8×10−3s−1, respectively. The hot compaction conducted at the process temperature of 698K produces nanocomposite samples with relative densities up to 98%. The micro-structural and mechanical behaviors of samples such as micro hardness and stress–strain curves under quasi-static and dynamic loadings are also investigated. The results show improvement in micro hardness of the material. The quasi-static and dynamic tests are carried out using Instron testing machine and split Hopkinson pressure bar (SHPB), respectively. The improvement is believed to be due to strengthening mechanisms such as Orowan and enhanced dislocation density induced by thermal mismatch phenomenon and not by grain refinement described by the Hall–Petch effect. The results from quasi-static compressive tests reveal higher strength for quasi-static hot pressed samples than those produced under dynamic compaction. However, the dynamic compressive tests using SHPB indicate similar compressive strength for both quasi-statically and dynamically compacted samples.

Microstructure and mechanical properties of Ti–22Al–25Nb alloy fabricated by vacuum hot pressing sintering

Abstract A study has been undertaken to verify the feasibility of using a powder metallurgy (P/M) approach to fabricate Ti–22Al–25Nb alloys. Pre-alloyed powders with a nominal composition of Ti–22Al–25Nb (at%) obtained by argon atomization were sieved to the spherical size less than 180 μm and used for the fabrication of P/M Ti–22Al–25Nb alloys via hot pressing in vacuum. Vacuum hot pressing sintering was carried out in a temperature range of 950–1200 °C with a pressure of 35 MPa for 1 h followed by furnace cooling. Microstructure and phase composition examinations of the as-atomized powders and hot pressed (HP׳ed) samples were conducted by applying optical microscopy, back-scatter electron imaging and X-ray diffraction analysis. Tensile tests were studied at room temperature and 650 °C, respectively. The results showed that all HP׳ed samples were composed of coarse equiaxed B2 grains, fine lamellar O phase inside the B2 grains, and some α2 along B2 grain boundaries. The elongations of HP׳ed samples were less than 3.95%, indicating the bad ductility at room temperature. However, the elongations were improved as the tensile temperature increased to 650 °C. The sample sintered at 1050 °C exhibited a better ductility with the elongation of 7.97% at 650 °C than that of other samples.

Solid-state processing of oxidation-resistant molybdenum borosilicide composites for ultra-high-temperature applications

The high-temperature capabilities of multi-phase composites based on Mo5Si3Bx are examined after solid-state processing and pulsed laser deposition (PLD) coating fabrication approaches. These composites are prepared by mechanical alloying of elemental powders and densified by vacuum hot pressing, which is a scalable processing approach. Chemical analyses of the hot-pressed compacts reveal a consistent 15–22 percent loss of boron, which is primarily due to the high-temperature hot-pressing step. Composites possessing sufficient amounts of boron are evaluated by thermogravimetric studies in temperatures up to 1650 °C in air. One composition demonstrates oxidative stability after long-term (100 h) isothermal conditions, as well as thermal cycling to simulate solar-thermal operation. Hot-pressed samples of composites consisting of Mo5Si3Bx + MoSi2 + MoB are also employed as deposition targets for PLD trials. X-ray diffraction analysis of the resulting films indicates the absence of long-range crystallographic order.

Thermoelectric properties of semi conducting compound Sb71.22Co2.97Fe20.77Ce5.04

Hot-pressed samples of the semi-conducting compound Sb71.22Co2.97Fe20.77Ce5.04 were prepared and characterized by X-ray thermal differential analysis and microprobe analysis. Thermoelectric characterization was done through measurements of the electrical and thermal conductivities as well as the Seebeck coefficient between room temperature and 900 K. All samples had p-type conductivity. The dimensionless thermoelectric figure of merit ZT increases with increasing temperature and reaches a maximum value of 1.22 at 873 K.

Thermoelectric properties of Zn doped Cu2SnSe3

Zn doped ternary compounds Cu2ZnxSn1−xSe3 (x = 0, 0.025, 0.05, 0.075) were prepared by solid state synthesis. The undoped compound showed a monoclinic crystal structure as a major phase, while the doped compounds showed a cubic crystal structure confirmed by powder XRD (X-Ray Diffraction). The surface morphology and elemental composition analysis for all the samples were studied by SEM (Scanning Electron Microscopy) and EPMA (Electron Probe Micro Analyzer), respectively. SEM micrographs of the hot pressed samples showed the presence of continuous and homogeneous grains confirming sufficient densification. Elemental composition of all the samples revealed an off-stoichiometry, which was determined by EPMA. Transport properties were measured between 324 K and 773 K. The electrical resistivity decreased up to the samples with Zn content x = 0.05 in Cu2ZnxSn1−xSe3, and slightly increased in the sample Cu2Zn0.075Sn0.925Se3. This behavior is consistent with the changes in the carrier concentration confirmed by room temperature Hall coefficient data. Temperature dependent electrical resistivity of all samples showed heavily doped semiconductor behavior. All the samples exhibit positive Seebeck coefficient (S) and Hall coefficient indicating that the majority of the carriers are holes. A linear increase in Seebeck coefficient with increase in temperature indicates the degenerate semiconductor behavior. The total thermal conductivity of the doped samples increased with a higher amount of doping, due to the increase in the carrier contribution. The total and lattice thermal conductivity of all samples showed 1/T dependence, which points toward the dominance of phonon scattering at high temperatures. The maximum 1/TZT = 0.48 at 773 K was obtained for the sample Cu2SnSe3 due to a low thermal conductivity compared to the doped samples.

Fractographical assessment of densification mechanisms in hot pressed ZrB2-SiC composites

The controlling densification mechanisms of hot pressed monolithic ZrB2 ceramics and ZrB2-based composites, containing 15 and 30vol% SiC, at different consolidating temperatures were investigated, based on scanning electron microscopy micrographs of fracture surfaces, relative densities, and average grain size of ZrB2. For the hot pressed samples at 1700°C, particles fragmentation in the composite samples, mechanical interweaving, and rearrangement without sizeable chemical bonding were appointed as dominant densification mechanisms. Neck formation between ZrB2/ZrB2 was observed at 1850°C and plastic deformation of ZrB2 grains was nominated as controlling densification mechanism. Reduction of porosity in the hot pressed specimens at 2000°C was related to grain boundary diffusion mechanism. Colossal grain growth in monolithic ZrB2 ceramic proposed the occurrence of detrimental mechanisms such as grain coarsening and evaporation/condensation. Presence of intergranular SiC particles between ZrB2 grains impeded extremist grain growth.

Relationship between microstructure and thermoelectric properties of Bi2Sr2Co2Ox bulk materials

Abstract

Dielectric and thermal properties of isotactic polypropylene/hexagonal boron nitride composites for high-frequency applications

Dielectric composites aimed for high frequency applications were prepared by using anisotropic hexagonal boron nitride (hBN) particles as a fillers and isotactic polypropylene (iPP) as polymer matrix. Dielectric and thermal properties of the composites were studied, focusing on the filler orientation in the plate-shape specimens and filler concentration up to 40vol%. The degree of orientation of the filler was controlled by the composite fabrication process. Hot-pressing gave relatively random orientation of the filler in the matrix, while injection molding induced a high orientation. Dielectric constant (εr) of the composites ranged between 2.25 and 3.39. The estimation of εr based on the Bruggeman mixing model agreed well with the measured value. Low dielectric losses (tanδ) at microwave frequencies, on the order of 10−4, were obtained for all the compositions. Through-thickness thermal conductivity (k) of the hot-pressed samples showed a drastic increase with increasing the filler concentration, reaching up to 2.1W/mK at 40vol% of hBN. The filler concentration dependence of k was less significant for the injection molded composites. In-plane thermal expansion was almost independent on the filler orientation, while the coefficient of thermal expansion for the thickness direction of the hot-pressed sample was reduced to approximately half of the injection molded counterpart. These differences in thermal conductivity and thermal expansion are thought to arise from the difference in hBN filler orientation.

Microstructural and mechanical properties of Cu-7vol.%ZrB2 alloy produced by powder metallurgy techniques

Abstract The Cu-7vol.%ZrB2 alloy examined in this study was consolidated via powder metallurgy (PM) processing by combining mechanical alloying and hot pressing. Powder mixture with composition: 94.78 wt.% copper, 4.1 wt.% zirconium, and 1.12 wt.% boron was used as a starting material. Mechanical alloying of powder mixture was performed at various times: 5, 10, 15, 20, 25, and 30 h. Structural changes that occur in the samples of copper alloy with 7vol.%ZrB2 after milling and during hot pressing process were studied with the use of X-ray diffraction. Scanning electron microscopy (SEM) equipped with an energy dispersive X-ray spectrometry (EDS) was applied to examine the morphological properties and elemental analysis of the hot-pressed samples as a function of milling times. Also, hardness of the Cu-7vol.%ZrB2 alloy was investigated, and the results showed that hardness of the samples increased as the milling time increased. The compressive properties at room temperature are correlated to the corresponding hardness results. Distribution of ZrB2 particles and presence of agglomerates in the Cu matrix directly depend on the milling time and show strong influence on compressive properties and fracture of Cu-7vol.%ZrB2 alloy.

Effect of High-Pressure Torsion on Texture, Microstructure, and Raman Spectroscopy: Case Study of Fe- and Te-Substituted CoSb3

Fe0.05Co0.95Sb2.875Te0.125, a double-element-substituted skutterudite, was prepared by induction melting, annealing, and hot pressing (HP). The hot-pressed sample was subjected to high-pressure torsion (HPT) with 4 GPa pressure at 673 K. X-ray diffraction was performed before and after HPT processing of the sample; the skutterudite phase was observed as a main phase, but an additional impurity phase (CoSb2) was observed in the HPT-processed sample. Surface morphology was determined by high-resolution scanning electron microscopy. In the HP sample, coarse grains with sizes in the range of approximately 100 nm to 300 nm were obtained. They changed to fine grains with a reduction in grain size to 75 nm to 125 nm after HPT due to severe plastic deformation. Crystallographic texture, as measured by x-ray diffraction, indicated strengthening of (112), (102) poles and weakening of the (123) pole of the HPT-processed sample. Raman-active vibrational modes showed a peak position shift towards the lower energy side, indicating softening of the modes after HPT. The distortion of the rectangular Sb-Sb rings leads to broadening of Sb-Sb vibrational modes due to local strain fluctuation. In the HPT process, a significant effect on the shorter Sb-Sb bond was observed as compared with the longer Sb-Sb bond.

Influence of hot pressing temperature on thermoelectric properties of type‐I clathrates

It is well known that the process variables for sample synthesis affect the electrical, magnetic, mechanical, etc. properties of the final product. In this study, we present the influence of the hot pressing temperature on the thermoelectric properties of a type‐I clathrate of nominal composition Ba8Cu4.5Ge35.5Si6. The samples are prepared in a high‐frequency furnace, annealed, and subject to a long‐time ball milling. Finally, the nanopowders from ball milling are hot pressed at different temperatures. All hot pressed samples are characterized by X‐ray diffraction, energy dispersive X‐ray spectroscope, scanning electron microscopy, and transport property measurements. The results show that lower hot pressing temperatures are desired to prevent the nanopowders from coarsening, but are unfavorable for the densification: Samples hot pressed at lower temperatures have smaller crystallite sizes, larger porosities, and lower relative densities and, consequently, have higher electrical resistivity, and lower thermal conductivity. No significant differences are seen in ZT as the positive effect of reduced thermal conductivity is compensated by the enhanced electrical resistivity. The highest ZT of 0.42 is reached at 873 K for a sample hot pressed at 780 °C.

LiCoPO4 mechanical properties evaluated by nanoindentation

LiCoPO4 is a promising cathode material for use in high voltage Li-ion batteries. One of the problems with LiCoPO4 is limited cycle life. One possible cause for the limited cycle life of LiCoPO4 is mechanical degradation. As a consequence, the mechanical properties, elastic modulus, E, and fracture toughness, KIC, of hot-pressed dense (~98%) polycrystalline (15–20μm) single phase LiCoPO4 were investigated. E for the hot-pressed LiCoPO4 specimen is ~137GPa while the E value for the LiCoPO4 specimen after annealing at 600°C is ~106GPa. The fracture toughness of the hot-pressed LiCoPO4 sample is ~0.41MPam1/2, which increased to ~0.53MPam1/2 after annealing. These low KIC values reveal that LiCoPO4 is a brittle material. It is believed that the decrease in E and increase in KIC with annealing is associated with a reduction in residual stress.

TiO2 sol-gel for formaldehyde photodegradation using polymeric support: photocatalysis efficiency versus material stability

Photocatalysts supported on polymers are not frequently used in heterogeneous photocatalysis because of problems such as wettability and stability that affect photocatalysis conditions. In this work, we used polypropylene as support for TiO2 sol-gel to evaluate its stability and efficiency under UV radiation. We also tested the effect of the thermo-pressing PP/TiO2 system on the photocatalytic efficiency and stability under UV radiation. The films were characterized by scanning electron microscopy (SEM), UV-Vis spectroscopy and X-ray diffraction (XRD). The SEM micrographs showed that the films of TiO2 sol-gel onto PP has approximately 1.0-μm thick and regular surface and the generation of polypropylene nanowires on hot-pressed samples. XRD showed the formation of TiO2 anatase on the surface of the films made by dip-coating. All photocatalysts were tested in decontaminating air-containing gaseous formaldehyde (70 ppmv) presenting degradation of the target compound to the limit of detection. The photocatalysts showed no deactivation during the entire period tested (30 h), and its reuse after washing showed better photocatalytic performance than on first use. The photocatalyst showed the best results were tested for 360 h with no observed deactivation. Aging studies showed that the film of TiO2 causes different effects on the photostability of composites, with stabilizing effect when exposed to most energetic UVC radiation (λmax = 254 nm) and degradative effects when exposed to UVA radiation (λmax = 365 nm).

Mechanical properties of Ca3Co4O9 bulk oxides intended to be used in thermoelectric generators

The lamellar thermoelectric (TE) oxides Ca3Co4O9 (Ca349) are promising candidates for energy conversion in a temperature range of 300–1200K in air. To be well-suited for being integrated in TE devices, Ca349 bulk materials must show high mechanical reliability to withstand the assembly constrains and in-service conditions. In the aim of optimizing TE performances of these materials, specimens were elaborated by using Hot-Pressing (HP) and Spark Plasma Sintering (SPS). Indentation measurements were operated on these ceramics using both micro-hardness testing and depth-sensing nano-indentation. Fracture characteristics were assessed by 3 point bending tests. Nano-hardness (nH), elastic modulus (E), strength (σR) and fracture toughness (KIc) were shown to drastically enhance versus the pressures PHP and PSPS applied during HP and SPS treatments, respectively, which is ascribed to higher densification and, to a lesser extent, to the texture strengthening and grain boundary density decrease in the direction perpendicular to the pressing axis. The contribution to micro-hardness (µH) of both later factors was estimated to ~30% for the hot-pressed sample under PHP=30MPa which depicted (nHxy=2.1±0.4GPa, Exy=56±4GPa) and (nHz=2.3±0.2GPa, Ez=85±5GPa) in its respective planes perpendicular and parallel to the pressing axis, revealing an anisotropy of the elastic modulus. It presented σR=251±12MPa and KIc=2.3±0.4MPam1/2. Although the lower size of the intrinsic flaws was found for the HP materials, the largest mechanical characteristics were achieved by SPS under PSPS≥50MPa. The elastic recovery H/E of Ca349 ceramics was found among the highest ratios reported for oxides and one order of magnitude larger compared to the half-Heusler or skutterudites potential TE materials.

Thermoelectric Properties of Cu2HgSnSe4-Cu2HgSnTe4 Solid Solution

Copper-based semiconductors from the family Cu2-II-IV-VI4 have recently attracted a great deal of attention because of their promising thermoelectric (TE) properties. Polycrystalline samples from the Cu2HgSnSexTe4−x (x = 0, 0.8, 2, 3.2, 4) solid solution were prepared and structurally characterized by powder x-ray diffraction. The samples from this solid solution crystallize in the stannite structure (space group \(I\bar{4}2m\)). Transport, TE, and thermal properties of hot-pressed samples are reported. About a 20 % reduction in calculated lattice thermal conductivities, compared to the lattice thermal conductivities of pure components of the alloys (i.e. Cu2HgSnSe4 and Cu2HgSnTe4), was observed for Cu2HgSnSe2Te2 alloy. The maximum ZT of the Cu2HgSnSe2Te2 sample reaches 0.6 at 575 K.

Thermoelectric properties of a Pb0.2Sn0.8Te solid solution prepared through thermal decomposition of salts

We have studied the thermoelectric properties of microcrystalline samples of the Pb0.2Sn0.8Te solid solution prepared by hot-pressing fine powders obtained through the thermal decomposition of lead acetate trihydrate and tin oxalate in the presence of tellurium powder. The electrical conductivity, Seebeck coefficient, and thermal conductivity of the samples have been measured at room temperature and in the temperature range 300–700 K and correlated with the particle size of the powders and heat-treatment conditions. We have compared the thermoelectric properties of hot-pressed samples from powders prepared through the thermal decomposition of salts and by mechanical grinding of ingots.

Shock compression behavior of bi-material powder composites with disparate melting temperatures

Laser driven experiments were used to investigate the shock compression behavior of powder processed Bismuth/Tungsten (Bi/W) composite samples. The constituents provide different functionality to the composite behavior as Bi could be shock melted at the pressures attained in this work, while the W could not. Samples were prepared by uniaxial pressing, and the relative density was measured as a function of particle size, applied pressure, and composition for both hot and cold pressing conditions. This resulted in sample densities between 73% and 99% of the theoretical maximum density, and also noticeable differences in microstructure in the hot and cold pressed samples. The compression waves were generated with a 1.3 × 1.3 mm square spot directly onto the surface of the sample, using irradiances between 1012 and 1013 W/cm2, which resulted in calculated peak pressures between 50 and 150 GPa within a few micrometers. Sample recovery and post-mortem analysis revealed the formation of a crater on the laser drive surface, and the depth of this crater corresponded to the depth to which the Bi had been melted. The melt depth was found to be primarily a function of residual porosity and composition, and ranged from 167 to 528 μm. In general, a higher porosity led to a larger melt depth. Direct numerical simulations were performed, and indicated that the observed increase in melt depth for low-porosity samples could be largely attributed to increased heating associated with work done for pore collapse. However, the relative scaling was sensitive to composition, with low volume fraction Bi samples exhibiting a much stronger dependence on porosity than high Bi content samples. Select samples were repeated using an Al foil ablator, but there were no noticeable differences ensuring that the observed melting was indeed pressure-driven and was not a result of direct laser heating. The resultant microstructures and damage near the spall surface were also investigated qualitatively.

Synthesis of Ag-ZnO composites via ball milling and hot pressing processes

Ag — 8 wt. % ZnO composites were synthesized by ball milling, heat treating and hot pressing of silver and zinc oxide powder mixtures. The crystalline size and microstrain of the milled powders before and after heat treatment were determined by Debye-Scherrer andWilliamson-Hall methods. It was shown that heat treatment resulted in decrease of microstrain and increase in the crystallite size of the milled powders. The effect of uniaxial pressure magnitude and duration of hot pressing at 550 °C on the final density of the powder compacts were investigated. The results showed that both plastic flow and atomic diffusion mechanisms affected densification of the composite powders during the hot pressing process. However, the latter one had more effective role on the density of the hot-pressed samples. The synthesized composites showed homogenous microstructure with relatively high density and hardness.

유성구볼밀을 이용한 La2O3-Gd2O3-ZrO2계 서스펜션준비와 서스펜션 플라즈마용사를 이용한 (La1-xGdx)2Zr2O7코팅증착과 특성

Lanthanum/gadolinium zirconate coatings are deposited via suspension plasma spray with suspensions fabricated by a planetary mill and compared with hot-pressed samples via solid-state reaction. With increase in processing time of the planetary mill, the mean size and BET surface area change rapidly in the case of lanthanum oxide powder. By using suspensions of planetary-milled mixture between lanthanum or gadolinium oxide and nano zirconia, dense thick coatings with fully-developed pyrochlore phases are obtained. The possibilities of these SPS-prepared coatings for TBC application are also discussed.