High-pressure Sintering Research Articles

Thermoelectric Properties of Fe1+y Te Prepared by a High Pressure Sintering Method

Enhancing thermoelectric properties by utilizing topological properties of topological insulators has attracted increasing attention. Here, powder metallurgy technology combined with high pressure sintering (HPS) was used to prepare Fe1+y Te (y = 0.1, 0.15, 0.1978, 0.22) alloys at 1173 K. The prepared products were identified by x-ray diffraction, and their microstructures were examined by field-emission scanning electron microscopy (FE-SEM). Electric conductivities (σ) and Seebeck coefficient (S) were measured in the temperature range of 303–623 K. The influence of variation in Fe content on thermoelectric properties was studied. The experimental results show that all HPS samples are nanometer composites consisting of Fe3Te2 and FeTe2. Fe1+y Te have electrical charges on the boundaries of topological insulators, which have the potential to be studied in the future. The Fe1+y Te with y = 0.1 shows the best thermoelectric properties at room temperature.

Thermoelectric Properties of Mn1+x Te-Based Compounds Densified Using High-Pressure Sintering

In this paper, powder metallurgy technology combined with high-pressure sintering (HPS) were used to fabricate Mn1+xTe (x = −0.02, 0, 0.04) alloys at 743 K. The products of synthesis were identified by x-ray diffraction; microstructures were examined by field-emission scanning electron microscopy (FE-SEM); the electrical conductivity and the Seebeck coefficient were measured in the temperature range of 300–800 K. The results show that all HPS samples consisted of nanoparticles. Mn1+xTe (x = −0.02, 0, 0.04) thermoelectric materials with a high Seebeck coefficient and low electrical conductivity have potential to warrant further study in the future. The peak figure of the merit, ZT, reaches 0.59 for Mn0.98Te at 773 K.

A new class of magnetorheological elastomers based on waste tire rubber and the characterization of their properties

This paper proposes a new type of magnetorheological elastomer (MRE) using rubber from waste tires and describes its performance characteristics. In this work, scrap tires were utilized as a primary matrix for the MRE without incorporation of virgin elastomers. The synthesis of the scrap tire based MRE adopted a high-temperature high-pressure sintering technique to achieve the reclaiming of vulcanized rubber. The material properties of the MRE samples were investigated through physical and viscoelastic examinations. The physical tests confirmed several material characteristics—microstructure, magnetic, and thermal properties–while the viscoelastic examination was conducted with a laboratory-made dynamic compression apparatus. It was observed from the viscoelastic examination that the proposed MRE has magnetic-field-dependent properties of the storage modulus, loss modulus, and loss tangent at different excitation frequencies and strain amplitudes. Specifically, the synthesized MRE showed a high zero field modulus, a reasonable MR effect under maximum applied current, and remarkable damping properties.

Interfacial Lithium-Ion Transfer Between Graphite Negative Electrode and Sulfide Solid Electrolyte

Introduction All-solid-state lithium secondary batteries using inorganic solid electrolytes instead of liquid electrolytes have been expected as next-generation secondary batteries with high safety and high reliability. Among the inorganic solid electrolytes, sulfide-based solid electrolyte such as Li2S-P2S5 based glass solid electrolyte has received much attention due to the high lithium-ion conductivities and room temperature pressure sintering [1]. Graphite has been used as the negative electrode materials in commercial lithium-ion batteries, and the improvement in interfacial lithium-ion transfer reaction between graphite and liquid electrolyte is recognized as an important issue and this issue should be considered in all-solid-state lithium secondary batteries. In this study, interfacial reaction between graphite and sulfide solid electrolyte was investigated. Experimental A layered electrolyte consists of 75Li2S-25P2S5 glass pellet and 1 mol dm-3 LiClO4/propylene carbonate (PC) was used in order to use lithium metal electrode as reference electrode. A three-electrode cell was assembled by using graphite sheet as working electrode, lithium metal as counter and reference electrodes. Cyclic voltammetry and electrochemical impedance spectroscopy were carried out. Results Figure 1 shows the cyclic voltammogram of the three-electrode cell using graphite sheet as working electrode. An oxidation-reduction current was observed between 0 – 0.5 V (vs. Li/Li+). Since lithium ion cannot intercalate into graphite sheet in LiClO4/PC, this oxidation-reduction peak indicates that lithium-ion intercalation/de-intercalation proceeded at the interfacial between graphite sheet and 75Li2S-25P2S5 glass pellet. In the Nyquist plot, one semi-circle was observed and assigned to interfacial lithium-ion transfer process. In the meeting, the activation energy for the interfacial lithium-ion transfer process will be discussed. Reference [1] A. Sakuda et al., Sci. Rep., 3, 2261 (2013). Acknowledgement This work was financially supported by ALCA-SPRING of the JST. Figure 1

Microstructure and phase evolution in spark-plasma-sintered Al86Ni6Y4.5Co2La1.5 glassy alloy

Al86Ni6Y4.5Co2La1.5 amorphous powders synthesized after 200 h of mechanical alloying were spark plasma sintered at 25 and 400 MPa at 400 °C for 15 min. The sample sintered at 25 MPa showed precipitation of FCC-Al grains of sub-micron size attributed to a higher amount of localized high-temperature regions which favoured faster long-range atomic diffusion. On the contrary, the 400-MPa sintered sample showed the presence of nanocrystals (5–20 nm) distributed in an amorphous matrix. This is attributed to higher pressure sintering favouring local and short-range atomic redistributions. The higher amount of retained amorphous phase in the 400-MPa sintered sample could be accounted for by a smaller amount of localized high-temperature regions in comparison to those in 25-MPa sintered sample. High-pressure sintering induces shear fracturing and fragmentation of the brittle amorphous particles and provided better surface contact and packing density. The retention of a larger amount of amorphous phase and a higher relative density (96%) contributes to high microhardness (278 Hv) and nanoindentation hardness (3.7 GPa) of the 400-MPa sintered sample.

Preparation of W-Cu alloy with high density and ultrafine grains by mechanical alloying and high pressure sintering

W-20 wt%Cu alloy, with a high sintering density of 99.2% theoretical density (%TD), ultrafine grain size of 500nm, and hardness of 310HV, was fabricated by mechanical alloying and high pressure sintering (HPS). The changes of morphology and structural evolution of W-Cu powders during mechanical alloying process were observed and studied, and the effects of temperature and pressure on the density of sintered compacts were studied. Results showed that nanostructured (NS) mechanical alloyed (MA) W-Cu powders were obtained after a 40h milling at a milling speed of 400rpm, and the W-Cu alloy had nanocrystalline structure and shape retention with nearly full densification when the HPS of NS MA W-Cu powders was carried out at 1050°C with a pressure of 40MPa. The densification behavior of NS MA W-Cu powders during HPS was also studied, including a nanosintering within the MA powders during heating and a global rearrangement and sintering when an external pressure was applied.

Influence of SiC on the Properties of PcBN Cutting Tools Fabricated by High Pressure and High Temperature Sintering

SiC-Al3BC3-cBN composites with different contents of cBN were fabricated by high pressure and high temperature sintering (HPHT) at 1450 °C for 3 min under a pressure of 5.0 GPa using SiC, Al, B4C and C as additives. The effect of SiC content on the density and mechanical properties of SiC-Al3BC3-cBN composites was investigated. X-ray diffraction (XRD) and Scanning electron microscope (SEM) were used to analyze phases and micro-structure of the sintered samples. The hardness of SiC-Al3BC3-cBN composites decreased with the increasing of SiC content, However, the fracture strength of SiC-Al3BC3-cBN composites increased with the increasing of SiC content.

The Oxidation Resistance of ZrB<sub>2</sub>-SiC Composites Fabricated by High Pressure and High Temperature Sintering

Dense ZrB2-SiC composites were fabricated by high pressure and high temperature sintering(HPHT) at 1500 oC for 3 min under a pressure of 5 GPa. The vickers’ hardness of ZrB2-SiC composite is 25 GPa. The flexural strength of the ZrB2-SiC composite is 300 MPa with the amount of SiC increased to 30wt.%, which increased after the composites oxidized at high temperature. The generated glass phase like SiO2 sealed pores and cracks on the surface of the ZrB2-SiC composite to increase the mechanical properties of the ZrB2-SiC composite. After oxdized at 1500oC for 45 mins, the dense oxidized film formed on the surface of ZrB2-SiC composites to improve the resistance oxi dation of ZrB2 ceramics.

High Pressure and High Temperature Sintered PcBN Using Al, B<sub>4</sub>C and C as Sintering Additive

Polycrystalline cubic boron nitride (PcBN) composites were sintered by high pressure and high temperature sintering (HPHT) at 1450 °C for 3 min under a pressure of 5.0 GPa. Aluminium,boron carbide and carbon in the starting mixture reacts with cubic boron nitride (cBN) to form Al3BC3 and AlN bonding among cBN grains during sintering. X-ray diffraction (XRD) and Scanning electron microscope (SEM) were used to analyze phases and micro-structure of the sintered samples. The dense structure of super hard cBN grains bonded together with Al3BC3 and AlN offers superior hardness and high strength. The Vickers hardness of PcBN composites was 45±5 GPa, and the strength of PcBN composites was 345±15 MPa.

Microstructures and Mechanical Properties of β-Sialon-cBN Composites

β-Sialon-cubic boron nitride (cBN) composites were prepared by the high temperature (1700°C) and high pressure (5GPa) sintering. β-Sialon chemical general composition can be represented as Si6-zAlzOzN8 –z (Z is the number of Al atoms which are substituted by Si atoms, and the range of z value is usually between 0-4.2[1].).The effects of different formula with corresponding to z values on microstructure and mechanical properties of β-Sialon-cBN composites were studied. The relative test results were verified by computer simulation. The study result showed: with the z values of β-Sialon increasing, the aluminum and oxygen content increased. The densification and mechanical properties of β-Sialon-cBN composites decreased gradually.

High-Pressure Preparation of High-Density Cu2ZnSnS4 Materials**Supported by the National Natural Science Foundation of China under Grant Nos 10874178, 11074093, 61205038 and 11274135, the National Found for Fostering Talents of Basic Science under Grant No J1103202, the Ph.D. Programs Foundation of Ministry of Education of China under Grant No 20120061120011, the Open Project of State Key Laboratory of Superhard Materials of

High-density Cu2ZnSnS4 (CZTS) materials are prepared via the mechanical alloying and high pressure sintering method using Cu2S, ZnS and SnS2 as the raw materials. The morphological, structural, compositional and electrical properties of the materials are investigated by using x-ray diffraction, scanning electron microscopy, and energy dispersive x-ray spectroscopy, as well as by the Raman scattering and the Hall Effect measurements. The CZTS synthesized under 5 GPa and 800°C shows a p-type conductivity, with a resistivity of 9.69 × 10−2 Ω·cm and a carrier concentration of 1.45 × 1020 cm−3. It is contributed to by the large grains in the materials reducing the grain boundaries, thus effectively reducing the recombination of the charge carriers.

Physicochemical Properties and Stress-Strain Compression Behaviors of a Waste based Magnetorheological Elastomers

This study reports a reclamation method of ground tire rubber (GTR) and electronic wastes into a tunable stiffness composite namely magnetorheological elastomers (MREs). The composite matrix was the GTR that has been separated from its metals and yarns. Meanwhile, the fillers were fabricated from magnetic cores of the cathode ray tubes (CRTs) monitor waste through high energy ball milling in wet condition. Some physicochemical characterizations of the powders were initially conducted to collect the physical and chemical properties. Based on the examination, the ground powders were identified as pure magnetite (Fe3O4) having low remnant and high magnetization. The MREs samples were prepared by high-temperature high-pressure sintering. The mixed of crumb rubber and magnetite powder was vulcanized under simultaneous pressure, and temperature in the absence of magnetic fields resulted in an isotropic MREs type. The specimens were examined their abilities in responding to the external magnetic field through static and dynamic compression test. The experimental investigation showed that the static and dynamic stresses of the stiffness increased by the increment of magnetic fields until a particular point of flux density. The tunability of the stiffness indicated that the elastomers can provide wide ranges of the storage and loss modulus.

Process and composition dependence of optical properties of zirconium, hafnium and tantalum borides for solar receiver applications

Ultra-high temperature boride ceramics have proved to show promising properties for novel solar receivers. The present work shows a further step towards their actual application, investigating how sintering technique and starting powders composition affect the properties of final materials. Thus we report on the comparative characterization of ZrB2, HfB2 and TaB2 produced by high pressure and pressureless techniques and with different amounts of MoSi2 sintering aid. We investigate microstructural, mechanical and optical properties, in the perspective to assess the material potential for novel solar absorbers operating at higher temperatures than those currently available. Moreover, a systematic study has been carried out on ZrB2, producing with fixed high pressure sintering technique, a series of samples with MoSi2 compositions in the range 5–50vol%. We show that the content of silicide and silicide-related secondary phases in the final pellets affects either the mechanical performance and the optical behavior. Thus, as far as the optical properties are concerned, the MoSi2 amount should be the lowest as possible to ensure a proper material consolidation whilst enhancing the absorbance/spectral selectivity.

Behaviors of Zn2GeO4 under high pressure and high temperature**Project supported by the Joint Fund of the National Natural Science Foundation of China and Chinese Academy of Sciences (Grant No. U1332104).

The structural stability of Zn2GeO4 was investigated by in-situ synchrotron radiation angle dispersive x-ray diffraction. The pressure-induced amorphization is observed up to 10 GPa at room temperature. The high-pressure and high-temperature sintering experiments and the Raman spectrum measurement firstly were performed to suggest that the amorphization is caused by insufficient thermal energy and tilting Zn–O–Ge and Ge–O–Ge bond angles with increasing pressure, respectively. The calculated bulk modulus of Zn2GeO4 is 117.8 GPa from the pressure-volume data. In general, insights into the mechanical behavior and structure evolution of Zn2GeO4 will shed light on the micro-mechanism of the materials variation under high pressure and high temperature.

Preparation and characterization of cBN-based composites from cBN-Ti3AlC2 mixtures

Cubic boron nitride (cBN)-based composites were prepared at ~5.5GPa and ~1350°C with different mass ratios of cBN-Ti3AlC2 mixtures as starting materials. Ti3AlC2 decomposed and reacted with cBN during high pressure and high temperature (HPHT) sintering. TiC, TiC0.7N0.3, TiB2, and AlN were then formed. The bending and compressive strengths of cBN-based composites first increased and then decreased when the Ti3AlC2 content increased from 5wt.% to 40wt.%; hardness was determined by both the compactness and binder content. cBN-based composites sintered with 20wt.% Ti3AlC2 possessed the highest bending strength (422.45MPa), compressive strength (3969.14MPa), and hardness (3381.58HV); they also had the most compact microstructure. The initial oxygenation temperature of the cBN-based composites sintered with 20wt.% Ti3AlC2 was about 850°C. After isothermal oxidation at 1200°C for 1200s, titanium and aluminum oxides formed on the surface with pores and cracks. The isothermal oxidation process of the cBN-based composites could be described by a parabolic law.

Unique mechanical properties of nano-grained YAG transparent ceramics compared with coarse-grained partners

In order to improve the inherent brittleness of the conventional coarse-grained YAG transparent ceramics, nano-grained YAG transparent ceramics were fabricated at 5GPa/450°C using the super high pressure sintering technique. Their mechanical properties were investigated by nanoindentation and the Vickers indentation measuring methods. It was found that the nano-grained YAG transparent ceramics exhibited much better ductility compared with the coarse-grained counterparts. The grain-boundary sliding provides the main contribution to plastic deformations of nano-grained YAG transparent ceramics.

Microstructure and mechanical behavior of NS/UFG aluminum prepared by cryomilling and spark plasma sintering

We report on a study of the microstructure and mechanical behavior of a bulk nanostructured (NS)/ultra-fine grained (UFG) aluminum fabricated by cryomilling and high-pressure spark plasma sintering (SPS). The compressive yield stress of the consolidated material is determined to be 380 MPa, which is significantly higher than that of commercial strain hardened aluminum (124 MPa). Microstructural studies reveal that the nanometric grains are embedded inside a matrix of ultra-fine grains in the bulk material. The corresponding average grain size is approximately 125 nm and with a distribution of grains that are smaller than 500 nm. Moreover, chemical analysis of the powder particles and the consolidated samples indicates that carbon and oxygen levels remain unchanged, and that there is a slight decrease in the nitrogen level and a significant reduction in the hydrogen level after spark plasma sintering. Semi-quantitative analysis suggests that the mechanisms that contribute to the strength of the consolidated material include: grain boundary strengthening, second phase strengthening and dislocation strengthening.

A strategy to optimize the thermoelectric performance in a spark plasma sintering process.

Spark plasma sintering (SPS) is currently widely applied to existing alloys as a means of further enhancing the alloys’ figure of merit. However, the determination of the optimal sintering condition is challenging in the SPS process. This report demonstrates a systematic way to independently optimize the Seebeck coefficient S and the ratio of electrical to thermal conductivity (σ/κ) and thus achieve the maximum figure of merit zT = S2(σ/κ)T. Sb2−xInxTe3 (x = 0–0.2) were chosen as examples to validate the method. Although high sintering temperature and pressure are helpful in enhancing the compactness and electrical conductivity of pressed samples, the resultant deteriorated Seebeck coefficient and increasing thermal conductivity eventually offset the benefit. We found that the optimal sintering temperature coincides with temperatures at which the maximum Seebeck coefficient begins to degrade, whereas the optimal sintering pressure coincided with the pressure at which the σ/κ ratio reaches a maximum. Based on this principle, the optimized sintering conditions were determined, and the zT of Sb1.9In0.1Te3 is raised to 0.92 at 600 K, showing an approximately 84% enhancement. This work develops a facile strategy for selecting the optimal SPS sintering condition to further enhance the zT of bulk specimens.

Study on High Pressure Sintering of Nanocrystalline Diamond with Nano Silicon as Additives

In this paper, nanocrystalline diamond with the average grain size of 50nm was prepared under different sintering pressure, temperature and sintering time. The microstructure of the sample was analyzed by SEM, EDS and XRD, and the mechanical properties tested by micro-hardness tester and wear ratio instrument. The results show that the sample sintered under the optimum conditions of oil pressure 87MPa, heating power 4000W and sintering time 120s possessed hardness of 706.41HV and wear ratio of 3280. It indicated that high performance n-PCD sintered from nanodiamond and silicon system can be formed hardly with diamond to diamond bonding but can be formed with diamond to silicon carbide bonding. The poor mechanical properties of the samples were due to the surface adsorption groups and surface graphitization of nanodiamond during high pressure sintering.

The high-entropy alloys with high hardness and soft magnetic property prepared by mechanical alloying and high-pressure sintering

The equiatomic multiprincipal CoCrFeCuNi and CoCrFeMnNi high-entropy alloys (HEAs) were consolidated via high pressure sintering (HPS) from the powders prepared by the mechanical alloying method (MA). The structures of the MA'ed CoCrFeCuNi and CoCrFeMnNi powders consisted of a face-centered-cubic (FCC) phase and a minority body-centered cubic (BCC) phase. After being consolidated by HPS at 5 GPa, the structure of both HEAs transformed to a single FCC phase. The grain sizes of the HPS'ed CoCrFeCuNi and CoCrFeMnNi HEAs were about 100 nm. The alloys keep the FCC structure until the pressure reaches 31 GPa. The hardness of the HPS'ed CoCrFeCuNi and CoCrFeMnNi HEAs were 494 Hv and 587 Hv, respectively, much higher than their counterparts prepared by casting. Both alloys show typical paramagnetism, however, possessing different saturated magnetization. The mechanisms responsible for the observed influence of Cu and Mn on mechanical behavior and magnetic property of the HEAs are discussed in detail.