High-pressure Sintering Technique Research Articles

Enhanced mechanical properties of nanocrystalline B4C–SiC composites by in-situ high pressure reactive sintering

A unique optimized of core–shell structural B4C nanopowder, sintering aid additive of Si, and high-pressure sintering technique has been used to process nanocrystalline B4C–SiC ceramics with enhanced mechanical properties. C-coated B4C nanopowder was initially uniformly mixed with micron Si of different content by ball-milling. B4C–SiC composites with a homogenous distribution of SiC in B4C matrix were subsequently obtained by sintering the mixed powders at 6 GPa and 1600 °C. The added Si reacted with submicron amorphous carbon layer and amorphous carbon nanoshell to form dispersed SiC nanocrystals and Si–C phase filled at B4C grain boundaries and pores, respectively. The prepared composite had the most outstanding mechanical properties when the Si content in the precursor was 15 wt%, with a hardness reaching 37.8 GPa and a fracture toughness reaching 7.3 MPa·m1/2. Microstructural characterizations indicated that the multi deflection of nanoscale crack caused by intergranular fracture, the covalent bonding of Si–C phase at the grain boundary, and the abundant nanotwin substructure were jointly responsible for the superior performance in hardness and fracture toughness.

High Pressure Drives Microstructure Modification and zT Enhancement in Bismuth Telluride-Based Alloys.

Manipulating and integrating the microstructures at different scales is crucial to tune the electrical and thermal properties of a given compound. High-pressure sintering can modify the multiscale microstructures and thus empower the cutting-edge thermoelectric performance. In this work, the high-pressure sintering technique followed by annealing is adopted to prepare Gd-doped p-type (Bi0.2Sb0.8)2(Te0.97Se0.03)3 alloys. First, the high energy of high-pressure sintering promotes the reduction of grain size, thus increasing the content of 2D grain boundaries. Next, high-pressure sintering induces strong interior strain, where 1D dense dislocations are generated near the strain field. More interestingly, the rare-earth element Gd with a high melting temperature is dissolved into the matrix via high-pressure sintering, thus promoting the formation of 0D extrinsic point defects. This concurrently improves the carrier concentration and density-of-state effective mass, resulting in an enhanced power factor. In addition, the integrated 0D point defects, 1D dislocations, and 2D grain boundaries by high-pressure sintering strengthen phonon scattering, thereby achieving a low lattice thermal conductivity of 0.5 Wm-1 K-1 at 348 K. Consequently, a maximum zT value of ∼1.1 at 348 K is achieved in the 0.4 at % Gd-doped (Bi0.2Sb0.8)2(Te0.97Se0.03)3 sample. This work demonstrates that high-pressure sintering enables microstructure modification to enhance the thermoelectric performance of Bi2Te3-based and other bulk materials.

Impact of Alkali Ions Codoping on Magnetic Properties of La0.9A0.1Mn0.9Co0.1O3 (A: Li, K, Na) Powders and Ceramics

The aim of the work was to check how the introduction of alkali and cobalt ions into a manganese structure can affect the structural disorder and, in consequence, lead to the changes (improvements) of magnetic properties. The high-pressure sintering technique was applied to check if the external factor can modify the magnetization of manganites. Nanocrystalline La0.9A0.1Mn0.9Co0.1O3 (where A is Li, K, Na) powders were synthesized by the combustion technique. The respective powders were used for nanoceramics preparation by the high-pressure sintering technique. The structure and morphology of the compounds were studied by X-ray powder diffraction, scanning electron microscopy and energy-dispersive X-ray spectroscopy. Magnetization studies for all compounds were performed in order to check the changes induced by either codoping or the sintering pressure. It was found that the type of the dopant ion and sintering pressure produced significant changes to the magnetic properties of the studied compounds. Alkali ions lead to the stabilization of Co ions in the +2 oxidation state and the formation of positive exchange interactions Mn3+–Mn4+ and Co2+–Mn4+ and the subsequent increase in remanent magnetization. High sintering pressure leads to a decrease in grain size and reduction of long-range ferromagnetic order and lower magnetization.

The study of composition, structure and cathodoluminescent features of YAG:Eu3+ nanoceramics. Excitation capture efficiency of Eu3+ energy levels

The focus of this research paper is upon cathodoluminescent features of YAG:Eu3+ nanoceramics. The concentration series of samples was synthesized from nano powders using low temperature high pressure sintering technique. The composition of the samples has been studied using electron-probe microanalysis. The structure and the grain size of the samples have been studied by X-ray diffraction, scanning electron microscopy and electron backscattered diffraction. Cathodoluminescent study of the samples has been performed. The concentration dependence of cathodoluminescence intensity and kinetic dependencies of CL bands have been obtained. The excitation capture efficiency of the radiative energy levels was estimated based on cathodoluminescence intensity rise rate dependence on electron beam current density. Energy transfer mechanism between the energy levels will be discussed hereinafter.

Optical, Dielectric and Magnetic Properties of La1−xNdxFeO3 Powders and Ceramics

Nanocrystalline La1−xNdxFeO3 powders with different concentrations of Nd3+ have been synthesized using a modified Pechini method. Their structures were studied by X-ray powder diffraction (XRD). Furthermore, La1−xNdxFeO3 nanoceramics were prepared using a high pressure sintering technique. The luminescence spectra of the powders were investigated as a function of concentration of active dopant to check the possible energy transfers observed due to Nd3+ concentration changes. The electrical and magnetic properties of the powders and ceramics were investigated to determine the effect of Nd3+ doping on the dielectric permittivity and magnetization in the wide frequency range.

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.

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.

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.

Comprehensive study of photoluminescence and cathodoluminescence of YAG:Eu3+ nano- and microceramics

The nanosized YAG particles doped with Eu3+ as the luminescent probe, were prepared by the modified Pechini method. The series of samples with various Eu3+ concentrations in the range of 0.1% up to 4% and the different average grain size from 31 to 75nm were obtained. YAG:Eu3+ nanoceramics were fabricated by the low temperature high pressure sintering technique (LTHP). Comprehensive studied with microsized grains ceramic as a reference were performed. The photoluminescence and cathodoluminescence properties of YAG:Eu3+ nanoceramic were performed at room temperature. In particular, the influence of dopant concentration and grain size of nanocrystals on the cathodoluminescence and photoluminescence properties was analyzed. The changes of the luminescent asymmetric ratio associated with the change in local symmetry for nanocrystals, nanoceramics and microceramics was investigated.

Enhanced thermoelectric performance in n-type Bi2Te2.95Se0.05 bulks fabricated by high pressure sintering technique

Bi2Te2.95Se0.05 bulk materials were prepared by high pressure sintering (HPS) technique under varying pressures. The HPS samples were then annealed for 36h in vacuum at 633K. The phase composition, morphology and thermoelectric properties were determined. The results show that high pressure has a significant effect on the structure and thermoelectric properties of the samples. The samples consist of nanoparticles before and after annealing. HPS together with annealing can improve the TE properties of the sample by decreasing the thermal conductivity of the sample with nanostructures. The Bi2Te2.95Se0.05 fabricated under 6.0GPa at 673K and subsequently annealed at 633K for 36h exhibits good TE properties. The ZT values first increased from 0.9 at 298K to the maximum value of 1.11 at 373K, then decreased to 0.91 at 473K. Such results can be attributed to the good electrical property and small thermal conductivity of the samples. The results can be applied to the field of thermoelectric power generation.

Thermoelectric transport properties and crystal growth of BiSbTe3 bulk materials produced by a unique high-pressure synthesis

We report a significant enhancement in the figure of merit ZT of bulk BiSbTe3 fabricated by high pressure sintering technique under variable pressures. High pressure intervention during synthesis processes can effectively adjust the thermoelectric transport properties and crystal structures of the reaction products. The novel vibration modes produced by high pressure in Raman spectra can induce a positive role in the reduction of thermal conductivity. Meanwhile, the electrical resistivity can also obtain an efficient decrease due to the effect of high pressure on the texture and the preferred orientation. Because of these positive effects, the maximum ZT has reached 1.4 at 432 K from synthesized BiSbTe3 at 2.5 GPa in a short period. Therefore, we present that the appropriate high-pressure has a positive effect on the thermoelectric transport properties to produce the significant enhancement in thermoelectric properties of BiSbTe3.

Structural and thermoelectric characterizations of high pressure sintered nanocrystalline Bi2Te3 bulks

Using Bi2Te3 nanopowders prepared from ball milling of elemental Bi and Te, nanocrystalline Bi2Te3 bulks were fabricated with high pressure sintering technique under variable pressures. The structural and thermoelectric properties were characterized, revealing a strong correlation with the sintering pressure. The nanocrystalline Bi2Te3 bulk fabricated under 2GPa exhibits good thermoelectric properties with ZT over 0.8 from 300 to 460K and a peak ZT of 1.03 occurring at 403K, which can be attributed to the small thermal conductivity from enhanced phonon scattering by grain boundaries and defects, as well as to the good electrical property comparable to that of the zone melting material.

Optical Properties of Cr(III) doped YAG Nanoceramics

The Cr3+ doped Y3Al5O12 ceramics composed of nanosized grains were fabricated by the low temperature high pressure sintering technique. The photo - and cathodoluminescence spectra of Cr3+:Y3Al5O12 nanoceramics were measured. The effect of applied pressure on luminescence properties of Cr3+ doped Y3Al5O12 nanoceramics was investigated.

Al-doped ZnO ceramics fabricated by mechanical alloying and high-pressure sintering technique

Al-doped ZnO ceramics fabricated by mechanical alloying and high-pressure sintering technique

Unit cell dimensions of some synthetic orthopyroxene group solid solutions

Three series of solid solutions of orthopyroxene structure, (Fe, Mg )SiO3, (Co, Mg)SiO3 and (Ni, Mg)SiO3, were synthesized, using high pressure sintering technique. Unit cell parameters of the synthesized pyroxenes were determined at room temperature. The deviation from Vegard's law is remarkable for both iron and nickel pyroxene series, whereas almost absent for cobalt one. It is suggested that the dominant factors which determine the distribution of ions among nonequivalent sublattice sites are 1) the difference in effective sizes of both ions (for the case of (Fe, Mg )SiO3) and 2) the crystal field stabilization energy (for the case of (Ni, Mg )SiO3). The nearly ideal behavior of (Co, Mg )SiO3 is a result of the compensation between these two competing factors.