High-pressure Sintering Research Articles

Mechanical property enhancement of cubic boron nitride composites through additive diamond

In this study, we report the synthesis and characterization of cubic boron nitride (cBN)-diamond composites under high-temperature and high-pressure (HTHP) sintering. The effects of diamond content and particle size on phase composition, microstructure, and mechanical properties were investigated. A homogenous microstructure was obtained. Diamond as the second phase positively affected the mechanical properties of cBN composites. With the addition of 1 μm diamond, the highest bending strength and abrasive ratio of cBN composites sintered with 8 wt% diamond were 791 MPa and 2.56, respectively. The hardness of composites sintered with 12 wt% diamond reached up to 60.12 GPa.

Substantial enhancement of mechanical properties for SnSe based composites with potassium titanate whiskers

Highly dense p-type polycrystalline SnSe dispersed with x vol% K2Ti6O13 whiskers (x = 0, 1, 3, 5) were fabricated by a high-pressure sintering (HPS) method. The effects of the K2Ti6O13 whiskers additives on the thermoelectric as well as mechanical properties of the composites were characterized in detail. The electrical conductivity, Seebeck coefficient, and thermal conductivity were measured within the temperature range of 300–830 K. It was found that the K2Ti6O13 whiskers were distributed homogeneously throughout the matrix and contacted with the SnSe grains intimately. Mechanical properties, such as Vickers hardness, flexural strength, compressive strength and fracture toughness of the composites were improved significantly. Especially, upon the addition of 5 vol% K2Ti6O13 whiskers, the flexural strength and compressive strength were almost doubled as compared to K2Ti6O13-free samples. Such substantial enhancement of mechanical properties is an important benefit for the commercial application of thermoelectric devices.

Structurally nanocrystalline electrically monocrystalline Sb2Te3 with high thermoelectric performance

A highly textured pure Sb2Te3 is fabricated via a hydrothermal method combined with high-pressure spark plasma sintering. The nanoscale texture gives rise to an extremely low thermal conductivity in the out-of-plane direction, while shows weak influence on the carrier mobility which reaches the level of single crystal. Microstructure analysis reveals large amount of low-angle boundaries, whose discriminated effect on scattering phonons and carriers should account for the observed phenomenon. Consequently, the structurally nanocrystalline electrical monocrystalline behavior leads to a high average ZT with peak value above 1 at 525 K, which is the best TE performance for undoped Sb2Te3.

Optical and mechanical properties of transparent alumina fabricated by high-pressure spark plasma sintering

Transparent alumina was fabricated from untreated commercial powder by high-pressure spark plasma sintering (HPSPS) at temperatures of 1000, 1050 and 1100 °C under pressures of 250-800 MPa. It was established that transparency strongly depends on the HPSPS parameters. At all temperatures, there was a certain point when increasing the pressure led to decreasing transparency. At 1100 °C, relatively high pressure led to excessive grain growth, as well as the formation of creep-induced porosity at the center of the samples. Hardness values decreased with pressure due to grain growth, correlated with the Hall-Petch relationship. The optimal combination of optical and mechanical properties (68% in-line transmittance at a wavelength of 640 nm and a hardness value of about 2300 HV2) was achieved after sintering at 1050 °C under 600 MPa.

Impact of gas composition on thermal conductivity of glass foams prepared via high-pressure sintering

Abstract Glass foam is a cellular material with excellent thermal insulating ability. The thermal conductivity (λ) of cellular materials is complex to understand because it is determined by the composition of the solid and gas phases, pore size, solid microstructure (amorphous or crystalline, struts, and walls), and radiation. In this paper, we focus on the influence of gas composition on λ of glass foams. Glass foams were prepared by a physical foaming approach, where powder pellets of cathode ray tube (CRT) panel glass were sintered at 640 °C in Ar or N2 atmosphere at elevated pressure (5–25 MPa), and then cooled down to room temperature. When heating the sintered samples to the viscous state (approx. 108–106 Pa s corresponding to 650–740 °C), the high internal gas pressure in the closed pores expands the glass melt. We heat-treated each sintered sample multiple times in air at atmospheric pressure to gradually decrease the density, and thereby to obtain multiple λ values of each sample at different densities, while the chemistry of the sample remained almost identical. Gas chromatography revealed binary Ar-CO2 and N2-CO2 gas compositions in the Ar- and N2-sintered samples, respectively. The thermal conductivities of these gas mixtures (λgas,mix) were calculated to be 16.4 and 23.1 mW m−1 K−1, presenting 22 and 31% of the effective λ for the Ar- and N2-sintered samples, respectively. The lower λgas,mix resulted in a lower λ of the Ar-sintered samples compared to the N2-sintered ones at low density (

Cold Sintering of Fe–Ag and Fe–Cu Nanocomposites by Consolidation in the High-Pressure Gradient

The results of fabricating dense Fe–Ag and Fe–Cu nanocomposites from mixtures of powders consolidated by high-pressure cold sintering and from nanosized powders of silver (Ag), iron (Fe), and copper (Cu) are reported. The results of mechanical tests of Fe–Ag and Fe–Cu nanocomposites are presented. Nanocomposite powders were prepared by milling the micron powder of carbonyl iron (Fe) and nanosized silver oxide (Ag2O) powder, as well as iron and cuprous oxide (Cu2O) nanopowders in a high-energy attritor. The microstructure was investigated using a high-resolution scanning electron microscope. Compacts with a density of about 70% of the theoretical density were annealed in hydrogen to reduce silver oxide and cuprous oxide to metals and remove oxide films from the surface of iron powder particles. Then cold sintering followed—high-pressure consolidation at room temperature. The data on the pressure dependence of density of samples are found in a range of 0.25–3.0 GPa. Densities higher than 95% of the theoretical density are attained for all nanocomposites at a pressure of 3.0 GPa, while a density of about 100% is attained for Ag and Cu powders. High mechanical properties are found for all compositions in experiments for the three-point bending and for compression. It is established that mechanical properties of nanocomposites are noticeably higher than for composites formed from micron-sized powders. The higher plasticity was observed for Fe‒Ag and Fe–Cu nanocomposites when compared with the samples formed from nanostructured Fe.

Influence of processing parameters on the densification and the microstructure of pure zinc oxide ceramics prepared by spark plasma sintering

Due to the sensitivity of nanopowders and the challenges in controlling the grain size and the density during the sintering of ceramics, a systematic study was proposed to evaluate the densification and the microstructure of ZnO ceramics using spark plasma sintering technique. Commercially available ZnO powder was dried and sintered at various parameters (temperature (400–900 °C), pressure (250–850 MPa), atmosphere (Air/Vacuum) etc.). High pressure sintering is desirable for maintaining the nanostructure, though it brings a difficulty in obtaining a fully dense ceramic. Whereas, increasing the temperature from 600 to 900 °C results in fully densified ceramics of about 99% which shows to have big impact on the grain size. However, a high relative density of 92% is obtained at a temperature as low as 400 °C under a pressure of 850 MPa. The application of pressure during the holding time seems to lower the grain size as compared to ceramics pressed during initial stage (room temperature).

Preparation and properties of boron-doped diamond composites fabricated by high-pressure and high-temperature sintering

Dense Boron-doped diamond (BDD) composites were fabricated by high pressure and high-temperature sintering (HPHT) at 5 GPa and 1450 °C for 300 s in presence of Al, B and C as sintering additives. Phase compositions, microstructures, and electrochemical performances of sintered specimens were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and electrochemistry. Results suggested that Al3BC3 and Al4C were formed by in-situ reaction between sintering additives and diamond, which can promote densification of BDD. In addition, BDD composites containing 10 wt% sintering additives exhibited the highest electrical resistivity (4.40 × 10−4 Ω m) and hole concentration (4.55 × 1025/m3). The working potential windows of BDD composite electrodes in 0.1 M H2SO4, 0.1 M Na2SO4 and 0.1 M NaOH electrolytes were estimated to 1.9 V, 2.9 V and 2.3 V, respectively. Electron transfer coefficients of composite electrodes approached 0.5, confirming good reversibility. Finally, methylene blue was completely degraded by BDD composite electrodes within 120 min using NaCl as supporting electrolyte.

Fabrication of yttrium-doped barium zirconate ceramic by high pressure sintering

A high pressure sintering method was devised to fabricate high density and strength BaZr0.9Y0.1O3-δ (BZY) ceramic at 0.6 GPa and 1100 °C for the first time. The mechanical properties and electrochemical response of the ceramic in high pressure hydrothermal fluids were tested in a self-designed autoclave. Results shows that the ceramic can be processed into fixed shape, it remains stable and a rapid electrochemical response was observed after exposing to water vapor at 340 °C and 14 MPa for 20 h, which makes the product an excellent candidate electrolyte material for in-situ measurement of H2 in high pressure hydrothermal fluids.

Enhanced thermoelectric power factor of half-Heusler solid solution Sc1-xTmxNiSb prepared by high-pressure high-temperature sintering method

Abstract In an effort to find new thermoelectric materials, a series of half-Heusler alloys Sc1-xTmxNiSb (x = 0.0, 0.25, 0.5, 0.75, 1.0) was synthesized by arc melting and subsequent high-pressure high-temperature sintering. The products were examined by means of X-ray diffraction, scanning electron microscopy and energy-dispersive X-ray spectrometry. The crystal structure of each sample was established to be of the MgAgAs-type, and the cubic lattice parameter was found to behave in accordance to the Vegard's law. The electrical transport properties of the Sc1-xTmxNiSb alloys were determined via resistivity and thermoelectric power measurements performed at temperatures 2–950 K. For each sample a semiconducting-like behavior was found with rather small values of the room-temperature resistivity (ρ = 10–50 μΩm), and fairly large positive Seebeck coefficient (Smax = 80–136 μV/K). For the alloy Sc0.75Tm0.25NiSb, combination of relatively small ρ (similar to that of TmNiSb) with rather high S (comparable to that of ScNiSb) yielded the thermoelectric power factor PF = 1.2 × 10 mW/(m K2) at 650 K, which is about twice larger than that found for ScNiSb. Increasing the amount of Tm in Sc1-xTmxNiSb brings about significant reduction in the thermal conductivity at 350 K from about 6 W/(m K) to ca. 3 W/(m K), and consequently leads to sizable enhancement of the thermoelectric figure of merit.

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.

The grain growth in alumina compacts sintered under high pressure

Abstract In this study, alumina ceramics were produced with high density and nanograin size without additives from nano-size γ-Al2O3. The effects of sintering pressure (3 GPa and 7 GPa) and temperature (600 °C and 1 200 °C) on the grain size of sintered samples were studied. It was shown that high pressure increases the nucleation rate while reducing the growth rate of the transformed α phase. Pressure-assisted densification decreases grain size, so that nano-scale grains can be achieved in the sintered structure. The effect of various sintering conditions on grain growth in alumina was investigated during high-pressure sintering for better understanding of pressure and temperature effects on microstructural properties.

Spark plasma sintered Al-0.5 wt% MWCNT nanocomposite: Effect of sintering pressure on the densification behavior and multi-scale mechanical properties

The current study aims to understand the effect of sintering pressure on densification behaviour and mechanical properties of multi walled carbon nanotube (MWCNT) reinforced aluminum (Al) based nanocomposite synthesized via spark plasma sintering (SPS). Physio-chemical functionalization, a novel dispersion technique, was followed which led to homogenous dispersion of MWCNTs in Al matrix. The crystallite size and density of the synthesized Al-0.5 wt% MWCNT nanocomposites increased with increase in sintering pressure. The relative density increased from 90.4% to 95.1% upon increasing the sintering pressure from 30 MPa to 80 MPa. To understand the densification behaviour, the movement of graphite punch within the die filled with Al-MWCNT powder mixture during sintering was monitored. Higher sintering pressure led to early onset of sintering and the total punch displacement during sintering increased from 0.38 mm to 1.48 mm upon increasing the applied pressure from 30 MPa to 50 MPa. The impact of high sintering pressure, leading to enhanced densification, was also translated in the improvement in mechanical properties. Compressive strength and microhardness increased by ~30% and ~13%, respectively as sintering pressure was increased from 30 MPa to 80 MPa. Higher sintering pressure leading to significantly enhanced mechanical properties at micro and macro scale was also replicated at nano-scale as confirmed by the results of nanoindentation and nanoscratch tests.

Fabrication and characterization of CaKFe4As4 round wires sintered at high pressure

We fabricated CaKFe4As4 round wires through a powder-in-tube method followed by hot isostatic pressing up to 175 MPa. The transport critical current density, Jc, at 4.2 K almost reached the level for practical applications (100 kA cm−2) under self-field and reached 7.6 kA cm−2 at 100 kOe. Magneto-optical images confirmed that the grains were well-connected at low temperatures. Although high-pressure sintering at high temperatures enhanced the Jc and density of the core of the wires, X-ray diffraction and scanning electron microscopy–energy-dispersive X-ray spectroscopy analyses indicated that impurity phases were present in both the polycrystalline powder and the core of the wire.

Stress-enhanced dynamic grain growth during high-pressure spark plasma sintering of alumina

Applying high pressure during the sintering of ceramic materials is a common practice that allows for a reduction of the sintering temperature and the obtaining of fine-grained microstructures. In this work, we show that the final grain size of submicron alumina increased consistently with applied pressure during low temperature (1000–1050 °C), high pressure (500–800 MPa) spark plasma sintering. Grain size trajectories and microstructural observations indicated that stress-enhanced grain growth occurred during the final stage of the sintering process, whereas thermally controlled grain boundary migration was negligible. We suggest that this dynamic, stress-enhanced grain growth is controlled by grain-boundary sliding, grain rotation and coalescence. A strong correlation was found between calculated creep strain rates and grain growth rates, such as during superplastic deformation.

Effect of Sintering Conditions on the Mechanical Strength of Cu-Sintered Joints for High-Power Applications.

In this study, the feasibility of low-cost Cu-sintering technology for power electronics packaging and the effect of sintering conditions on the bonding strength of the Cu-sintered joint have been evaluated. A Cu paste with nano-sized Cu powders and a metal content of ~78% as a high-temperature bonding material was fabricated. The sinter-bonding reactions and mechanical strengths of Cu-sintered joints were evaluated at different sinter bonding pressures, temperatures, and durations during the sintering process. The shear strength of the Cu-sintered joints increased with increasing sintering pressure. Good interfacial uniformity and stable metallurgical microstructures were observed in the Cu joints sintered at a high sintering pressure of 10 MPa, irrespective of the sintering time. It was confirmed that a high-pressure-assisted sintering process could create relatively dense sintered layers and good interfacial uniformity in the Cu-sintered joints, regardless of the sintering temperatures being in the range of 225–300 °C. The influence of the sinter bonding pressure on the shear strengths of the Cu-sintered joints was more significant compared to that of the sintering temperature. Durations of 10 min (at 300 °C) and 60 min (at 225 and 250 °C) are sufficient for complete sintering reactions between the Si chip and the direct bond copper (DBC) substrate. Relatively good metallic bonding and dense sintered microstructures created by a high sintering pressure of 10 MPa resulted in high shear strength in excess of 40 MPa of the Cu-sintered joints.

Microstructure of AlCrFeSi Alloys Prepared by High-Pressure Spark Plasma Sintering

The rapidly solidified aluminium alloy with slowly diffusing transition metals (iron, chromium) prepared by powder metallurgy techniques is characterized by higher thermal stability in comparison with other common aluminium alloys thanks to hardening of dispersed intermetallic phases. The solubility of transition metals in aluminium is increased by rapid solidification, however, their content, in the alloy, is simultaneously significantly reduced due to the formation of hard and brittle intermetallic phases, which lessen plasticity, ductility, and strength of the material. This work is devoted to the description of a microstructure of AlCr6Fe2Si1 alloy prepared by gas atomization followed by consolidation by High-Pressure Spark Plasma Sintering, and comparison of the influence of various sintering conditions on the microstructure of alloy. The low-porosity compacted alloys are formed by quasi-crystalline phase Al95Fe4Cr, the crystalline phase Al13Cr2, and Al80Cr13.5Fe6.5 in the aluminium matrix. They are formed by powder particles with the different internal morphology of intermetallic phases (spherical clusters or snowflakes).

Phase transformation of alumina induced by high pressure spark plasma sintering (HP-SPS)

Spark plasma sintering of nanocrystalline γ-Al2O3 at very high pressure was investigated up to 5 GPa. A SPS pressure/temperature diagram of α-Al2O3 and γ-Al2O3 phase was plotted. An in-house high pressure spark plasma sintering equipment (HP-SPS) was used. The capabilities and parameters control of this equipment greatly reduced the phase transition and sintering temperatures to obtain ceramics in contrast to conventional high pressure and high temperature sintering (HP-HT). The transition temperature could be lowered to ~500 °C from 1 GPa. The grain growth in ceramic was retained by the HP-SPS sintering. The density was also demonstrated to be optimized with HP-SPS.

Liquid phase assisted high pressure sintering of dense TiC nanoceramics

To address the difficulty of achieving high density while effectively suppressing grain growth in the fabricating of nanoceramics, this paper demonstrates a novel integrated approach, consisting of a solid-liquid reaction, high pressure and low temperature sintering, to prepare dense nanocrystalline TiC ceramics. Using Si as the additive, a low-viscosity Si liquid phase is formed under the sintering condition of 1200 °C/4.5 GPa. It is shown that both of the sintering aid and high pressure are crucial in achieving high density nanostructured TiC ceramics and controlling their microstructure and thus their mechanical properties. The pure TiC can be sintered to reach 95.3% of its theory density and, with the assistance of liquid Si additive, it can be sintered to full densification without grain growth by high pressure technique.

Microstructures and mechanical properties of in situ TiC\u2013\u03b2\u2013Ti\u2013Nb composites with ultrafine grains fabricated by high-pressure sintering

In this study, an in situ β–Ti–Nb composites reinforced with TiC particles with an ultrafine grain size were fabricated using a powder metallurgical (PM) method. The microstructures and mechanical properties of the composites were characterized using X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and compression tests. TiC particles were formed in the ball-milled powders after annealing at 600 °C due to a chemical reaction between stearic acid and titanium. Using high-pressure sintering (HPS) on an apparatus with six tungsten carbide anvils, a fully dense β–Ti–Nb composite reinforced with fine in situ TiC particles was obtained. The TiC particles exhibit particle sizes of ~500 nm, uniformly distributed in the composite matrix, which had grain sizes of ~600 nm. Thus, the TiC–β–Ti–Nb composite show very high compression yield strength and relatively high plasticity contributed by grain refinement and TiC particles strengthening. The composite with 45 vol.% TiC exhibited excellent mechanical properties, with a yield compressive strength of 1990 MPa and plastic strain of 9.12%. More over, a modified rule-of-mixture (ROM) was presented to describe the combined strengthening effect of grain refinement and TiC particles.