High-pressure Sintering Research Articles

High critical current density in low-cost iron-based superconducting round wires annealed at ambient pressure

Superconducting round wires with isotropic architecture are preferred in fabrications of cables and magnets. To diminish the obstacles to supercurrent, e.g., voids, cracks and bubbles, over-pressure heat treatment or hot isostatic pressing is indispensable in the final annealing process. Here, we fabricated a stainless steel/Cu/Ag sheathed Ba0.6K0.4Fe2As2 superconducting round wire without the aid of high-pressure sintering. It was found that high-strength outer sheaths and groove rolling synergistically enhance the core density to ∼100%. Combined with the fiber and concentric texture of the superconducting core, the J c reaches 4.45 × 104 A cm−2 at 4.2 K and 10 T. Furthermore, the specially designed conductor architecture not only reduces material costs by lowering the proportion of Ag to ∼3.1%, but also provides high mechanical and thermal stability. This straightforward and cost-effective process can be scaled up for the massive production of long wires for high-field applications.

Fabrication of nanocrystalline SiO2–ZrO2 glass-ceramic via a high-pressure cold sintering process

The nanocrystalline 0.35SiO2-0.65ZrO2 (65 mol% monoclinic ZrO2) glass-ceramic was successfully fabricated via a high-pressure cold sintering process (CSP), displaying remarkable nano hardness and elastic modulus of 12.5 and 154.8 GPa, respectively. We conducted a thorough investigation of the influence of NaOH solution concentration, liquid-to-solid mass ratio, sintering temperature, and pressure on the phase compositions, microstructural evolution, and densification. Notably, excessive NaOH concentration (>0.7 M) results in the transformation of SiO2 from an amorphous phase to α-quartz, hindering densification. However, below 0.7 M, increasing NaOH concentration enhances the densification processes. Furthermore, higher temperature and pressure contribute to improved densification. It is worth mentioning that there is no grain growth for the ZrO2 phases during the CSP, indicating that the densification of the nanocrystalline 0.35SiO2-0.65ZrO2 glass-ceramic is primarily governed by the glass phase.

Crystal structure of Ti4Ni2C.

Single crystals of the inter-metallic phase with composition Ti4Ni2C were serendipitously obtained by high-pressure sinter-ing of a mixture with initial chemical composition Ti2Ni. The Ti4Ni2C phase crystallizes in the Fd m space group and can be considered as a partially filled Ti2Ni structure with the C atom occupying an octa-hedral void. Ti4Ni2C is isotypic with Ti4Ni2O, Nb4Ni2C and Ta4Ni2C, all of which were studied previously by means of powder diffraction.

Cold Sintering Mediated Engineering of Polycrystalline SnSe with High Thermoelectric Efficiency.

Despite the attractive thermoelectric properties in single crystals, the fabrication of high-performance polycrystalline SnSe by a cost-effective strategy remains challenging. In this study, we prepare the undoped SnSe ceramic with remarkable thermoelectric efficiency by the combination of a cold sintering process (CSP) and thermal annealing. The high sintering pressure during CSP induces not only highly oriented grains but also a high concentration of lattice dislocations and stacking faults, which leads to large lattice strain that can shorten the phonon relaxation time. Meanwhile, the thermal annealing breaks the highly resistive SnOx layers at grain boundaries, which improves the electrical conductivity and power factor. In addition, the grain growth during annealing further turns the broken SnOx layers into nanoparticles, which further lowers the thermal conductivity by enhanced scattering. As a result, a peak ZT of 1.3 at 890 K and a high average ZT of 0.69 are achieved in the polycrystalline SnSe, suggesting great potential in mid-temperature power generation. This work may pave the way for the mass production of SnSe-based ceramics for thermoelectric devices.

Exploiting mechanochemical activation and molten-salt-assisted reduction-diffusion approach in bottom-up synthesis of Sm2Fe17N3

Sm2Fe17N3 powders were prepared by a novel molten-salt mechanochemically assisted reduction-diffusion (RD) approach. CaCl2, plays a critical role during mechanochemical processing as a dispersant, facilitating the RD process by dissolving the precursors in a molten flux and further assisting during the washing step by efficient removal of by-products thus minimizing the surface oxidation of Sm2Fe17N3 powder particles. Various milling times and RD temperatures were examined, and synthesis conditions were optimized to achieve pure-phase Sm2Fe17N3 powders with low aggregation of magnetic particles. Powders synthesized at 950 °C RD exhibited the highest hard-magnetic properties: coercivity Hc of 13.5 kOe, and maximum energy product (BH)max of 19.4 MGOe. This was attributed to the formation of fine single-phase Sm2Fe17N3 particles as well as minimal oxidation of particle surface. Furthermore, by densifying the Sm2Fe17N3 powders with high-pressure spark plasma sintering, a bulk magnet with (BH)max of 16.5 MGOe and a relative density of 86 % was produced indicating that the obtained Sm2Fe17N3 powders had low oxygen content.

A magnetite reference material for in situ Fe isotope analysis

A high-temperature and high-pressure sintering method was used to prepare magnetite (MtFe-1), which can be used as a matrix-matched bracketing standard for Fe isotope analysis of natural magnetite using LA-MC-ICP-MS.

Regulating the resistivity of rare-earth nickelates electronic phase transition perovskites via NiO composition

Although rare-earth nickelates (ReNiO3) exhibit widely adjustable metal-to-insulator transition (MIT) properties, it is yet difficult to control their material resistivities to cater for potential applications in correlated electronics. Herein, we demonstrate a strategy to regulate the material resistivities of ReNiO3 while maintaining their relative abrupt MIT properties by compositing with the NiO nanoparticles and afterward high oxygen pressure sintering. Introducing the more insulating NiO as a secondary phase within ReNiO3 (e.g., NdNiO3 and SmNiO3) elevates their material resistivities across two orders of magnitudes while maintaining a similar critical temperature (TMIT) and its resultant resistive change at a moderate compositing ratio (e.g., <80 %). The nearest-neighbors-hopping (NNH) model and linear model were used to fit the low-temperature electric transportation properties for both the insulating and metallic phases of ReNiO3, and it indicates that the pristine carrier transportation modes can be well preserved despite a high compositing ratio of NiO up to 80 %. Similar effects cannot be achieved by using other more insulating compositing oxides, such as BaBiO3, LaAlO3, and NiFe2O4, as the abruption in resistive change across TMIT was not preserved at a similar high compositing ratio (e.g., >20 %), while their carrier transportation modes varied at much smaller composition ratio. It is worth noticing that the compositing with NiO also improves the mechanical strength of ReNiO3, and this benefits their practical applications in electronic devices.

Influences of Sintering Temperature and Pressure on Microstructure, Optical and Mechanical Properties of Hot-Pressing-Sintered Gd2O3–MgO Nanocomposites

Low temperature combined with high application pressure sintering technology is one of effective approaches to fabricate ceramics with fine microstructure. We reported that the Gd2O3–MgO nanocomposite ceramics were obtained by hot-pressing sintering using nanopowders derived from sol–gel combustion method in this paper. The high optical transmittance (80.1%–83.5%) over 2-6 μm and the high values for Vickers hardness (11.1 ± 0.3 GPa) and bending strength (192.6 ± 8.1 MPa) were obtained by hot-pressing sintering at low sintering temperature (1300 μC) and high application pressure (65 MPa).

Mechanical properties of a novel tungsten fiber-reinforced tungsten composite prepared through powder extrusion printing and high-pressure high-temperature sintering

Tungsten is one of the most promising candidate materials for plasma-facing materials (PFM) because of its high melting point, superior thermal conductivity, and low sputtering rate. However, its significant brittleness and high ductile-to-brittle transition temperature constrain its broader engineering applications. To address these limitations, we developed a novel tungsten fiber-reinforced tungsten (Wf//W) composite. This composite utilizes commercially available black tungsten wire mesh as the fiber reinforcement phase with a mass ratio of 1 %. The material was synthesized through a combined Powder Extrusion Printing and High-Pressure, High-Temperature sintering process, achieving a relative density exceeding 99 %. Additionally, the composite exhibits a maximum hardness of 589 ± 10 HV, attributable to the presence of high-density dislocations in the matrix. Its compressive strength reaches up to 1530 MPa, and the plastic deformation strain is as high as 15.8 %, both of which are outstanding compared to existing research. These findings offer an alternative route for producing high-density Wf//W materials, in addition to chemical vapor deposition method and powder metallurgy technique.

Mechanisms and mechanical properties of high-temperature high-pressure sintered vanadium carbide ceramics

Vanadium carbide (VC) is extensively employed as a reinforcing agent in coating materials and a grain refiner in hard alloys, owing to its exceptional mechanical properties. A series of bulk VC ceramics were fabricated via high-temperature high-pressure sintering, followed by a comprehensive and systematic investigation of various properties of the sintered samples. The results revealed significant influences of processing temperature on the properties of sintered VC at 5.0 GPa. However, the exact nature of temperature dependence varied depending on the specific property. It was found that VC ceramics sintered at 1100 °C and 5.0 GPa exhibited the best overall performance, providing a relatively optimal temperature condition for synthesis. The Vickers hardness of the material reached an astonishing 43.2 GPa under a 9.8 N load, placing it within the category of superhard materials. However, the sintered sample exhibited a gradually lower hardness value of 30.4 GPa under a higher load of 29.4 N. Highly dense with a relative density of 99.8%, exhibiting near full density, the vanadium carbide ceramics possess an impressive Young's modulus of 544 GPa, indentation fracture resistance of 5.4 MPa m1/2, and an oxidation initiation temperature of 758 °C. This study offers practical guidance for the design of novel transition metal carbide superhard materials and provides valuable insights for the exploration of visualizing directions in the next generation of superhard materials.

Improving corrosion resistance of spark plasma sintering-produced AlCoCrFeNi2.1 high entropy alloy via in-situ process pressure and temperature control

The ability to control phases in eutectic high-entropy alloys (EHEAs) remains a formidable challenge. This study illustrates a simple approach that applies various sintering temperature and pressure to achieve precise control of phase transformation in AlCoCrFeNi2.1 EHEA, particularly to further enhance its corrosion resistance. The result display that at high sintering pressure, pore density in sintered material could be reduced significantly, and at high sintering temperature, BCC phase is seen a notable growth, which both determine the corrosion performance of studied alloy. Moreover, it is found that large BCC phase size could reduce micro-galvanic effect between FCC and BCC and then improve corrosion resistance. This work may provide a technological implication in the EHEA design for anti-corrosion.

Fabrication of dense YSZ ceramic via vat photopolymerization with low ceramic particles loading

In this work, dense Yttria stabilized zirconia (YSZ) ceramic was printed by vat photopolymerization. We first formulated a slurry with good ceramic particle dispersion and modified the sintering process of the printed part. We improved the dispersion of the yttria stabilized zirconia (YSZ) particles by mixing with dispersant before preparing the slurry. After printing with the slurry, the printed part was sintered under a pressure of 20 KPa. No pores could be observed in the cross-section view indicating we successfully fabricated ultra-dense YSZ ceramic by vat polymerization and high-pressure sintering process.

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.

Experimental Study on ELID Grinding of Silicon Nitride Ceramics for G5 Class Bearing Balls

This study has focused on analyzing the impact of material characteristics and grinding conditions on the surface roughness in ELID grinding of ceramic materials intended for bearing balls. The main research objective was to examine the feasibility of achieving the required surface roughness for G5 class bearing balls through a high-efficiency and high-precision ELID grinding process. Three types of silicon nitride specimens and two types of grinding wheels with cBN and diamond abrasives were prepared for the experiments. An HP (high-pressure) specimen was fabricated through high-temperature and high-pressure sintering at 1700 °C for 2 h, containing a composition of Y2O3 and MgO in Si3N4, while GPS 1hr and GPS 6hr specimens were prepared using gas-pressure sintering for 1 h and 6 h, respectively. From the experimental results, it has been confirmed through surface morphology and surface roughness analysis that material characteristics and grinding parameters affect the surface roughness of silicon nitride ceramics during the grinding process. The surface ground with a #2000 diamond wheel is at a level that can satisfy the required surface roughness, 0.014 um or less in G5 class bearing balls. Based on the analysis of surface morphology and roughness in grinding processes, the #325 cBN wheel exhibited excellent performance in rough grinding, while the #2000 diamond wheel demonstrated highly effective surface finishing performance, indicating that the combination of these two abrasives can be effectively utilized for high-efficiency and high-precision nanosurface machining of silicon nitride ceramics.

Powder metallurgy process enables production of high-strength conductive Cu-based composites reinforced by Cu50Zr43Al7 metallic glass

This study showcases the preparation of Cu50Zr43Al7 metallic glass-reinforced ultrafine grain copper matrix composites using a powder metallurgical process to achieve a superior combination of strength and electrical conductivity. The mechanical ball milling (BM) process is utilized to effectively refine the Cu matrix grains to the nanoscale. Furthermore, high-pressure spark plasma sintering (SPS) is employed to prevent grain growth and improve the interface bonding between two phases. Increasing the Cu50Zr43Al7 addition content improved the mechanical properties under a load transfer strengthening strategy. Notably, the 20 wt% Cu50Zr43Al7/Cu composite demonstrates excellent mechanical properties and high electrical conductivity. This is attributed to the uniformly-dispersed Cu50Zr43Al7 metallic glass particles in the ultrafine grain Cu matrix with well-established interfacial bonding. The synergistic strengthening of the Cu50Zr43Al7/Cu composites represents a novel choice for switch contact materials application, maintaining excellent mechanical properties while satisfying conductive requirements.

High-pressure sintering of full-dense B6O ceramic with excellent hardness and fracture toughness

B6O is an extremely prospective hard material for industrial applications. Herein, the densification behavior, microstructure and mechanical properties of high-pressure sintered B6O ceramics at 6 GPa are investigated. The sintered B6O ceramics exhibit enhanced densification as the sintering temperature increases, and the ceramic sintered at 1700 °C possesses nearly full-dense characteristics with strong bonding at grain boundaries and exhibits outstanding mechanical properties (HV: 38.5 ± 0.7 GPa, HN: 45.7 ± 0.8 GPa, KIc: avg. 4.8 MPa·m1/2). At higher temperatures, however, the increase in sublimation pressure is counterproductive to densification. The enhanced fracture toughness of the full-dense B6O ceramics is mainly attributed to the high strength grain boundaries and the abundance of defects within the grains, which generate substantial crack bridging, crack deflection and grain pull-out.

High pressure sintering of fully dense tantalum carbide ceramics with limited grain growth

High pressure sintering of fully dense tantalum carbide ceramics with limited grain growth

Dense nanocrystalline ReB2 bulk with ultrahigh hardness

Microcrystalline ReB2 with a high hardness of 48 GPa measured under a small load of 0.49 N has been reported as a superhard material. Nanocrystalline ReB2 is expected to achieve better mechanical properties than its microcrystalline counterparts. Here we report dense nanocrystalline ReB2 bulks synthesized by modified mechanical alloying and high-pressure sintering techniques. The modified mechanical alloying technique reduces the content of WC impurity from 7.75 wt% to 1.59 wt% in as-synthesized nanocrystalline ReB2 powders with an average grain size of 6.6 ± 2.0 nm. The extrinsic high-pressure (6 GPa) and the intrinsic nanocrystalline structure greatly promote the densification process of the ReB2 bulks during sintering. Our optimized nanocrystalline ReB2 bulk is ∼99% dense and with an average crystallite size of 40 nm, resulting in a high hardness of ∼33 GPa under a load of 9.8 N and indentation fracture toughness of ∼5 MPa m1/2. The high hardness can be ascribed to the synergistic effect of high density and nanocrystalline structure of ReB2 bulks. This work provides promising strategies to largely enhance the mechanical properties of different ceramics for practical applications.

Review on high-pressure spark plasma sintering and simulation of the impact of die/punch material combinations on the sample temperature homogeneity

This paper provides a thorough review on high-pressure spark plasma sintering (HP-SPS) experiments conducted to date, with a focus on different tooling materials and their impact on microstructural, mechanical and optical properties of the resulting materials. Any SPS experiment conducted with a pressure over 200 MPa was considered as “high-pressure”. This review was supported by a comprehensive finite-element modelling study of the temperature distribution in the sample and tool set-up as a function of the used geometries, tooling material combinations, temperature window and sample material properties. The effect of the electrical and thermal conductivities of the tool and sample material on the temperature gradients in some specific sample-tool material combinations was elucidated. Electrically and thermally conductive samples in combination with SiC tools showed the lowest temperature gradients during processing, whereas electrical insulators in combination with high pressure WC, steel or TZM tools with an inner graphite die showed the smallest temperature gradients over the sample.

Highly transparent cubic γ-Al2O3 ceramic prepared by high-pressure sintering of home-made nanopowders

Highly transparent gamma-aluminum oxide (γ-Al2O3) ceramics were fabricated for the first time, by combining homogeneous precipitation and high-pressure sintering in the absence of exogenous dopants. The resulting cubic γ-Al2O3 transparent ceramic material exhibits a promising replacement for single-crystal sapphire. The optimum optical properties are achieved in response to sintering at 5 GPa and a temperature of 300 °C and include maximum transmittance of 86% in the range of 0.6–1.2 µm which are properties that are comparable to those of single-crystal sapphire (∼86%). Vickers hardness (16 GPa) and compressive strength (350 MPa) in response to high-pressure sintering are also similar to those of a conventional sapphire single crystal. Meanwhile, the dielectric constant (9.46) is comparable to that of sapphire in the C-axis direction. These findings will facilitate further development of transparent Al2O3 ceramics for use in a wider range of optical applications.