Vacuum Hot Pressing Research Articles

Highly conductive polystyrene/carbon nanotube/PEDOT:PSS nanocomposite with segregated structure for electromagnetic interference shielding

Highly conductive polymer nanocomposites (CPNs) are promising alternatives to metals for electromagnetic interference (EMI) shielding applications. However, constructing a well-established conductive network within a polymer matrix using conventional processes is still challenging. This research aimed to improve the EMI shielding performance of CPNs by developing highly conductive segregated structures through a facile innovative dispersion mixing process. The nanocomposites were fabricated by dispersing polystyrene beads (PS), CNT, and PEDOT:PSS in deionized water, followed by vacuum filtration, solvent treatment, and hot press molding. The employed technique effectively constructed a highly conductive network in the PS/CNT nanocomposite, resulting in the lowest ever reported percolation threshold of 0.009 vol% among CNT-based segregated structures. Moreover, adding PEDOT:PSS to the nanocomposite as an additional constituent significantly promoted the conductive network by improving the dispersion of CNTs and the interparticle contact. The PS/CNT/PEDOT:PSS (100:2:4 w/w/w) exhibited a high electrical conductivity of 2.352 S/cm with notable specific EMI shielding effectiveness (SE) of 55.7 dB/mm (with dominant absorption mechanism), which is among the best performance reported for CNT-based conductive segregated structures, to the best of our knowledge. In brief, this work proposed a novel approach of using a facile, cost-effective, and eco-friendly method to fabricate highly CPNs for EMI shielding applications.

Beryllium intermetallics: Industrial experience on development and manufacture

Ulba Metallurgical Plant JSC has been leading several research and development programs aimed at producing beryllides for both structural and fusion applications. The main focus has been on the development of tantalum beryllide as a high-temperature material, and chromium and titanium beryllides as neutron multipliers for EU DEMO. Utilizing vacuum hot pressing, billets of tantalum, chromium, and titanium beryllides were successfully produced, with their key properties such as density, phase composition, and microstructure extensively analyzed. These results show promising potential for expanding the use of beryllides and developing new compositions, highlighting the continued importance of research in this area.

Cr3+ doped CaF2 transparent ceramic with effective broadband near-infrared emission

Cr3+ has attracted much attention as activator because of the merits such as, broadband near-infrared emission and tunable emission wavelength. However, the mixed valence of Cr3+ in some matrix materials greatly affected its luminescence. In this work, we synthesized Cr3+ doped CaF2(CaF2:Cr3+) precursor powders with a single valence of Cr3+ ion by coprecipitation and successfully prepared CaF2:Cr3+ transparent ceramics by vacuum hot-press sintering. The obtained ceramics showed high transmittance with 83% at 1200 nm and effective broadband near-infrared emission covering 680–980 nm with a full width at half-maximum (FWHM) of 130 nm. Moreover, the photoluminescence properties of ceramics were tuned by the concentration of Cr3+ ions.

Arc erosion resistance of WC/CuCr30 composites based on changing of Cr particle morphology and orientation

CuCr30 composite was commonly applied in medium and low voltage vacuum interrupters. The WC/CuCr30 composites based on changing of Cr particle morphology and orientation were successfully prepared by vacuum hot press sintering and hot extrusion, and the arc erosion resistance behavior at different voltages along the deformation direction of Cr particles was investigated on the JF04C electrical contact material testing system. The microstructure characterization of WC/CuCr30 composites shows that the Cr particles in the WC/CuCr30 composite have been elongated and the long axis direction is close to parallel to the extrusion direction after the hot extrusion deformation. Compared with the longitudinal section, the thermal conductivity of CuCr30 and WC/CuCr30 composites in the cross section is increases by 3.75% and 3.63%, respectively. The electrical contact testing results under different voltages shows that the addition of WC particles can extend the service life of the CuCr30 composite, and the operation times of WC/CuCr30 composite (4850 times) increases by 34.72% compared to CuCr30 composite (3600 times) at 30 V DC. The welding force decreases more obviously with the increase of the voltage, and the average welding force of WC/CuCr30 composite decreases by 8.57% and 20.9% at 24 V DC and 30 V DC compared with CuCr30, respectively. The arc erosion mechanism analysis shows that addition of WC particles is beneficial for the decrease of arc duration. And WC particles can effectively inhibit the arc erosion process and delay the formation of bumps and pits.

Evolution of crystallographic texture and grain boundary character in hot press sintered tungsten heavy alloy as a function of sintering temperature

Using the vacuum hot press sintering method, tungsten heavy alloys (93 W − 4.9Ni − 2.1Cu) was produced. The study explored the effect of sintering temperature on W-Ni-Cu heavy alloy. Studies in metallography reveals that an increase in sintering temperature led to the coalescence of tungsten grains and affected their mechanical, electrical properties. As a result, an alloy sintered at a high temperature (1450 °C) has the highest hardness (410 Hv0.5), electrical conductivity (19.72% of IACS), and relative density (96.6%).

Microstructure and Mechanical Properties of Ni-20Cr-Eu2O3 Composites Prepared by Vacuum Hot Pressing.

Ni-20Cr-Eu2O3 composites were designed as new control rod materials and were synthesized from Ni, Cr, and Eu2O3 mixture powders via ball milling and vacuum hot pressing. During ball milling, Eu2O3 was fined, nano-crystallized, amorphized, and then dissolved into matrix. The effect of Eu2O3 content on the microstructure and mechanics was researched, and the corresponding mechanism was discussed. The relative densities, grain sizes, and microhardness increased when Eu2O3 content increased. According to the TEM observations, Eu2O3 particles showed a semi-coherent relationship with the matrix. The results of mechanical property testing showed that the ultimate tensile strength, yield strength, and elongation decreased with the Eu2O3 content increased. The maximum ultimate tensile strength, yield strength, and elongation were 741 MPa, 662 MPa, and 4%, respectively, with a 5 wt.% Eu2O3 addition. The experimental strengths were well matched with the theoretical values calculated by the strengthening mechanisms indicating that this method was highly effective for predicting the mechanical properties of Ni-20Cr-Eu2O3 composites.

Ink casting and 3D-extrusion printing of the thermoelectric half-Heusler alloy Nb1-xCoSb

Additive manufacturing of lattices with ∼600 μm diameter struts is achieved via extrusion printing of an ink containing prealloyed powder of the half-Heusler alloy Nb1-xCoSb, followed by debinding and vacuum sintering to a relative density of ∼70-80%. The ink can also be poured and cast into square blocks, which, after debinding and sintering, achieve a similar relative density. Sintering in a Sb-rich atmosphere reduces Sb sublimation and decomposition of the half-Heusler phase. This is reflected in electrical conductivity values, for ink-cast specimens, similar to those of a sample created by vacuum hot-pressing of dry powders (1080 S/cm 298 K). However, a lower Seebeck coefficient for the ink-cast specimens (92 vs. 150 µV/K for a hot-pressed sample) leads to a reduced figure of merit (zTmax= 0.10±0.015 vs. 0.26±0.04 at 873 K for a hot-pressed sample), where secondary phases, NbSb2 and CoSb3 were observed; these might be minimized, or even eliminated, via further optimization of the sintering conditions.

Effect of Bi-B-Si-Zn-Al glass additive on the properties of low-temperature sintered silicon carbide ceramics

In this study, the performance of low-temperature sintered Bi-B-Si-Zn-Al glass/SiC composites by vacuum hot-press sintering between 700°C and 1000°C was investigated. The specimen had a relatively preferable density of 95.5% and thermal conductivity of 8.660 Wm−1K−1 after sintering at 900°C for 2 h. The dielectric constant and dielectric loss tangle of the composite were 32.9 and 0.57 at 20 GHz, respectively. The XRD analysis indicated the formation of Bi, Zn and ZnAl2O4 crystals, and the microstructure showed the low contact angle of the glass and SiC grain. Such low-temperature sintered SiC ceramic may have a promising application in the electronic field.

Effects of nano scale Y2O3 additions on microstructural stability and mechanical properties of equiatomic CoCrCuFeNi based high entropy alloys

The use of a blend of pre-alloyed powder aggregate with a measured amount of nano-dispersoids is a powerful method to fabricate near-net-shaped components via the Powder metallurgy route. In the current investigation, CoCrCuFeNi -based multicomponent alloys with controlled addition of Y2O3 (Y2O3 wt. % = 0, 1, 2 and 3) were synthesized by mechanical alloying (MA) and subsequently consolidated by vacuum hot pressing (VHP). The effect of the Y2O3 addition of the as-sintered Y2O3 dispersed CoCrCuFeNi aggregate on the microstructural and mechanical characteristics was investigated to better understand their behaviour. The XRD analysis confirms that the as-prepared oxide dispersion strengthened high entropy alloy (ODS-HEAs) including the base HEA, and established a metastable nanostructure after the MA process. The microstructure eventually turned up into a thermodynamically stable aggregate once it is hot pressed. In addition, following 30 h of MA in CoCrCuFeNi HEA with 2 wt% and 3 wt% Y2O3, a stable intermetallic phase of Co14.2Cr2.7Y2 and an FCC phase were observed, but with 0 wt% and 1 wt% Yttria addition, only an FCC phase was observed. During the milling of CoCrCuFeNi HEA, the average particle size decreases from 13.864 μm to 9.541 μm (start to the end of the milling process). Furthermore, for 3 wt % Y2O3 dispersed CoCrCuFeNi ODS-HEA, the change in particle size is from 15.582 μm to 5.712 μm at the start and end of milling, respectively, indicating that the particle size has decreased to much lower values compared to base HEA. After increasing the amount of Y2O3 added from 0 to 3 wt %, there was a significant increase in the microhardness of CoCrCuFeNi HEA from 370 ± 6 HV to 548 ± 5 HV. Higher Yttria wt.% (2 wt% and 3 wt%) was found to result in the formation of complex intermetallic Co14.2Cr2.7Y2 particles, which were able to pin the migration of the grain boundaries during sintering due to their high thermal stability, and were thus responsible for the increase in hardness for these alloys.

Mechanical and thermal properties of Cu-coated diamond reinforced Cu matrix bioinspired laminated composites

Cu-coated diamonds reinforced Cu matrix laminated composites were successfully prepared by combining flake powder metallurgy and vacuum hot press sintering. The interface strength, mechanical properties and thermal properties of Cu-coated diamonds reinforced Cu matrix laminated composites were studied. Results show that the Cu flakes obtained by flake powder metallurgy is the key to realize the laminated structure. Cu-coated diamonds have higher dispersion and higher interfacial strength when mixed with Cu matrix due to tight bonding interface. The best compressive strength, tensile strength and thermal conductivity are obtained when the content of Cu-coated diamond is 3.0 wt%. On the one hand, the diamond particles are obviously refined to achieve fine grain strengthening; on the other hand, the laminated structure is the most obvious at this time, which can provide more heat conduction channels and thus increase the thermal conductivity. In addition, the high strength of diamond can effectively prevent crack propagation through the load transfer effect at the interface. This study provides a new idea for the preparation of functional structural materials with high mechanical properties and thermal conductivity.

The Role of Al4C3 Morphology in Tensile Properties of Carbon Fiber Reinforced 2024 Aluminum Alloy during Thermal Exposure.

The aluminum alloy drill pipe suffers long-term high-temperature conditions during ultra-deep well drilling. In this paper, the samples were prepared by vacuum hot pressing, followed by hot extrusion and T6 heat treatment. The mechanical properties of short carbon fiber reinforced 2024 aluminum alloy composites (SCFs/2024 Al) and the microstructure evolution at the interface region thermal exposure at 160 °C for 500 h are discussed. The experimental results showed that the effect of short carbon fiber on 2024 aluminum alloy remained steady throughout the whole process of the heat exposure experiment. The distribution and volume of interface products (Al4C3) changed with the prolonging of heat exposure time, and connected after coarsening. The evolution of the morphology of Al4C3 relieved the stress of the interface between carbon fiber and aluminum alloy matrix and enhanced the mechanical properties of the composite.

Fabrication and properties of multistage gradient doping Yb:YAG laser ceramics

AbstractIn order to fully pump and smoothen the temperature gradient of the gain medium, multistage gradient doping Yb:YAG laser ceramics were designed. The composite green bodies were fabricated by tape casting, and multistage gradient doping Yb:YAG ceramics with high optical quality were prepared by vacuum sintering and hot isostatic pressing. For samples pre‐sintered at 1740°C for 30 h and then HIP‐ed at 1700°C for 3 h in argon at 200 MPa, the in‐line transmission values at 1100 nm of YAG, 0.6 at.%Yb:YAG, and 1.5 at.%Yb:YAG ceramics were found to be 83.9%, 84.1%, and 83.3%, respectively. Finally, the 940 nm laser diode was used as the pump source to realize the 1030 nm laser output of multistage gradient doping Yb:YAG ceramic slab with a total energy of 3.43 J. The corresponding optical‐to‐optical conversion and slope efficiencies were 30% and 45%.

Structural Modeling and Thermal Conductivity of Graphite Film Reinforced Aluminum Matrix Laminated Composites

Abstract: Excellent thermal conductivities of thermal management materials are expected to ensure the timely heat dissipation in lots of engineering applications and electronic devices. High in-plane thermal conductivity of laminated composites has become increasing significant for high energy and power density electronic devices. In this study, the continuous graphite film/aluminum (Gr film/Al) laminated composites were fabricated by vacuum hot pressing. In-plane and out-of-plane thermal conductivity of Gr film/Al laminated composites are tested. Two-dimensional structural models of Gr film/Al laminated composites are established, in which volume fraction, interfacial property, punching zone and orientation angle of Gr films can be controlled according to their actual composite microstructures. The effects of volume fraction and interfacial property on the thermal conductivity of Gr film/Al laminated composites are investigated. Two ways to reduce anisotropy of thermal conductivity are introduction of punching zones and control of Gr orientation, which are verified to be effective. On basis of the analysis above, a good understanding can be brought out for extensive thermal management applications of Gr/Al composites.

Microstructures, mechanical and thermal properties of diamonds and graphene hybrid reinforced laminated Cu matrix composites by vacuum hot pressing

Graphene nanoplatelets (GNPs) and diamond are considered as reinforcements due to their high strength and thermal conductivity. The key problem is the interfacial bonding between the reinforcement and the matrix and the construction of the laminated structure in this study. The interfacial bonding between the reinforcement and the matrix is improved by coating copper on the surfaces of the reinforcement. Lamellar composites were successfully prepared by the combination of flake powder metallurgy and vacuum hot pressing. The results show that the mechanical and thermal properties of lamellar composites are better than the non-lamellar composites. With the increase of GNPs content, the laminated structure becomes more and more obvious, and the strength and thermal conductivity increase. When the content of GNPs reaches 1.5 wt%, the tensile strength, compressive strength and thermal conductivity of X–Y direction is 267 MPa, 663.33 MPa, 402 W/mK, respectively. In summary, copper-coated diamond and copper-coated GNPs improve the bonding with the matrix, and the lamellar structure extends the crack path and provides more heat conduction channels, thus increasing the mechanical and thermal properties of the composites. This work provides an effective method for the development of new thermal management structures and functional materials.

Microstructure and Mechanical Properties of Steel and Ni-Based Superalloy Joints for Rotors of High-Speed Electric Motors.

High-speed electric motors, e.g., axially laminated anisotropic synchronous reluctance motors (ALA-SynRM), use a solid rotor manufactured by joining alternating layers of magnetic and non-magnetic metallic sheets. The strength of the dissimilar metallic joints is critical for the rotor’s ability to withstand the operating conditions of the high-speed electrical machine. In this work, various dissimilar metallic joint configurations that can be used in high-speed ALA-SynRM rotors are studied by analyzing the shear strength, microstructure, hardness, and composition of the joints. Metallic joints of structural steels and Inconel® alloys fabricated by vacuum brazing and hot isostatic pressing (HIP) are studied. Finite element analysis (FEA) was performed to calculate the maximum shear stress of the joints that were subjected to various high speed operating conditions. The shear strength of the test specimens was measured and compared with FEA results. The microstructure and chemical composition of the joints were studied by using optical microscopy, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) on SEM. The results show that the hot isostatic pressed S1100MC-IN718 joint achieved the highest ultimate shear strength (233.3 MPa) followed by vacuum brazed S355MC-IN600 joint (230.1 MPa) and HIP S355-IN718 (203.5 MPa), thereby showing that vacuum brazing and HIP can be viable manufacturing methods to fabricate a high-speed ALA-SynRM rotor.

Microstructures and interfacial interactions of Al2O3 whiskers and graphene nano-platelets co-reinforced copper matrix composites

The mechanical properties and microstructures of Al2O3 whiskers and graphene nano-platelets (GNPs) co-reinforced Cu-matrix composites were studied. Cu-matrix composites with a variation of GNPs amount were fabricated by mechanical alloying followed by vacuum hot-pressing sintering and hot isostatic pressing. The Cu-matrix composite with 0.5 wt.% GNPs (GNPs-0.5) suggests a good interfacial bonding of both Cu/C and Cu/Al2O3 interfaces. Both the hardness and compressive strength of Cu-matrix composites show a consistent tendency that firstly increases to a critical value and then decreases with increasing GNPs amount. It is suggested that the most possible strengthening mechanisms of both GNPs and Al2O3 whisker working in the Cu-matrix composites involve energy dissipating and load transfer, as well as grain refinements for GNPs. The synergetic effect of GNPs and Al2O3 whiskers is highlighted that the embedded GNPs would hinder the crack path generated at the Al2O3/Cu interface and enhance the already outstanding strengthening effect that Al2O3 whiskers provide.

Microstructure, mechanical properties and high temperature tribological behavior of NiAl-based composites

NiAl-based composites were fabricated in-situ using vacuum hot-press sintering technology. The Ti3AlC2, TiC and Ti3Al were formed during the sintering process, and the intensity of peaks grow higher with the addition of Al, Ti and C powder contents, respectively. The microhardness of the composites was significantly improved by adding these powders and gradually increased with the incremental content of Al, Ti and C powders. Meanwhile, the tribological properties of the composites at high temperature were also improved. The worn surface formed during the tribo-oxidation reactions and produced the glaze film in the high-temperature friction process, which could improve the composites' tribological properties. The TiO2 and Al2O3 phases were observed on the worn surface after the friction test, which could act as a high-temperature lubricant and improve the composites’ tribological properties.

Unidirectional Carbon Fiber Reinforced Cyanate‐Based Shape Polymer Composite with Variable Stiffness

Cyanate‐based shape memory polymer (CSMP) has the advantages of good mechanical properties and excellent shape memory properties. However, the mechanical properties of polymers are relatively poor at high temperature. This article aims to improve the mechanical properties of CSMP by adding varying contents of unidirectional carbon fibers. The carbon fiber reinforced CSMP composites (CSMPCs) are prepared by vacuum hot pressing of prepregs. Compared with CSMP, the tensile, flexural, and compressive properties of the CSMPCs have significant improvement. By changing fiber content and adjusting temperature, the composites can achieve variable stiffness characteristic. In addition, the CSMPC with fiber content of 15% has excellent shape memory performances including higher recovery rates and faster recovery speeds. A shape memory spring based on CSMPC has strong deformation ability, which can realize large compression and stretching deformation. The CSMPCs make up for the shortcoming of decreased mechanical properties at high temperatures of CSMPs and broaden the application used in high‐temperature environments.

Fabrication and characterizations of Tm:Lu2O3 transparent ceramics for 2 μm laser applications

2 at.% Tm:Lu2O3 ceramics were fabricated by vacuum sintering plus hot isostatic pressing (HIP) from nano-powders without sintering aids. The resultant powders consist of spherical particles with the diameter of about 100 nm. The HIP post-treated ceramic sample has fine grains and clean grain boundaries, whose in-line transmittance reaches 82.1% at 2000 nm (1.3 mm thickness). The mechanical, thermal, and spectral properties were also investigated systematically. The thermal conductivity of the Tm:Lu2O3 transparent ceramics is 10.2 W·m−1·K−1 at room temperature. The absorption and emission cross-sections are 0.35 × 10−20 cm2 at 796 nm and 1.13 × 10−20 cm2 at 1943 nm, respectively. The laser performance was tested in quasi-continuous wave (QCW) conditions. A maximum laser output power of 0.45 W was obtained with a slope efficiency of 16.0%. Results indicate that Tm:Lu2O3 transparent ceramics is a promising gain medium for 2 μm laser applications.

A Quantitative Investigation of Dislocation Density in an Al Matrix Composite Produced by a Combination of Micro-/Macro-Rolling

An aluminum matrix composite with dispersed carbon nanotubes (CNTs) was produced via flake powder metallurgy using a micro-rolling process and vacuum hot pressing (VHP), followed by conventional rolling using a macro-rolling process. The microstructure and mechanical properties of the produced composites were studied. In addition, a new quantitative model was introduced to study the dislocation density based on the microstructural parameters. The results revealed that the distribution characteristics of the CNTs in the Al matrix and the Al-CNT interfaces were the two main governing parameters of dislocation density. Moreover, the dependence of dislocation density on the geometry of the grains and crystallographic texture was shown in this model. The microstructural evolution revealed that a lamellar grain structure had been achieved, with a high capacity for the storage of dislocation. A uniform distribution of CNTs with high bonding quality was also seen in the final microstructure.