Advantages and disadvantages of graphite addition on the characteristics of hot-pressed ZrB2–SiC composites

ZrB₂–SiC–graphite composites with 0–35 vol% graphite flakes were densified via hot-pressing route at the temperature of 1800 °C under the uniaxial pressure of 40 MPa for 1 h. Consolidation, mechanical properties, and microstructure of hot-pressed composites were investigated by variation of graphite content. By the addition of graphite, the relative density of composites increased, and at this hot pressing condition, fully densified composites were fabricated. The highest flexural strength of 366 MPa was measured for composite containing 7.5 vol% graphite, while the maximum Vickers hardness resulted in 2.5 vol% graphite doped one, and its value was equal to 20.8 GPa. Phase analysis of hot-pressed samples revealed the formation of the Zr₃C₂ and B₄C phases besides the main existing ZrB₂, SiC, and graphite phases. The newly carbide phases formed at the surface of ZrB₂ grains. The addition of graphite into the ZrB₂–SiC composites improved the sintering process and caused a fine-grained microstructure.     

Conventional HP sintering of asymmetric hexagonal structure Yb3+-doped Sr5(PO4)3F transparent ceramic without additives

The 2 at.% Yb³⁺:Sr₅(PO₄)₃F (S-FAP) polycrystalline transparent ceramic with asymmetric hexagonal structures has been synthesized by vacuum hot-pressing the nanoparticles prepared via coprecipitation method. X-ray diffraction results of powder and ceramic indicate that their phase peaks are well matched to the crystal structure of S-FAP. The average particle size of 35.5 nm has been exhibited by powder scanning electron microscopy images, and subsequent images of the ceramic cross section and surface morphology revealed a homogenous and compact microstructure with an average grain size of around 220 nm. The relationship between the optical loss caused by the scattering of anisotropic ceramic grains and the optical transmittance of ceramics was revealed in the hexagonal S-FAP transparent ceramics with different thicknesses. The in-line transmittance of hot-pressed ceramics with 1.5-mm thickness achieved 79.95% at 1100-nm wavelength, and the room-temperature absorption and emission spectra of Yb³⁺ in S-FAP polycrystalline ceramic matrix were measured using a spectrofluorometer.     

Influence of post-HIP temperature on microstructural and optical properties of pure MgAl2O4 spinel: From opaque to transparent ceramics

Transparent MgAl₂O₄ spinel ceramics were processed from sub-micrometric commercial powder by applying a two-step procedure: pressureless sintering under vacuum followed by hot isostatic pressing. To limit grain growth and to avoid secondary reactions or impurities, no additives or sintering aids were added to the powder. First, pressureless sintering at 1500 °C during 2 h under vacuum led to opaque samples due to a high level of porosity. To improve the optical quality of the MgAl₂O₄ ceramics and the in-line transmission in the visible range, a post-treatment by hot isostatic pressing was applied. Highly transparent ceramics were obtained after a post-treatment at 1800 °C for 10 h with an in-line transmission of 81% at 400 nm and 86% from 950 to 3000 nm for a thickness of 2 mm (98.8% of the theoretical transmission).     

Characterization of Pore Size Distribution by Infrared Scattering in Highly Dense ZnS

A model based upon Mie theory was developed to calculate the amount of spectral infrared scattering caused by the presence of a porous second phase. Various in-line transmission curves were calculated and used to characterize the scattering effects of pore size and concentration. The in-line transmission from 2.5 to 10 fjim of ZnS samples hot-pressed at 137.8,172.3, and 206.7 MPa was measured using Fourier transform infrared spectrophotometry and compared with calculated transmission curves. Good agreement with measured results was obtained only when a size distribution effect was included. From the analysis, a bimodal distribution of pores was found to give the best agreement which duplicates the measured transmission.     

Ultrafast high-temperature sintering of Li7La3Zr1.75Nb0.25Al0.15O12 (LLZO)

Rapid sintering of Li₇La₃Zr_1.75Nb_0.25Al_0.15O₁₂ (LLZO) is reported. The selection of heating elements, the effect of powder preparation and MgO additions in rapid sintered LLZO are described. Annealing LLZO powder at 750 ºC for 2 h in argon immediately before pressing helped to minimize porosity. A 15–20 s hold at 1372 ºC was sufficient to achieve densities >97%. The total sintering schedule time for rapid sintering represents a 99.7% decrease in sintering time compared to conventional sintering. At 70 °C under a pressure of 4.12 MPa cells had a critical current density of 1020 µA/cm².     

Transmission Properties of Translucent Polycrystalline Alumina

Effects of residual pores and optical birefringence on transmission through translucent polycrystalline alumina have been studied with the Mie scattering theory. The in-line and total forward transmissions of the translucent polycrystalline alumina were simulated as a function of porosity, pore radius, and grain size. The model revealed that porosity has a significant effect on both the total forward scattering and the in-line transmission, whereas grain size affects only in-line transmission. The calculated transmittance is in good agreement with the experimental values. The in-line transmission gradually increased with a decrease in grain size, and the effect of birefringence on the total forward transmission is negligible in the visible spectrum. The total forward transmission in the visible spectrum is mainly governed by residual pores.     

改性大豆分离蛋白/淀粉降解材料研究

对大豆蛋白和淀粉进行辐(照辐照剂量20 kGy)处理,制备了大豆蛋白/淀粉生物降解材料,研究了热压温度、时间、压力对大豆蛋白/淀粉降解材料力学性能、透光率、吸水率的影响,并利用FTIR对其进行分析。结果表明:热压温度130℃、时间15 min、压力15 MPa时,所制备材料的断裂伸长率为377.4%、拉伸强度为7.21 MPa透、光率为28.5%吸、水率为46.26%。     

Rapid Hot-Pressing of Ultrafine PSZ Powders

The process of compaction and densification of ultrafine (40- to 60-nm grain size) powder of partially stabilized zirconia with 3 mol% of Y₂O₃ (Y3-PSZ) during rapid hot-pressing was investigated. A special apparatus was designed to allow rapid application of 1.6 GPa of quasi-isostatic pressure at temperatures of 1100° to 1300°C to powder compacts encapsulated in glass under vacuum. Pressure was applied for 10 s, then the samples were rapidly cooled to room temperature, removed from the encapsulating glass, and characterized using SEM, TEM, and X-ray diffraction. Density and mechanical properties of the prepared materials were measured and compared with those of similar materials fabricated using conventional hot-pressing. SEM and TEM observations revealed that the ultrafine grains of the starting powder coarsened rapidly during the initial heating, and the compacts developed large (> 10 μm) and small (< 1 μm) pores. The process of densification under pressure consisted of closing of the large pores, whereas the small pores were relatively unaffected by the application of pressure at all investigated temperatures. The major mechanism of densification during the rapid pressing appears to be rearrangement and sliding of grains around the large pores. The material prepared by rapid pressing at 1300°C had higher hardness ( H _v= 1400 versus 1300 kg/mm²) but somewhat lower fracture toughness ( K _{I C} = 5.5 versus 6.0 MPa · m^1/2) compared with the conventionally hot-pressed Y3-PSZ. Density of the material pressed at 1300°C was 97% of theoretical density.     

Production of Hydroxyapatite/Bioactive Glass Biomedical Composites by the Hot-Pressing Technique

Biomedical composites of hydroxyapatite (HA) and bioactive glass (BG) have been difficult to obtain as a dense body without the undesirable occurrence of thermal reactions and phase degradation. Herein, HA–BG dense composites were produced by the hot-pressing technique. A range of HA–BG powder mixtures (30–50 wt% BG) was fully densified by hot pressing at temperatures as low as ∼700°–800°C. On the other hand, the HA–BG composites could not be densified by pressureless sintering because their composition was degraded due to a severe thermal reaction. The hot-pressed composites had significantly improved flexural strengths (∼60 MPa) as compared with those subjected to pressureless sintering (∼30 MPa) or the pure HA control (∼40 MPa). The hot-pressed HA–BG composites showed significantly enhanced bioactivity in a simulated body fluid, as well as osteoblast cell activity with respect to the pure HA, confirming their excellent in vitro biocompatibility.     

Effect of Sintering Temperature on the Densification of Al2O3

Alumina powder compacts sintered at various temperatures were isostatically hot-pressed. The specimens sintered to the closed-pore state can be densified further by hot isostatic pressing. If the open pores are eliminated during sintering, sintering at a low temperature is desirable to achieve a full densification after hot-pressing. Sintering at high temperatures causes pores to be trapped inside the grains; these pores are difficult to eliminate by subsequent hot-pressing.     

Bulk, Dense, Nanocrystalline Yttrium Aluminum Garnet by Consolidation of Amorphous Powders at Low Temperatures and High Pressures

Amorphous Al₂O₃–37.5% Y₂O₃ powders, prepared using spray pyrolysis followed by partial or complete thermal decomposition, were hot-pressed at 315°–640°C and 500 or 750 MPa uniaxial pressure. Hot pressing of fully decomposed amorphous powder at 450°–640°C at pressures up to 750 MPa led to densification (up to 96%) as well as nanocrystallization of yttrium aluminum garnet (YAG). When the pressure was applied during heating, instead of after reaching the final temperature, higher relative densities resulted. Fully crystalline starting powder did not densify. The low true density of the amorphous phase (3.1 g·cm⁻³) was believed to be responsible for the densification through enhanced ionic mobilities.     

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.     

Influence of pressure and dwell time on pressure-assisted sintering of calcium cobaltite

Calcium cobaltite Ca₃Co₄O₉, abbreviated Co349, is a promising thermoelectric material for high-temperature applications in air. Its anisotropic properties can be assigned to polycrystalline parts by texturing. Tape casting and pressure-assisted sintering (PAS) are a possible future way for a cost-effective mass-production of thermoelectric generators. This study examines the influence of pressure and dwell time during PAS at 900°C of tape-cast Co349 on texture and thermoelectric properties. Tape casting aligns lentoid Co349. PAS results in a textured Co349 microstructure with the thermoelectrically favorable ab-direction perpendicular to the pressing direction. By pressure variation during sintering, the microstructure of Co349 can be tailored either toward a maximum figure of merit as required for energy harvesting or toward a maximum power factor as required for energy harvesting. Moderate pressure of 2.5 MPa results in 25% porosity and a textured microstructure with a figure of merit of 0.13 at 700°C, two times higher than the dry-pressed, pressureless-sintered reference. A pressure of 7.5 MPa leads to 94% density and a high power factor of 326 µW/mK² at 800°C, which is 11 times higher than the dry-pressed reference (30 MPa) from the same powder.     

Fabrication and characterization of silica ceramic compact prepared using Aloe-Vera mucilage as a binder

In this study, silica compacts were fabricated through a powder processing route at different compaction pressure, using Aloe-Vera (AV) mucilage as a binder. The silica compacts were prepared at 90, 100, and 110 MPa compaction pressure using 0%–16 wt% of AV binder. The optimum amount of AV binder was 14 wt% for both 90 and 100 MPa and 12 wt% for 110 MPa. The maximum achieved green density and green strength of silica compacts at the optimum binder amount were 62.3% and 4 MPa, respectively at 110 MPa compaction pressure. The green silica compacts prepared at 110 MPa compaction pressure exhibited a minimum porosity of 21% and maximum flexural strength of 15 MPa after sintering at 1400°C. The green silica compacts with the optimum amount of binder were strong enough for machining. The Fourier transform infrared spectroscopy analysis revealed the functional groups present in AV mucilage. The binder burnout characteristic of AV mucilage in the silica compact was determined by thermogravimetric analysis and differential thermal analysis. Additionally, AV gel acted as a binder and solvent simultaneously for ceramic compaction.     

Densification of ordered microporous carbons and controlling their micropore size by hot-pressing

Ordered microporous carbons synthesized by the template method using zeolite Y were hot-pressed at 573 K up to 147 MPa, aiming at densification of the templated carbons for the future use as energy storage media. Upon the hot-pressing, the powdery templated carbons were easily pelletized without any binder and the bulk density was significantly increased from 0.2 g/cm³ to 0.7-0.9 g/cm³. As a result, both the surface area and micropore volume per unit volume were increased. Surprisingly, it was found that the hot-pressing treatment reduced the average micropore size and it was tunable by simply adjusting a pressure in the treatment. In contrast, such changes in the density and the pore structure were not observed for commercial KOH-activated carbons when they were hot-pressed under the same conditions. The observed peculiar behavior of the templated carbons upon the hot-pressing can be explained by their unique molecular structure as compared to the conventional activated carbons.     

Processing and room temperature mechanical properties of a zirconium carbide ceramic

Zirconium carbide (ZrC) powder, batched to a ratio of 0.98 C/Zr, was prepared by carbothermal reduction of ZrO₂ with carbon black. Nominally phase-pure ZrC powder had a mean particle size of 2.4 μm. The synthesized powder was hot-pressed at 2150°C to a relative density of > 95%. The mean grain size was 2.7 ± 1.4 μm with a maximum observed grain size of 17.5 μm. The final hot-pressed billets had a C/Zr ratio of 0.92, and oxygen content of 0.5 wt%, as determined by gas fusion analysis. The mechanical properties of ZrC_0.92O_0.03 were measured at room temperature. Vickers’ hardness decreased from 19.5 GPa at a load of 0.5 kgf to 17.0 GPa at a load of 1 kgf. Flexural strength was 362.3 ± 46 MPa, Young's modulus was 397 ± 13 MPa, and fracture toughness was 2.9 ± 0.1 MPa•m^1/2. Analysis of mechanical behavior revealed that the largest ZrC grains were the strength-limiting flaw in these ceramics.     

One-step cold sintering process towards translucent BaF2 ceramics

BaF₂ ceramics were prepared using a one-step cold sintering process with an ultra-low sintering temperature of 150 °C and uniaxial pressures ranging from 450 to 900 MPa. The relative density and microstructure improved steadily with the increasing pressure, and a fully densified microstructure with a relative density of 97.2% was achieved at 900 MPa. For BaF₂ ceramics with a thickness of 1 mm, the optimum in-line transmittance in the visible light region (58.5%) was achieved at a wavelength of 720 nm, and the maximum value (65.3%) was obtained at 1864 nm. The permittivity of the ceramics increased gradually from 6.18 to 7.09 with increasing pressure, and the dielectric loss was optimized from 0.01 to 0.003. Additionally, the mechanical properties improved continuously with the increasing pressure, and the optimal compressive strength (257 MPa), hardness (2.01 GPa), and Young's modulus (54.8 GPa) were achieved when cold sintered at 900 MPa.     

Joining of silicon carbide ceramics using a silicon carbide tape

This paper reports the joining of liquid-phase sintered SiC ceramics using a thin SiC tape with the same composition as base SiC material. The base SiC ceramics were fabricated by hot pressing of submicron SiC powders with 4 wt% Al₂O₃–Y₂O₃–MgO additives. The base SiC ceramics were joined by hot-pressing at 1800-1900°C under a pressure of 10 or 20 MPa in an argon atmosphere. The effects of sintering temperature and pressure were examined carefully in terms of microstructure and strength of the joined samples. The flexural strength of the SiC ceramic which was joined at 1850°C under 20 MPa, was 343 ± 53 MPa, higher than the SiC material (289 ± 53 MPa). The joined SiC ceramics showed no residual stress built up near the joining layer, which was evidenced by indentation cracks with almost the same lengths in four directions.     

From nanometric zirconia powder to transparent polycrystal

Coprecipitated zirconia-yttria (8 mol%) gel subjected to hydrothermal treatment at 240 °C resulted in the solid solution powder of 8 nm particle sizes and specific surface area 132.7 m²/g. Uniaxial compaction followed by cold isostatic pressing under 300 MPa resulted in samples of the extremely small and narrow pore size distribution. Such samples start to shrink at about 200 °C which is related to the desorption of water layers surrounding particles. The state of closed porosity is achieved at 1150 °C. Pore closing was performed in air or oxygen atmosphere. Hot isostatic pressing at 1150 °C for 2 h under 250 MPa argon pressure led to transparent materials. Some pores remained in the material whose preliminary pore closure was performed in air. The samples initially sintered in oxygen atmosphere show no porosity and higher light transmittance than the former ones.     

Consolidation behavior and hardness of P/M molybdenum

In this study, the consolidation behavior and hardness of commercially available molybdenum powder were investigated. In order to analyze the compaction response of the powder theoretically, an elastoplastic constitutive equation based on the yield function presented by Shima and Oyane was applied to predict the compact density under uniaxial pressure from 100 MPa to 700 MPa. The compacts were sintered at 1400-1600 °C for 20-60 min. The sintered density and grain size of molybdenum were increased with increasing the compacting pressure and processing temperature and time. The effect of the porosity and grain size on the hardness of the specimens was explained based on the modified plasticity theory of porous material and the Hall-Petch type equation.