Dense nanocrystalline BaTiO3 ceramics with a homogeneous grain size of 30 nm was obtained by pressure assisted sintering. The ferroelectric behaviour of the ceramics was characterized by the dielectric peak at around ...Dense nanocrystalline BaTiO3 ceramics with a homogeneous grain size of 30 nm was obtained by pressure assisted sintering. The ferroelectric behaviour of the ceramics was characterized by the dielectric peak at around 120 ℃, the P-E hysteresis loop and some ferroelectric domains. These experimental results indicate that the critical grain size for the disappearance of ferroelectricity in nanocrystalline BaTiO3 ceramics fabricated by pressure assisted sintering is below 30 nm. The ferroelectric property decreasing with decreasing grain size can be explained by the lowered tetragonality and the ＇dilution＇ effect of grain boundaries.展开更多
Polycrystalline cubic boron nitride（Pc BN） compacts, using the mixture of submicron cubic boron nitride（c BN） powder and hexagonal BN（h BN） powder as starting materials, were sintered at pressures of 6.5–10.0 G...Polycrystalline cubic boron nitride（Pc BN） compacts, using the mixture of submicron cubic boron nitride（c BN） powder and hexagonal BN（h BN） powder as starting materials, were sintered at pressures of 6.5–10.0 GPa and temperature of1750℃ without additives. In this paper, the sintering behavior and mechanical properties of samples were investigated.The XRD patterns of samples reveal that single cubic phase was observed when the sintering pressure exceeded 7.5 GPa and h BN contents ranged from 20 vol.% to 24 vol.%, which is ascribed to like-internal pressure generated at grain-to-grain contact under high pressure. Transmission electron microscopy（TEM） analysis shows that after high pressure and high temperature（HPHT） treatments, the submicron c BN grains abounded with high-density nanotwins and stacking faults, and this contributed to the outstanding mechanical properties of Pc BN. The pure bulk Pc BN that was obtained at 7.7 GPa/1750℃ possessed the outstanding properties, including a high Vickers hardness（～ 61.5 GPa）, thermal stability（～ 1290℃ in air）,and high density（～ 3.46 g/cm^3）.展开更多
As an important A_(2)B_(2)O_(7)-type ceramic,(La_(0.2)Nd_(0.2)Sm_(0.2)Gd_(0.2)Eu_(0.2))_(2)Zr_(2)O_(7)high-entropy pyrochlore pos-sesses promising properties such as high melting point,high chemical durability,and low...As an important A_(2)B_(2)O_(7)-type ceramic,(La_(0.2)Nd_(0.2)Sm_(0.2)Gd_(0.2)Eu_(0.2))_(2)Zr_(2)O_(7)high-entropy pyrochlore pos-sesses promising properties such as high melting point,high chemical durability,and low thermal conductivity.However,the low sintering ability limits its application in thermal barrier coating and radioactive waste immobilization.It usually needs long-term high-temperature soaking to achieve full density,but with inevitable grain growth.In this work,dense and grain-refined nanocrystalline(La_(0.2)Nd_(0.2)Sm_(0.2)Gd_(0.2)Eu_(0.2))_(2)Zr_(2)O_(7)ceramics were prepared with ultra-high pressure sintering(UHPS)method under 10 GPa at a low temperature of 800℃.The densification behavior,microstructure evo-lution,and properties of the UHPS-ed samples were then investigated.The grain size of as-prepared(La_(0.2)Nd_(0.2)Sm_(0.2)Gd_(0.2)Eu_(0.2))_(2)Zr_(2)O_(7)ceramic was only 151 nm,which is 40%smaller than that of raw pow-der.In addition,it exhibited advantageous properties including both high hardness and aqueous durabil-ity.Plastic deformation under ultra-high pressure was believed as the dominant densification mechanism responsible for grain refinement and property improvement.展开更多
W/ Cu functionally gradient materials （FGMs） containing 1% La2O3 and 1% TiC were prepared using graded sintering under tdtra-high pressure （GSUHP）. The specimens have been found to exhibit low porosity （11.57% an...W/ Cu functionally gradient materials （FGMs） containing 1% La2O3 and 1% TiC were prepared using graded sintering under tdtra-high pressure （GSUHP）. The specimens have been found to exhibit low porosity （11.57% and 11.35%, respectively）. Shearing strength of the specimens between layers is good. Moreover, the specimens have still demonstrated good performance in testing thermal-shock resistance. When power density of laser is 200MWm^-2, the specimens have been tested for thermal-shock resistance （1000 times）; the specimens that contained 1%La2O3 were not subjected to damage, whereas those that contained 1%TiC began to crack. Finally, effect of additives on thermal-shock resistance was also preliminarily discussed.展开更多
（Bi_2Te_3）_（0.2）（Sb_2Te_3）_（0.8） thermoelectric material was sintered via a field activated and pressure assisted sintering（FAPAS） process.By applying different current intensity（0,60,320 A/cm^2） in the si...（Bi_2Te_3）_（0.2）（Sb_2Te_3）_（0.8） thermoelectric material was sintered via a field activated and pressure assisted sintering（FAPAS） process.By applying different current intensity（0,60,320 A/cm^2） in the sintering process,the effects of electric current on the microstructure and thermoelectric performance were investigated.This demonstrated that the application of electric current in the sintering process could significantly improve the uniformity and density of（Bi_2Te_3）_（0.2）（Sb_2Te_3）_（0.8） samples.When the current intensity was raised to 320 A/cm^2,the preferred orientation of grains was observed.Moreover,positive effects on the thermoelectric performance of applying electric current in the sintering process were also confirmed.An increase of 0.02 and 0.11 in the maximum figure of merit ZT value could be acquired by applying current of 60 and 320 A/cm^2,respectively.展开更多
Ultra-high temperature ceramics (UHTCs) are most recently getting much attention for structural parts of hypersonic missiles with their cruising speed of more than Mach 5. Most of the UHTCs are poor sinterability carb...Ultra-high temperature ceramics (UHTCs) are most recently getting much attention for structural parts of hypersonic missiles with their cruising speed of more than Mach 5. Most of the UHTCs are poor sinterability carbides, nitrides, and borides. Therefore, they have been studied and developed for a long time. However, there are still many problems to solve. In this paper, based on the solid-state reaction presented as an equation of (x + y)·ZrC + 2·y·B → x·ZrC + y·ZrB<sub>2</sub> + y·C, three-phase ZrC/ZrB<sub>2</sub>/C composites have been fabricated from ZrC and amorphous B powders using pulsed electric-current pressure sintering at 1373 to 2173 K for 6.0 × 10<sup>2</sup> s under 50 MPa in a vacuum. ZrC/ZrB2/C = 30/70/C~70/30/C vol% composites with the relative densities D<sub>r</sub> of 96.6 to 98.7% were obtained at 2073 K. The 60/40/C vol% composite revealed high bending strength σ<sub>b</sub> (554 MPa), Vickers hardness H<sub>v</sub> (19.2 GPa) and moderate fracture toughness K<sub>IC</sub> (5.25 MPa·m<sup>1/2</sup>) at room temperature. Furthermore, all composites showed elastic deformation up to 1873 K and revealed σ<sub>b</sub> more than 600 MPa at this temperature, in addition, some composites showed higher σ<sub>b</sub> than 900 MPa at the same temperature. These high mechanical behaviors are discussed with those of the simple binary ZrC/ZrB<sub>2</sub> composites which were fabricated under the same conditions except for their starting materials. The best mechanical properties of binary composites were σ<sub>b</sub> (474 MPa), H<sub>v</sub> (18.5 GPa), and K<sub>IC</sub> (4.45 MPa·m<sup>1/2</sup>) at room temperature, and σ<sub>b</sub> of 400 - 700 MPa at 1873 K. Overall, three-phase composites, nevertheless including soft carbon, have higher mechanical properties than the binary composites.展开更多
文摘Dense nanocrystalline BaTiO3 ceramics with a homogeneous grain size of 30 nm was obtained by pressure assisted sintering. The ferroelectric behaviour of the ceramics was characterized by the dielectric peak at around 120 ℃, the P-E hysteresis loop and some ferroelectric domains. These experimental results indicate that the critical grain size for the disappearance of ferroelectricity in nanocrystalline BaTiO3 ceramics fabricated by pressure assisted sintering is below 30 nm. The ferroelectric property decreasing with decreasing grain size can be explained by the lowered tetragonality and the ＇dilution＇ effect of grain boundaries.
文摘Polycrystalline cubic boron nitride（Pc BN） compacts, using the mixture of submicron cubic boron nitride（c BN） powder and hexagonal BN（h BN） powder as starting materials, were sintered at pressures of 6.5–10.0 GPa and temperature of1750℃ without additives. In this paper, the sintering behavior and mechanical properties of samples were investigated.The XRD patterns of samples reveal that single cubic phase was observed when the sintering pressure exceeded 7.5 GPa and h BN contents ranged from 20 vol.% to 24 vol.%, which is ascribed to like-internal pressure generated at grain-to-grain contact under high pressure. Transmission electron microscopy（TEM） analysis shows that after high pressure and high temperature（HPHT） treatments, the submicron c BN grains abounded with high-density nanotwins and stacking faults, and this contributed to the outstanding mechanical properties of Pc BN. The pure bulk Pc BN that was obtained at 7.7 GPa/1750℃ possessed the outstanding properties, including a high Vickers hardness（～ 61.5 GPa）, thermal stability（～ 1290℃ in air）,and high density（～ 3.46 g/cm^3）.
基金financially supported by the National Natu-ral Science Foundation of China(nos.92163208,51902233,and 51972243)the National Key Research and Development Plan of China(no.2021YFB3701400)the Independent Innovation Projects of the Hubei Longzhong Laboratory(no.2022ZZ-11).
文摘As an important A_(2)B_(2)O_(7)-type ceramic,(La_(0.2)Nd_(0.2)Sm_(0.2)Gd_(0.2)Eu_(0.2))_(2)Zr_(2)O_(7)high-entropy pyrochlore pos-sesses promising properties such as high melting point,high chemical durability,and low thermal conductivity.However,the low sintering ability limits its application in thermal barrier coating and radioactive waste immobilization.It usually needs long-term high-temperature soaking to achieve full density,but with inevitable grain growth.In this work,dense and grain-refined nanocrystalline(La_(0.2)Nd_(0.2)Sm_(0.2)Gd_(0.2)Eu_(0.2))_(2)Zr_(2)O_(7)ceramics were prepared with ultra-high pressure sintering(UHPS)method under 10 GPa at a low temperature of 800℃.The densification behavior,microstructure evo-lution,and properties of the UHPS-ed samples were then investigated.The grain size of as-prepared(La_(0.2)Nd_(0.2)Sm_(0.2)Gd_(0.2)Eu_(0.2))_(2)Zr_(2)O_(7)ceramic was only 151 nm,which is 40%smaller than that of raw pow-der.In addition,it exhibited advantageous properties including both high hardness and aqueous durabil-ity.Plastic deformation under ultra-high pressure was believed as the dominant densification mechanism responsible for grain refinement and property improvement.
文摘W/ Cu functionally gradient materials （FGMs） containing 1% La2O3 and 1% TiC were prepared using graded sintering under tdtra-high pressure （GSUHP）. The specimens have been found to exhibit low porosity （11.57% and 11.35%, respectively）. Shearing strength of the specimens between layers is good. Moreover, the specimens have still demonstrated good performance in testing thermal-shock resistance. When power density of laser is 200MWm^-2, the specimens have been tested for thermal-shock resistance （1000 times）; the specimens that contained 1%La2O3 were not subjected to damage, whereas those that contained 1%TiC began to crack. Finally, effect of additives on thermal-shock resistance was also preliminarily discussed.
基金Project support by the National Research Program of China(No.50975190)
文摘（Bi_2Te_3）_（0.2）（Sb_2Te_3）_（0.8） thermoelectric material was sintered via a field activated and pressure assisted sintering（FAPAS） process.By applying different current intensity（0,60,320 A/cm^2） in the sintering process,the effects of electric current on the microstructure and thermoelectric performance were investigated.This demonstrated that the application of electric current in the sintering process could significantly improve the uniformity and density of（Bi_2Te_3）_（0.2）（Sb_2Te_3）_（0.8） samples.When the current intensity was raised to 320 A/cm^2,the preferred orientation of grains was observed.Moreover,positive effects on the thermoelectric performance of applying electric current in the sintering process were also confirmed.An increase of 0.02 and 0.11 in the maximum figure of merit ZT value could be acquired by applying current of 60 and 320 A/cm^2,respectively.
文摘Ultra-high temperature ceramics (UHTCs) are most recently getting much attention for structural parts of hypersonic missiles with their cruising speed of more than Mach 5. Most of the UHTCs are poor sinterability carbides, nitrides, and borides. Therefore, they have been studied and developed for a long time. However, there are still many problems to solve. In this paper, based on the solid-state reaction presented as an equation of (x + y)·ZrC + 2·y·B → x·ZrC + y·ZrB<sub>2</sub> + y·C, three-phase ZrC/ZrB<sub>2</sub>/C composites have been fabricated from ZrC and amorphous B powders using pulsed electric-current pressure sintering at 1373 to 2173 K for 6.0 × 10<sup>2</sup> s under 50 MPa in a vacuum. ZrC/ZrB2/C = 30/70/C~70/30/C vol% composites with the relative densities D<sub>r</sub> of 96.6 to 98.7% were obtained at 2073 K. The 60/40/C vol% composite revealed high bending strength σ<sub>b</sub> (554 MPa), Vickers hardness H<sub>v</sub> (19.2 GPa) and moderate fracture toughness K<sub>IC</sub> (5.25 MPa·m<sup>1/2</sup>) at room temperature. Furthermore, all composites showed elastic deformation up to 1873 K and revealed σ<sub>b</sub> more than 600 MPa at this temperature, in addition, some composites showed higher σ<sub>b</sub> than 900 MPa at the same temperature. These high mechanical behaviors are discussed with those of the simple binary ZrC/ZrB<sub>2</sub> composites which were fabricated under the same conditions except for their starting materials. The best mechanical properties of binary composites were σ<sub>b</sub> (474 MPa), H<sub>v</sub> (18.5 GPa), and K<sub>IC</sub> (4.45 MPa·m<sup>1/2</sup>) at room temperature, and σ<sub>b</sub> of 400 - 700 MPa at 1873 K. Overall, three-phase composites, nevertheless including soft carbon, have higher mechanical properties than the binary composites.
