Low-intensity ultrasound was applied to the pressureless consolidation of AlSi10Mg powders in a broad temperature range from 600 to 860℃.Under static conditions,the consolidation of AlSi10Mg powders can only be achie...Low-intensity ultrasound was applied to the pressureless consolidation of AlSi10Mg powders in a broad temperature range from 600 to 860℃.Under static conditions,the consolidation of AlSi10Mg powders can only be achieved at 860℃,but still with the presence of some residual unconsolidated regions.The introduction of low-intensity ultrasound at this temperature eliminates the unconsolidated regions and transforms the columnar grains observed in original directional solidification into equiaxed or globular grains.Remarkably,the application of low-intensity ultrasound significantly reduces the consolidation temperature to 620℃,without compromising the microhardness of the resulting samples when compared to static conditions.Furthermore,by lowering the temperature to 600℃,a well-sintered porous material is obtained through the assistance of the low-intensity ultrasound.展开更多
In this paper,we consider the inhomogeneous pressureless Euler equations.First,we present a class of self-similar analytical solutions to the 1D Cauchy problem and investigate the large-time behavior of the solutions,...In this paper,we consider the inhomogeneous pressureless Euler equations.First,we present a class of self-similar analytical solutions to the 1D Cauchy problem and investigate the large-time behavior of the solutions,and particularly,we obtain slant kink-wave solutions for the inhomogeneous Burgers(InhB)type equation.Next,we prove the integrability of the InhB equation in the sense of Lax pair.Furthermore,we study the spreading rate of the moving domain occupied by mass for the 1D Cauchy problem with compact support initial density.We find that the expanding domain grows exponentially in time,provided that the solutions exist and smooth at all time.Finally,we extend the corresponding results of the inhomogeneous pressureless Euler equations to the radially symmetric multi-dimensional case.展开更多
We consider the Cauchy problem for the three-dimensional pressureless Navier-Stokes/Navier-Stokes system,which consists of the pressureless Navier-Stokes equations for(n,w)coupled with the isentropic compressible Navi...We consider the Cauchy problem for the three-dimensional pressureless Navier-Stokes/Navier-Stokes system,which consists of the pressureless Navier-Stokes equations for(n,w)coupled with the isentropic compressible Navier-Stokes equations for(ρ,u)through a drag force term n(w−u).We prove the global existence of strong solutions to the coupled system when the initial data are small perturbations of the constant equilibrium state.However,due to the pressureless structure,one can only deal with the density n of the pressureless flow through the transport equation and it is crucial to obtain the exact time-decay rates for the corresponding velocity w of the pressureless flow.To this end,we make use of the spectral analysis,low-high frequency decomposition and time-weighted energy method to deduce the large time behavior of(w,ρ,u)and consequently establish the Lyapunov stability of the density n in Sobolev space.展开更多
High entropy boride ceramics have great potential as structural materials serving in extreme environ-ments.However,their applications are limited by the difficulty of sintering.In the present study,dense(Ti_(0.2)Zr_(0...High entropy boride ceramics have great potential as structural materials serving in extreme environ-ments.However,their applications are limited by the difficulty of sintering.In the present study,dense(Ti_(0.2)Zr_(0.2)Hf_(0.2)Nb_(0.2)Ta_(0.2))B_(2)ceramics with B_(4)C additions were prepared through pressureless sintering at as low as 1900℃.Calculations based on the CALPHAD approach predict that(Ti_(0.2)Zr_(0.2)Hf_(0.2)Nb_(0.2)Ta_(0.2))B_(2)starts to melt at about 3315℃whilst B_(4)C additions reduce the temperature and broaden the tempera-ture region where solid and liquid coexist.Results showed that the introduction of B_(4)C could trigger the densification of(Ti_(0.2)Zr_(0.2)Hf_(0.2)Nb_(0.2)Ta_(0.2))B_(2)at a lower temperature and promote their densification signif-icantly.The relative density of samples with 5 wt%of B_(4)C additions sintered at 1900 and 2000℃was 97.7%and 99.7%,respectively.While the sintering temperature was further increased to 2100℃,the liquid phase was reactively formed,leading to the rapid grain coarsening in samples with B_(4)C additions.Strengthened by well-dispersed B_(4)C grains,the sample with 5 wt%B_(4)C sintered at 2000℃exhibited excellent mechanical properties with the Vickers hardness,flexural strength,and fracture toughness of 21.07±2.09 GPa,547±45 MPa,and 5.24±0.14 MPa m^(1/2),which are comparable or even higher than counterparts sintered under pressure.展开更多
Ti3SiC2-reintbrced Ag-maJxix composites are expected to serve as eleclrical contacts. In this study, the wettability of Ag on a Ti3SiC2 subslxate was measured by the sessile drop melkod. The Ag-Ti3SiC2 composites were...Ti3SiC2-reintbrced Ag-maJxix composites are expected to serve as eleclrical contacts. In this study, the wettability of Ag on a Ti3SiC2 subslxate was measured by the sessile drop melkod. The Ag-Ti3SiC2 composites were prepared from Ag mad Ti3SiC2 powder mix- tures by pressureless sintering. The effects of compacting pressure （100-800 MPa）, sintering temperature （850-950~C）, mad soaking time （0.5-2 h） on the microslxucture mad properties of the Ag-Ti3SiC2 composites were investigated. The experimental results indicated that Ti3SiC2 paxticulates were uniformly distxibuted in flae Ag matrix, wiflaout reactions at the interthces between flae two phases. The prepared Ag-10wt%Ti3SiC2 had a relative density of 95% mad an electrical resistivity of 2.76 x 10 3 m~）＇cm when compacted at 800 MPa mad sintered at 950~C for 1 h. The incorporation of Ti3SiC2 into Ag was found to improve its hardness without substantially compromising its electrical conductivity; INs behavior was attxibuted to the combination of ceramic and metallic properties of the Ti3SiC2 reinforcement, suggesting its potential application in electrical contacts.展开更多
A multicomponent(TiZrHfNbTaMo)C ceramic has been fabricated by pressureless sintering at temperatures from 2100℃to 2500℃,using an equimolar multicomponent carbide powder synthesized by carbothermal reduction as the ...A multicomponent(TiZrHfNbTaMo)C ceramic has been fabricated by pressureless sintering at temperatures from 2100℃to 2500℃,using an equimolar multicomponent carbide powder synthesized by carbothermal reduction as the starting material.Influence of sintering temperature on densification,microstructure and mechanical properties of the ceramics was investigated.The relative density increases with increasing sintering temperature,and a nearly fully dense sample is achieved by pressureless sintering at 2500℃.Average grain size increases from 3.7 to 15.2μm with increasing sintering temperature from 2300 to 2500℃.The(TiZrHfNbTaMo)C ceramic sintered at 2400℃exhibits a single phase fcc structure with homogeneous chemical composition,an average grain size of 7.0μm and a relative density of96.5%,while its measured hardness is 33.2 GPa at 100 mN and 23.2 GPa at 9.8 N.展开更多
In this study, the results of measurements on pressureless sintering behavior of Ag-SnO_2(88%wt Ag,12%wt SnO_2) pellets were reported. Dilatometric measurements, relative densities, hardness values, rupture transverse...In this study, the results of measurements on pressureless sintering behavior of Ag-SnO_2(88%wt Ag,12%wt SnO_2) pellets were reported. Dilatometric measurements, relative densities, hardness values, rupture transverse strength and electrical conductivities function of sintering temperatures were presented. A constant thermal expansion coefficient was determined, and a threshold temperature of densification(T_d) was exhibited. Sintering kinetics were reported for different temperatures. Hardness values were measured, and no increase in hardness is found under Td. Three-points bending tests were used to determine the transverse rupture strength whose evolution appears importantly well under Td. In the same manner, the increase in initial electrical conductivities begins well under Td. Under the threshold temperature, the relative increase in electrical conductivity is found to be independent of initial density of green compact pellets. This work highlights different evolutions in function of sintering temperature for the electrical conductivity and transverse rupture strength on the one hand, and for the densification and hardness on the other hand.展开更多
Abstract： An alloy steel/alumina composite was successfully fabricated by pressureless infiltration of X10CrNil8-8 steel melt on 30% （mass fraction） Ni-containing alumina based composite ceramic （Ni/Al2O3） at 1 6...Abstract： An alloy steel/alumina composite was successfully fabricated by pressureless infiltration of X10CrNil8-8 steel melt on 30% （mass fraction） Ni-containing alumina based composite ceramic （Ni/Al2O3） at 1 600 ℃. The infiltration quality and interfacial bonding behavior were investigated by SEM, EDS, XRD and tensile tests. The results show that there is an obvious interfacial reaction layer between the alloying steel and the Ni/Al2O3 composite ceramic. The interfacial reactive products are （FexAly）3O4 intermetallic phase and （AlxCry）2O3 solid solution. The interracial bonding strength is as high as about 67.5 MPa. The bonding mechanism of X10CrNi 18-8 steel with the composite ceramic is that Ni inside the ceramic bodies dissolves into the alloy melt and transforms into liquid channels, consequently inducing the steel melt infiltrating and filling in the pores and the liquid channels. Moreover, the metallurgical bonding and interfacial reactive bonding also play a key role on the stability of the bonding interface.展开更多
The wetting behavior of copper alloys on SiC substrates was studied by a sessile drop technique. The microstructure of SiCp/Cu composites and the pressureless infiltration mechanism were analyzed. The results indicate...The wetting behavior of copper alloys on SiC substrates was studied by a sessile drop technique. The microstructure of SiCp/Cu composites and the pressureless infiltration mechanism were analyzed. The results indicate that Ti and Cr are effective elements to improve the wettability, while Ni, Fe, and Al have minor influence on the improvement of wettability. Non-wetting to wetting transition occurs at 1210 and 1190℃ for Cu-3Al-3Ni-9Si and Cu-3Si-2Al-1Ti, respectively. All the copper alloys react with SiC at the interface forming a reaction layer except for Cu-3Al-3Ni-9Si. High Si content favors the suppression of interracial reaction. The infiltration mechanism during pressureless infiltration is attributed to the decomposition of SiC. The beneficial effect of Fe, Ni, and Al is to favor the dissolution of SiC. The real active element during pressureless infiltration is Si.展开更多
The grain growth kinetics and densification mechanism of(TiZrHfVNbTa)C high-entropy carbide ceramic are investigated in this work.A single phase carbide with a rock-salt structure is formed until 2300°C,below whi...The grain growth kinetics and densification mechanism of(TiZrHfVNbTa)C high-entropy carbide ceramic are investigated in this work.A single phase carbide with a rock-salt structure is formed until 2300°C,below which an apparent aggregation of V,Zr and Hf exists.It is associated with the slow diffusion rate of V element as well as the relatively poor solubility of VC in HfC(as well as ZrC).The grain growth mechanism gradually changes from surface diffusion to volume diffusion and then grain boundary diffusion with increasing sintering temperature.This is attributed to the variation of activation energy of grain growth.The densification mechanism is principally dominated by the mass transport through lattice diffusion with the activation energy of 839±53 k J/mol.Through the design of two-step sintering,it is verified that the solid solution formation can effectively promote the densification process.展开更多
Fabrication of Gd2O2S：Pr scintillation ceramics by 2Gd2O3.（Gd,Pr）2（SO4）3.mH2O precursor was made Gd2O3, Pr6O11 and H2SO4 as the starting materials pressureless reaction sintering was investigated. The by hydrothe...Fabrication of Gd2O2S：Pr scintillation ceramics by 2Gd2O3.（Gd,Pr）2（SO4）3.mH2O precursor was made Gd2O3, Pr6O11 and H2SO4 as the starting materials pressureless reaction sintering was investigated. The by hydrothermal reaction using commercially available Then single phase Gd2O2SO4：Pr powder was obtained by calcining the precursor at 750℃ for 2 h. The Gd2O2SO4：Pr powder compacts can be sintered to single phase Gd2O2S：Pr ceramics with a relative density of 99% and mean grain size of 30um at 1750℃ for 2 h in flowing hydrogen atmosphere. Densification and microstructural development of the Gd2O2S：Pr ceramics were examined. Luminescence spectra of the Gd2O2S：Pr ceramic under 309 nm UV excitation and X-ray excitation show a green emission at 511 nm as the most prominent peak, which corresponds to the ^3p0-3H4 transition of Pr^3＋ ions.展开更多
A1203/5%SIC nanocomposites were fabricated by pressureless sintering using MgO as a sintering aid and then post hot-isostatic pressed （HIP）, which can subsequently break through the disadvantage of hot-pressing proc...A1203/5%SIC nanocomposites were fabricated by pressureless sintering using MgO as a sintering aid and then post hot-isostatic pressed （HIP）, which can subsequently break through the disadvantage of hot-pressing process. The MgO additive was able to promote the densification of the composites, but could not induce the grain growth of A1203 matrix due to the grain growth inhibition by nano-sized SiC particles. After HIP treatment, A12OJSiC nanocomposites achieved full densification and homogeneous distribution of nano-sized SiC particles. Moreover, the fracture morphology of HIP treated specimens was identical with that of the hot-pressed A1203/SiC nanocomposites showing complete transgranular fracture. Consequently, high fracture strength of 1 GPa was achieved for the A1203/5%SIC nanocomposites by pressureless sintering and post HIP process.展开更多
We study the existence and uniqueness problem for the nonhomogeneous pressureless Euler system with the initial density being a Radon measure. Our uniqueness result is obtained in the same space as the existence theor...We study the existence and uniqueness problem for the nonhomogeneous pressureless Euler system with the initial density being a Radon measure. Our uniqueness result is obtained in the same space as the existence theorem. Besides, by counterexample we prove that Huang-Wang’s energy condition is also necessary for our nonhomogeneous system.展开更多
ZrN-SiAlON composite materials were synthesized at 1 550 ℃ for 6 h via a carbothermal reduction nitridation route using fly ash （≤74 μm）,zircon （≤ 44 μm） and active carbon as starting materials.The processed ...ZrN-SiAlON composite materials were synthesized at 1 550 ℃ for 6 h via a carbothermal reduction nitridation route using fly ash （≤74 μm）,zircon （≤ 44 μm） and active carbon as starting materials.The processed ZrN-SiAlON composite micropowders were mixed with polyvinyl alcohol as binder to prepare ZrN （ZrON）-SiAlON composite ceramics by carbon-embedded pressureless firing at 1 450,1 500 and 1 550 ℃ for 1 h,respectively.Influences of firing temperature on the phase compositions,microstructure and sintering properties of the ceramics were investigated.The results show that：（1） β-SiAlON based composite ceramics with different compositions can be prepared by controlling firing temperature,and the main crystalline phases of the specimen fired at 1 550 ℃ for 1 h involve ZrN,ZrON and β-SiAlON （z =2,Si4Al2O2N6）; （2） ZrN （ZrON）,β-SiAlON and a Fe-Si based compound can be observed in the microstructures of the specimens fired at different temperatures.ZrN （ZrON） particles distribute homogeneously in the β-SiAlON matrix; （3） raising firing temperature can increase the shrinkage ratio of the ceramics,and the volume shrinkage ratio increases from 19.4％ to 40.3％ when the firing temperature rises from 1 450 to 1 550 ℃.展开更多
Porous silicon nitride（Si_3N_4） ceramics were fabricated by low temperature pressureless sintering, using carbonized rice husk and α-Si_3N_4 powders as raw materials, and MgO-CeO_2 as sintering additives. The effec...Porous silicon nitride（Si_3N_4） ceramics were fabricated by low temperature pressureless sintering, using carbonized rice husk and α-Si_3N_4 powders as raw materials, and MgO-CeO_2 as sintering additives. The effects of CeO_2 concentration and sintering temperature on phase composition, microstructure, porosity and flexural strength of the sintered products were investigated by X-ray diffraction（XRD）, scanning electronic microscopy（SEM）, Archimedes＇ displacement method and three-point bending strength, respectively. The results suggested that MgO-CeO_2 was a much more effective sintering additive for Si_3N_4 than MgO alone. When CeO_2 concentration was 4 wt.%–5 wt.% and sintering temperatures were in a range of 1500 to 1550 oC, the obtained porous silicon nitride ceramics had the porosity of 45.78%–42.81% and flexural strength of 49.44–84.09 MPa. Moreover, when sintering temperature was 1550 oC and CeO_2 concentration was 5%, large elongated β-Si_3N_4 grains were well developed.展开更多
We introduced a modified pressureless sintering strategy by SPS with a new T-shape die and tapered punches, which helps the evaporation of melted Al and reduces the sample sticking with the inner wall of the die. Thus...We introduced a modified pressureless sintering strategy by SPS with a new T-shape die and tapered punches, which helps the evaporation of melted Al and reduces the sample sticking with the inner wall of the die. Thus, the die breaking risk in the sintering process or the de-molding process is avoided at all. At a low temperature and short holding time, a high purity of Cr_2 AlC was obtained in this SPS process from the optimization of different molar ratios of raw materials. Simultaneously, the high porosity of the as-obtained sample was also a distinguishing feature worth noticing. The reaction mechanism for this process was also discussed in detail. This study presented a new venue for future development of high purity "MAX" materials and others related materials by a modified pressureless sintering strategy.展开更多
The microstructure of a pressureless infiltrating 55vol% oxidized SiC preform by Al-8Mg alloy was characterized by transmission electron microscopy （TEM）, high resolution TEM （HRTEM）, field emission scanning elect...The microstructure of a pressureless infiltrating 55vol% oxidized SiC preform by Al-8Mg alloy was characterized by transmission electron microscopy （TEM）, high resolution TEM （HRTEM）, field emission scanning electron microscopy （FE-SEM）, and X-ray diffraction. The TEM image of the interface between Al and SiC shows that the surface of SiC is covered by a rough nanocrystal layer of MgAl2O4, Al2O3, and Si, produced by the interfacial reaction of Al, Mg, and SiO2 on the surface of SiC. The Al-SiC interface is also examined by HRTEM to be better understood how MgAl2O4 and Al2O3 are produced. Dendritic Al2O3 crystals are embedded in the pores of the composite generated from the mutual bonding of SiO2 on the surface of SiC. Columnar AlN crystals of about 250 nm in length are bunched vertically on the SiC particle surface.展开更多
The nanocrystalline Bi2O3-Y2O3 solid electrolyte material was synthesized by pressureless reactive sintering process with Bi2O3 and Y2O3 nano mixed powder as raw materials, which was prepared by a chemical coprecipita...The nanocrystalline Bi2O3-Y2O3 solid electrolyte material was synthesized by pressureless reactive sintering process with Bi2O3 and Y2O3 nano mixed powder as raw materials, which was prepared by a chemical coprecipitation process. The study on the behavior of nano δ-Bi2O3 formation and its grain growth showed that the solid solution reaction of Y2O3 and δ-Bi2O3 to form δ-Bi2O3 occurs mainly in the initial stage of sintering process, and nano δ-Bi2O3 crystal grains grow approximately following the rule of paracurve （（D-D0）^2=K.t） during sintering process. After sintered at 600℃ for 2 h, the samples could reach above 96% in relative density and have dense microstructure with few remaining pores, the δ-Bi2O3 grains are less than 100 nm in size.展开更多
Nanometer 3Y TZP/20%Al 2O 3 (mass fraction) composite powders prepared by the chemical coprecipitation method were pressureless sintered at 1550 ℃ for 2 h. Effects of calcining temperatures at 800 ℃, 1 000 ℃, and 1...Nanometer 3Y TZP/20%Al 2O 3 (mass fraction) composite powders prepared by the chemical coprecipitation method were pressureless sintered at 1550 ℃ for 2 h. Effects of calcining temperatures at 800 ℃, 1 000 ℃, and 1 200 ℃ on phase structure, relative density, and Vicker′s hardness of the sintered bodies were studied. The results show that 1 000 ℃ was the optimal calcining temperature,and the powder calcined was composed of tetragonal zirconia with the Scherrer crystalline size of 6.3 nm. The relative density was up to 98.5% under pressureless sintering, and the sintered body was t ZrO 2(without m ZrO 2)+α Al 2O 3 with the average size of 0.4 μm.展开更多
The pressureless sintering process of Sialon ceramics added ( 1.5% La 2O 3+ 4.5% Y 2O 3) as sintering aids was studied. It is found that a β′ Sialon with relative density over 99% is obtained by sintering at 1?750?...The pressureless sintering process of Sialon ceramics added ( 1.5% La 2O 3+ 4.5% Y 2O 3) as sintering aids was studied. It is found that a β′ Sialon with relative density over 99% is obtained by sintering at 1?750?℃ for 1?h. Crystal grain growth was observed at room temperature by SEM for the samples sintered at 1?800?℃ holding for different time. It is proved that grain growth along c axis of β′ phase is a first priority, since (210) and (001) lattice plans have different activation energy. On the present conditions, logarithm of grain aspect ratio / of average grain length to average width is in proportion to sintering time because of proportion of lg to sintering time and proportion of to sintering time.展开更多
基金National Natural Science Foundation of China(Nos.52101051,52130405,51872241,52101142)Key Research Plan of Shaanxi Province,China(No.2020ZDLGY13-03)+1 种基金Key Research and Development Program of Shaanxi Province,China(No.2023-YBGY-439)Fundamental Research Funds for the Central Universities,China(No.5000210653)。
文摘Low-intensity ultrasound was applied to the pressureless consolidation of AlSi10Mg powders in a broad temperature range from 600 to 860℃.Under static conditions,the consolidation of AlSi10Mg powders can only be achieved at 860℃,but still with the presence of some residual unconsolidated regions.The introduction of low-intensity ultrasound at this temperature eliminates the unconsolidated regions and transforms the columnar grains observed in original directional solidification into equiaxed or globular grains.Remarkably,the application of low-intensity ultrasound significantly reduces the consolidation temperature to 620℃,without compromising the microhardness of the resulting samples when compared to static conditions.Furthermore,by lowering the temperature to 600℃,a well-sintered porous material is obtained through the assistance of the low-intensity ultrasound.
基金Supported by the Henan Natural Science Foundation(242300421397)the Basic Research Projects of Key Scientific Research Projects Plan in Henan Higher Education Institutions(25ZX013)+2 种基金the Scientific Research Team Plan of Zhengzhou University of Aeronautics(23ZHTD01003)the National Natural Science Foundation of China(11971475)the FLASS Internationalization and Exchange Scheme(FLASS/IE−D09/19-20−FLASS).
文摘In this paper,we consider the inhomogeneous pressureless Euler equations.First,we present a class of self-similar analytical solutions to the 1D Cauchy problem and investigate the large-time behavior of the solutions,and particularly,we obtain slant kink-wave solutions for the inhomogeneous Burgers(InhB)type equation.Next,we prove the integrability of the InhB equation in the sense of Lax pair.Furthermore,we study the spreading rate of the moving domain occupied by mass for the 1D Cauchy problem with compact support initial density.We find that the expanding domain grows exponentially in time,provided that the solutions exist and smooth at all time.Finally,we extend the corresponding results of the inhomogeneous pressureless Euler equations to the radially symmetric multi-dimensional case.
基金supported by the National Natural Science Foundation of China(11931010,12226326,12226327)the Key Research Project of Academy for Multidisciplinary Studies,Capital Normal Universitysupported by the Anhui Provincial Natural Science Foundation(2408085QA031).
文摘We consider the Cauchy problem for the three-dimensional pressureless Navier-Stokes/Navier-Stokes system,which consists of the pressureless Navier-Stokes equations for(n,w)coupled with the isentropic compressible Navier-Stokes equations for(ρ,u)through a drag force term n(w−u).We prove the global existence of strong solutions to the coupled system when the initial data are small perturbations of the constant equilibrium state.However,due to the pressureless structure,one can only deal with the density n of the pressureless flow through the transport equation and it is crucial to obtain the exact time-decay rates for the corresponding velocity w of the pressureless flow.To this end,we make use of the spectral analysis,low-high frequency decomposition and time-weighted energy method to deduce the large time behavior of(w,ρ,u)and consequently establish the Lyapunov stability of the density n in Sobolev space.
基金supported by the National Natural Science Foundation of China(Nos.52022072,52293373,52332003,52202067)the National Key R&D Programs(No.2021YFB3701400)+1 种基金the Hubei Provincial Natural Science Foundation of China(Distinguished Young Scholars 2022CFA042)the Independent Innovation Projects of Hubei Longzhong Laboratory(No.2022ZZ-10).
文摘High entropy boride ceramics have great potential as structural materials serving in extreme environ-ments.However,their applications are limited by the difficulty of sintering.In the present study,dense(Ti_(0.2)Zr_(0.2)Hf_(0.2)Nb_(0.2)Ta_(0.2))B_(2)ceramics with B_(4)C additions were prepared through pressureless sintering at as low as 1900℃.Calculations based on the CALPHAD approach predict that(Ti_(0.2)Zr_(0.2)Hf_(0.2)Nb_(0.2)Ta_(0.2))B_(2)starts to melt at about 3315℃whilst B_(4)C additions reduce the temperature and broaden the tempera-ture region where solid and liquid coexist.Results showed that the introduction of B_(4)C could trigger the densification of(Ti_(0.2)Zr_(0.2)Hf_(0.2)Nb_(0.2)Ta_(0.2))B_(2)at a lower temperature and promote their densification signif-icantly.The relative density of samples with 5 wt%of B_(4)C additions sintered at 1900 and 2000℃was 97.7%and 99.7%,respectively.While the sintering temperature was further increased to 2100℃,the liquid phase was reactively formed,leading to the rapid grain coarsening in samples with B_(4)C additions.Strengthened by well-dispersed B_(4)C grains,the sample with 5 wt%B_(4)C sintered at 2000℃exhibited excellent mechanical properties with the Vickers hardness,flexural strength,and fracture toughness of 21.07±2.09 GPa,547±45 MPa,and 5.24±0.14 MPa m^(1/2),which are comparable or even higher than counterparts sintered under pressure.
基金financially supported by the National Natural Science Foundation of China(Nos.51731004,51671054,and 51501038)“the Fundamental Research Funds for the Central Universities”in China
文摘Ti3SiC2-reintbrced Ag-maJxix composites are expected to serve as eleclrical contacts. In this study, the wettability of Ag on a Ti3SiC2 subslxate was measured by the sessile drop melkod. The Ag-Ti3SiC2 composites were prepared from Ag mad Ti3SiC2 powder mix- tures by pressureless sintering. The effects of compacting pressure （100-800 MPa）, sintering temperature （850-950~C）, mad soaking time （0.5-2 h） on the microslxucture mad properties of the Ag-Ti3SiC2 composites were investigated. The experimental results indicated that Ti3SiC2 paxticulates were uniformly distxibuted in flae Ag matrix, wiflaout reactions at the interthces between flae two phases. The prepared Ag-10wt%Ti3SiC2 had a relative density of 95% mad an electrical resistivity of 2.76 x 10 3 m~）＇cm when compacted at 800 MPa mad sintered at 950~C for 1 h. The incorporation of Ti3SiC2 into Ag was found to improve its hardness without substantially compromising its electrical conductivity; INs behavior was attxibuted to the combination of ceramic and metallic properties of the Ti3SiC2 reinforcement, suggesting its potential application in electrical contacts.
基金financially supported by the National Natural Science Foundation of China(Nos.51972081,51602074,51872061,51621091)Natural Science Foundation of Heilongjiang Province(Grant No.E2016026)。
文摘A multicomponent(TiZrHfNbTaMo)C ceramic has been fabricated by pressureless sintering at temperatures from 2100℃to 2500℃,using an equimolar multicomponent carbide powder synthesized by carbothermal reduction as the starting material.Influence of sintering temperature on densification,microstructure and mechanical properties of the ceramics was investigated.The relative density increases with increasing sintering temperature,and a nearly fully dense sample is achieved by pressureless sintering at 2500℃.Average grain size increases from 3.7 to 15.2μm with increasing sintering temperature from 2300 to 2500℃.The(TiZrHfNbTaMo)C ceramic sintered at 2400℃exhibits a single phase fcc structure with homogeneous chemical composition,an average grain size of 7.0μm and a relative density of96.5%,while its measured hardness is 33.2 GPa at 100 mN and 23.2 GPa at 9.8 N.
基金financially supported by the French National Research Agency REF ANR(No.ANR-09-MAPR-0007-MAPR)
文摘In this study, the results of measurements on pressureless sintering behavior of Ag-SnO_2(88%wt Ag,12%wt SnO_2) pellets were reported. Dilatometric measurements, relative densities, hardness values, rupture transverse strength and electrical conductivities function of sintering temperatures were presented. A constant thermal expansion coefficient was determined, and a threshold temperature of densification(T_d) was exhibited. Sintering kinetics were reported for different temperatures. Hardness values were measured, and no increase in hardness is found under Td. Three-points bending tests were used to determine the transverse rupture strength whose evolution appears importantly well under Td. In the same manner, the increase in initial electrical conductivities begins well under Td. Under the threshold temperature, the relative increase in electrical conductivity is found to be independent of initial density of green compact pellets. This work highlights different evolutions in function of sintering temperature for the electrical conductivity and transverse rupture strength on the one hand, and for the densification and hardness on the other hand.
基金Project(2009ZM0296) supported by the Fundamental Research Funds for the Central Universities in China
文摘Abstract： An alloy steel/alumina composite was successfully fabricated by pressureless infiltration of X10CrNil8-8 steel melt on 30% （mass fraction） Ni-containing alumina based composite ceramic （Ni/Al2O3） at 1 600 ℃. The infiltration quality and interfacial bonding behavior were investigated by SEM, EDS, XRD and tensile tests. The results show that there is an obvious interfacial reaction layer between the alloying steel and the Ni/Al2O3 composite ceramic. The interfacial reactive products are （FexAly）3O4 intermetallic phase and （AlxCry）2O3 solid solution. The interracial bonding strength is as high as about 67.5 MPa. The bonding mechanism of X10CrNi 18-8 steel with the composite ceramic is that Ni inside the ceramic bodies dissolves into the alloy melt and transforms into liquid channels, consequently inducing the steel melt infiltrating and filling in the pores and the liquid channels. Moreover, the metallurgical bonding and interfacial reactive bonding also play a key role on the stability of the bonding interface.
基金supported by the National High-Tech Research and Development Program of China (No.2006AA03Z557)the Major State Basic Research and Development Program of China (No.2006CB605207)+1 种基金the National Nature Science Foundation of China (No.5063410)the MOE Program for Changjiang Scholars and Innovative Research Team in Universities (No.I2P407)
文摘The wetting behavior of copper alloys on SiC substrates was studied by a sessile drop technique. The microstructure of SiCp/Cu composites and the pressureless infiltration mechanism were analyzed. The results indicate that Ti and Cr are effective elements to improve the wettability, while Ni, Fe, and Al have minor influence on the improvement of wettability. Non-wetting to wetting transition occurs at 1210 and 1190℃ for Cu-3Al-3Ni-9Si and Cu-3Si-2Al-1Ti, respectively. All the copper alloys react with SiC at the interface forming a reaction layer except for Cu-3Al-3Ni-9Si. High Si content favors the suppression of interracial reaction. The infiltration mechanism during pressureless infiltration is attributed to the decomposition of SiC. The beneficial effect of Fe, Ni, and Al is to favor the dissolution of SiC. The real active element during pressureless infiltration is Si.
基金financially supported by the National Natural Science Foundation of China(Nos.51972081,52032002 and 51872061)Heilongjiang Touyan Team Programthe Foundation of Science and Technology on Particle Transport and Separation Laboratory。
文摘The grain growth kinetics and densification mechanism of(TiZrHfVNbTa)C high-entropy carbide ceramic are investigated in this work.A single phase carbide with a rock-salt structure is formed until 2300°C,below which an apparent aggregation of V,Zr and Hf exists.It is associated with the slow diffusion rate of V element as well as the relatively poor solubility of VC in HfC(as well as ZrC).The grain growth mechanism gradually changes from surface diffusion to volume diffusion and then grain boundary diffusion with increasing sintering temperature.This is attributed to the variation of activation energy of grain growth.The densification mechanism is principally dominated by the mass transport through lattice diffusion with the activation energy of 839±53 k J/mol.Through the design of two-step sintering,it is verified that the solid solution formation can effectively promote the densification process.
基金supported by the National Natural Sci-ence Foundation of China (Grant. No. 50672014)the National Science Fund for Distinguished Young Scholars,China (Grant No. 50425413)
文摘Fabrication of Gd2O2S：Pr scintillation ceramics by 2Gd2O3.（Gd,Pr）2（SO4）3.mH2O precursor was made Gd2O3, Pr6O11 and H2SO4 as the starting materials pressureless reaction sintering was investigated. The by hydrothermal reaction using commercially available Then single phase Gd2O2SO4：Pr powder was obtained by calcining the precursor at 750℃ for 2 h. The Gd2O2SO4：Pr powder compacts can be sintered to single phase Gd2O2S：Pr ceramics with a relative density of 99% and mean grain size of 30um at 1750℃ for 2 h in flowing hydrogen atmosphere. Densification and microstructural development of the Gd2O2S：Pr ceramics were examined. Luminescence spectra of the Gd2O2S：Pr ceramic under 309 nm UV excitation and X-ray excitation show a green emission at 511 nm as the most prominent peak, which corresponds to the ^3p0-3H4 transition of Pr^3＋ ions.
基金Project supported by Pusan National University Research GrantProject(2010-0008-276) supported by National Core Research Center Program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology
文摘A1203/5%SIC nanocomposites were fabricated by pressureless sintering using MgO as a sintering aid and then post hot-isostatic pressed （HIP）, which can subsequently break through the disadvantage of hot-pressing process. The MgO additive was able to promote the densification of the composites, but could not induce the grain growth of A1203 matrix due to the grain growth inhibition by nano-sized SiC particles. After HIP treatment, A12OJSiC nanocomposites achieved full densification and homogeneous distribution of nano-sized SiC particles. Moreover, the fracture morphology of HIP treated specimens was identical with that of the hot-pressed A1203/SiC nanocomposites showing complete transgranular fracture. Consequently, high fracture strength of 1 GPa was achieved for the A1203/5%SIC nanocomposites by pressureless sintering and post HIP process.
文摘We study the existence and uniqueness problem for the nonhomogeneous pressureless Euler system with the initial density being a Radon measure. Our uniqueness result is obtained in the same space as the existence theorem. Besides, by counterexample we prove that Huang-Wang’s energy condition is also necessary for our nonhomogeneous system.
基金the financial supports from the National Natural Science Foundation of China ( 51274057 )Fundamental Research Funds for the Central Universities ( N120402006)Educational Commission of Liaoning Province of China ( L2012079)
文摘ZrN-SiAlON composite materials were synthesized at 1 550 ℃ for 6 h via a carbothermal reduction nitridation route using fly ash （≤74 μm）,zircon （≤ 44 μm） and active carbon as starting materials.The processed ZrN-SiAlON composite micropowders were mixed with polyvinyl alcohol as binder to prepare ZrN （ZrON）-SiAlON composite ceramics by carbon-embedded pressureless firing at 1 450,1 500 and 1 550 ℃ for 1 h,respectively.Influences of firing temperature on the phase compositions,microstructure and sintering properties of the ceramics were investigated.The results show that：（1） β-SiAlON based composite ceramics with different compositions can be prepared by controlling firing temperature,and the main crystalline phases of the specimen fired at 1 550 ℃ for 1 h involve ZrN,ZrON and β-SiAlON （z =2,Si4Al2O2N6）; （2） ZrN （ZrON）,β-SiAlON and a Fe-Si based compound can be observed in the microstructures of the specimens fired at different temperatures.ZrN （ZrON） particles distribute homogeneously in the β-SiAlON matrix; （3） raising firing temperature can increase the shrinkage ratio of the ceramics,and the volume shrinkage ratio increases from 19.4％ to 40.3％ when the firing temperature rises from 1 450 to 1 550 ℃.
基金Project supported by the research fund of Collaborative Innovation Center for Ecological Building Materials(GX2015304,CP201506)Jiangsu Provincial Natural Science Foundation(BK20150427)
文摘Porous silicon nitride（Si_3N_4） ceramics were fabricated by low temperature pressureless sintering, using carbonized rice husk and α-Si_3N_4 powders as raw materials, and MgO-CeO_2 as sintering additives. The effects of CeO_2 concentration and sintering temperature on phase composition, microstructure, porosity and flexural strength of the sintered products were investigated by X-ray diffraction（XRD）, scanning electronic microscopy（SEM）, Archimedes＇ displacement method and three-point bending strength, respectively. The results suggested that MgO-CeO_2 was a much more effective sintering additive for Si_3N_4 than MgO alone. When CeO_2 concentration was 4 wt.%–5 wt.% and sintering temperatures were in a range of 1500 to 1550 oC, the obtained porous silicon nitride ceramics had the porosity of 45.78%–42.81% and flexural strength of 49.44–84.09 MPa. Moreover, when sintering temperature was 1550 oC and CeO_2 concentration was 5%, large elongated β-Si_3N_4 grains were well developed.
文摘We introduced a modified pressureless sintering strategy by SPS with a new T-shape die and tapered punches, which helps the evaporation of melted Al and reduces the sample sticking with the inner wall of the die. Thus, the die breaking risk in the sintering process or the de-molding process is avoided at all. At a low temperature and short holding time, a high purity of Cr_2 AlC was obtained in this SPS process from the optimization of different molar ratios of raw materials. Simultaneously, the high porosity of the as-obtained sample was also a distinguishing feature worth noticing. The reaction mechanism for this process was also discussed in detail. This study presented a new venue for future development of high purity "MAX" materials and others related materials by a modified pressureless sintering strategy.
基金supported by the National Natural Science Foundation of China (No.51004010)
文摘The microstructure of a pressureless infiltrating 55vol% oxidized SiC preform by Al-8Mg alloy was characterized by transmission electron microscopy （TEM）, high resolution TEM （HRTEM）, field emission scanning electron microscopy （FE-SEM）, and X-ray diffraction. The TEM image of the interface between Al and SiC shows that the surface of SiC is covered by a rough nanocrystal layer of MgAl2O4, Al2O3, and Si, produced by the interfacial reaction of Al, Mg, and SiO2 on the surface of SiC. The Al-SiC interface is also examined by HRTEM to be better understood how MgAl2O4 and Al2O3 are produced. Dendritic Al2O3 crystals are embedded in the pores of the composite generated from the mutual bonding of SiO2 on the surface of SiC. Columnar AlN crystals of about 250 nm in length are bunched vertically on the SiC particle surface.
基金financially supported by the National Nature Science Foundation of China (No.20101006)the Nano Technology Special Foundation of Shanghai Science and Technology Committee (No.O452nm073)
文摘The nanocrystalline Bi2O3-Y2O3 solid electrolyte material was synthesized by pressureless reactive sintering process with Bi2O3 and Y2O3 nano mixed powder as raw materials, which was prepared by a chemical coprecipitation process. The study on the behavior of nano δ-Bi2O3 formation and its grain growth showed that the solid solution reaction of Y2O3 and δ-Bi2O3 to form δ-Bi2O3 occurs mainly in the initial stage of sintering process, and nano δ-Bi2O3 crystal grains grow approximately following the rule of paracurve （（D-D0）^2=K.t） during sintering process. After sintered at 600℃ for 2 h, the samples could reach above 96% in relative density and have dense microstructure with few remaining pores, the δ-Bi2O3 grains are less than 100 nm in size.
文摘Nanometer 3Y TZP/20%Al 2O 3 (mass fraction) composite powders prepared by the chemical coprecipitation method were pressureless sintered at 1550 ℃ for 2 h. Effects of calcining temperatures at 800 ℃, 1 000 ℃, and 1 200 ℃ on phase structure, relative density, and Vicker′s hardness of the sintered bodies were studied. The results show that 1 000 ℃ was the optimal calcining temperature,and the powder calcined was composed of tetragonal zirconia with the Scherrer crystalline size of 6.3 nm. The relative density was up to 98.5% under pressureless sintering, and the sintered body was t ZrO 2(without m ZrO 2)+α Al 2O 3 with the average size of 0.4 μm.
文摘The pressureless sintering process of Sialon ceramics added ( 1.5% La 2O 3+ 4.5% Y 2O 3) as sintering aids was studied. It is found that a β′ Sialon with relative density over 99% is obtained by sintering at 1?750?℃ for 1?h. Crystal grain growth was observed at room temperature by SEM for the samples sintered at 1?800?℃ holding for different time. It is proved that grain growth along c axis of β′ phase is a first priority, since (210) and (001) lattice plans have different activation energy. On the present conditions, logarithm of grain aspect ratio / of average grain length to average width is in proportion to sintering time because of proportion of lg to sintering time and proportion of to sintering time.
