800密度等级的渣土陶粒制备及性能研究

为进一步促进城市渣土资源化利用,本文研制了800密度等级,粒径不同(10~15 mm,15~25 mm)的渣土陶粒.探究了原料配方、烧制工艺对渣土陶粒性能的影响规律,同时采用超景深光学显微镜和扫描电镜对渣土陶粒的微观结构进行了分析.研究结果表明:(1)渣土:粉煤灰质量配比为75∶25时,预热温度500 ℃,预热时间20 min,焙烧时间15 min,焙烧温度1190 ℃下,可制备出不同粒径(10~15 mm,15~25 mm)的800密度等级渣土陶粒;(2)15~25 mm粒径的渣土陶粒,筒压强度为4.6 MPa,堆积密度729 kg/m3,表观密度1329 kg/m3,1 h吸水率为1.7%,烧失量1.4%;10~15 mm粒径的渣土陶粒,筒压强度为5.2 MPa,堆积密度760 kg/m3,表观密度1483 kg/m3,1 h吸水率为1.6%,烧失量1.4%;(3)不同粒径下的渣土陶粒微观结构均比较疏松,其中小粒径渣土陶粒内部结构相比大粒径较疏松,孔隙较多,孔径较大.     

Experimental study on high temperature performances of silica-based ceramic core for single crystal turbine blades

Silica-based ceramic cores are widely utilized for shaping the internal cooling canals of single crystal superalloy turbine blades. The thermal expansion behavior, creep resistance, and high temperature flexural strength are critical for the quality of turbine blades. In this study, the influence of zircon, particle size distribution, and sintering temperature on the high-temperature performance of silica-based ceramic cores were investigated. The results show that zircon is beneficial for narrowing the contraction temperature range and reducing the shrinkage, improving the creep resistance and high-temperature flexural strength significantly. Mixing coarse, medium and fine fused silica powders in a ratio of 5:3:2, not only reduced high temperature contraction, but effectively improved the creep resistance. Properly increasing the sintering temperature can slightly reduce the thermal deformation and improve the high-temperature flexural strength of the silica-based core, but excessively high sintering temperature negatively impacts the creep resistance and high-temperature flexural strength.     

Electrical and nonlinear current-voltage characteristics of La-doped BiFeO3 ceramics

Multiferroic Bi_1-xLa_xFeO₃(x=0, 0.05, 0.1, 0.2, and 0.3) ceramics with particle sizes of ~67–19 nm were prepared by a simple co-precipitation method. The effects of La dopant on the microstructure, giant dielectric response, and electrical properties were investigated. The grain size of Bi_1-xLa_xFeO₃ ceramics significantly decreased with increasing La doping ions. The Bi_0.95La_0.05FeO₃ ceramic exhibited the highest leakage current density value. Interestingly, it strongly decreased as the concentration of La increased. The nonlinear coefficient of La doped BFO slightly decreased with increasing La. This shows a space-charge-limited conduction mechanism, which is involved in low electric field regions for all samples investigated. La substitution significantly enhanced the breakdown field. It was found that the potential barrier height at the grain boundary was slightly reduced from 0.3 to 0.16 eV by substitution of La ions. Using impedance spectroscopy analysis, except for the Bi_0.7La_0.3FeO₃ ceramic, the grain boundary resistance at room temperature was affected by dc bias, whereas the grain resistance of all samples was independent of dc bias. This result was well consistent with the variation in low-frequency dielectric constant and loss tangent value due to the effect of dc bias. These results were closely related to the existence of the interfacial polarization at the grain boundary.     

Nanocomposite Derived from Core–Shell Nanoparticles via Creep Deformation of an Amorphous Silica Layer Below its Glass Transition Temperature

The microstructural design of ceramics is relevant to tune their properties. Nanostructuring can drastically modify ceramic properties because of enhanced interfacial effects, although the creation of such structures in ceramics is still challenging because of the interfacial reaction and grain growth at elevated temperatures during sintering. Here, we demonstrate densification of core – shell nanoparticles consisting of Fe₃O₄ (core particle, 20 nm diameter) and SiO₂ (shell layer, 2 nm thick) with over 90% of theoretical density below 500°C, which was achieved by facilitating plastic flow of amorphous SiO₂ under high pressure below its glass transition temperature. Thus, grain growth of the core nanoparticles was strongly suppressed, and the core nanoparticles remained separated by an amorphous layer in the final microstructure reflecting the original core–shell nanostructure. We also analyzed the densification behavior on the basis of a power law creep model, and estimated the pressures required to attain full density.     

Thermal conductivity of pressed powder compacts: tin oxide and alumina

Three aspects, which significantly reduce heat transfer through a polycrystalline material, are considered in this paper: porosity, grain boundary thermal resistance and the state of the grain–grain contacts. Tin oxide and alumina were chosen as model systems. Tin oxide, without a sintering additive, does not densify during thermal treatment but grain growth is not inhibited and consequently the microstructure can be varied. In alumina, variation of the thermal treatment conditions varies both grain size and porosity. Thermal conductivity measurements, using the laser-flash technique, reveal that the thermal resistance of a pressed powder compact is almost independent of temperature and at least a factor of 2.5 greater than a consolidated material with similar pore volume fraction and grain size. The reduced contact area of the grain–grain interfaces in the green body can explain this as demonstrated by numerical simulation. We also show that larger grain size increases the thermal conductivity of the porous ceramic.     

The influence of branch length on the deformation and microstructure of polyethylene

The length and number of side chain branches have a profound influence on the microstructure and physical properties of polyethylene (PE). For a series of linear PE copolymers: environmental stress cracking resistance (ESCR), melting points, creep resistance and modulus, and equilibrium spherulite size were all found to increase with increasing branch length (methyl to hexyl) at a given density and molecular weight. It is proposed that (at a fixed molecular weight) branch length and branch concentration determine spherulite size and, consequently, spherulitic boundary areas, in which the dry crazing/voiding occurs during the incubation period of environmental stress cracking (ESC). At a fixed density, decreased spherulite size contributes to greater spherulite boundary slip and increased creep at low (less than 2 MPa) stresses.     

Creep-Sintering and Microstructure Development of Heterogeneous MgO Compacts

Simultaneous creep and densification and the microstructure development of magnesium oxide powder compacts were studied at 125°C and for applied stresses of up to 0.25 MPa. Die-pressing the powder into compacts with a relative green density of ∼0.40 led to an approximately bimodal distribution of pores, with one fraction having sizes of the order of 10 times the (initial) particle size and the other fraction having pore sizes of the order of the particle size. The presence of the large pores in turn gave rise to rather unusual sintering effects. After first decreasing with relative density (ρ), the densification rate (dρ/dt) and the creep rate (dɛ/dt) then increased dramatically for 0.6 < ρ < 0.75. This range of ρ corresponded to the stage of microstructure development when grain growth and coalescence of the smaller pores have created a more uniform pore distribution. Above ρ∼ 0.75, both dρ/dt and dɛ/dt again decreased with ρ. These trends in the densification behavior are discussed in terms of material parameters such as the equilibrium dihedral angle and the pore coordination number.     

无压烧结Al2O3／SiC纳米复相陶瓷的研究

将粒径为30～35nm的β-SiC粉,加入亚微米尺寸的α-Al     

Hot Isostatic Pressing of Si3N4 Powder Compacts and Reaction-Bonded Si3N4

Hot isostatically pressed silicon nitride was produced by densifying Si₃N₄ powder compacts and reaction-bonded Si₃N₄ (RBSN) parts with yttria as a sintering additive. The microstructure was analyzed using scanning electron microscopy, X-ray diffraction, and density measurements. The influence of the microstructure on fracture strength, creep, and oxidation behavior was investigated. It is assumed that the higher amount of oxygen in the Si₃N₄ starting powder compared with the RBSN starting material leads to an increased amount of liquid phase during densification. This results in grain growth and in a larger amount of grain boundary phase in the hot isostatically pressed material. Compared with the hot isostatically pressed RBSN samples therefore, strength decreases whereas the creep rate and the weight gain during oxidation increase.     

Micromechanics modeling of creep fracture of zirconium diboride–silicon carbide composites at 1400–1700 °C

The creep deformation of the ultra-high temperature ceramic composite ZrB₂–20%SiC at temperatures from 1400 to 1700 °C was studied by a micromechanical mode in which the real microstructure was adopted in finite element simulations. Based on the experiment results of the change of activation energy with respect to the temperature, a mechanism shift from diffusional creep-control for temperatures below 1500 °C to grain boundary sliding-control for temperatures above 1500 °C was concluded from simulations. Also, the simulation results revealed the accommodation of grain rotation and grain boundary sliding by grain boundary cavitation for creep at temperatures above 1500 °C which was in agreement with experimental observations.     

Role of powder properties and shaping techniques on the formation of pore-free YAG materials

A key feature of transparent ceramics is the absence of residual porosity because boundary between pores and ceramic grains is the origin for light scattering. Powders characterized by a grain size in the nanometric range are generally adopted for obtaining transparent ceramics because of their superior reactivity, but the formation of undesired secondary phase related to the presence of impurities, is observed. The present study shows the results obtained with alternative, highly pure, micrometric powders with two shaping techniques, cold isostatic pressing (CIP) and slip casting (SC). The powder treatment and shaping process are easier when coarser powders are adopted. The influence of the powder properties and of the dispersant system on the particle packing, on the density and on the porosity are studied in relation to the two shaping techniques. The role of the aforementioned features on the final microstructure and on the optical properties are also discussed.     

Effect of particle size on the shaping of ceramics by slip casting

The effect of the nanometric-ranged particle size of the starting powder through a simple and well-established shaping method, slip casting, has been studied. Several alumina suspensions with the same viscosity (but different solid content suspensions) and different particle size (11, 44, 190 and 600 nm) were prepared and shaped into a dense body. The green and sintered densities ranged between 30–67% and 63–99% of the theoretical value, respectively. These values, together with the microstructure observations reveal the effect of the solid content of the suspensions and the characteristics of the ceramic powder, leading to the determination of an optimal particle size. Based on both processability (rheological behaviour) and microstructure (density and grain size) it has been determined that particles with sizes ranging 100–300 nm are the best for preparing concentrated suspensions with low viscosity and bodies with density close to the theoretical value when using conventional pressureless sintering densification.     

Influence of processing parameters on the densification and the microstructure of pure zinc oxide ceramics prepared by spark plasma sintering

Due to the sensitivity of nanopowders and the challenges in controlling the grain size and the density during the sintering of ceramics, a systematic study was proposed to evaluate the densification and the microstructure of ZnO ceramics using spark plasma sintering technique. Commercially available ZnO powder was dried and sintered at various parameters (temperature (400–900?°C), pressure (250–850?MPa), atmosphere (Air/Vacuum) etc.). High pressure sintering is desirable for maintaining the nanostructure, though it brings a difficulty in obtaining a fully dense ceramic. Whereas, increasing the temperature from 600 to 900?°C results in fully densified ceramics of about 99% which shows to have big impact on the grain size. However, a high relative density of 92% is obtained at a temperature as low as 400?°C under a pressure of 850?MPa. The application of pressure during the holding time seems to lower the grain size as compared to ceramics pressed during initial stage (room temperature).     

Transmittance enhancement of spark plasma sintered CaF2 ceramics by preheating commercial powder

Spark plasma sintering (SPS) is a convenient approach for preparing transparent CaF₂ ceramics. However, carbon contamination is a key issue that should be addressed to achieve high transparency. In this study, a commercially available CaF₂ powder was preheated under vacuum before performing SPS to mitigate carbon contamination. During the preheating of the CaF₂ powder, impurities adsorbed on the particle surface, such as H₂O, CO₂, and O₂, are desorbed. Moreover, the interdiffusion of carbon contaminants is suppressed due to the pre-sintering of the raw powder. The in-line transmittance of the CaF₂ ceramic prepared from the preheated powder increased to 85 % at the wavelength of 1100 nm, which is 38 % higher than that of the ceramic prepared without preheating. In addition, the in-line transmittance increased with increasing grain size of the ceramic, possibly because of the decrease in the number of scattering sources with the reduction in the grain boundary fraction.     

High-temperature creep of carbon nanofiber-reinforced and graphene oxide-reinforced alumina composites sintered by spark plasma sintering

Alumina (Al₂O₃) ceramic composites reinforced with either graphene oxide (GO) or carbon nanofibers (CNFs) were prepared using Spark Plasma Sintering. The effects of GO and CNFs on the microstructure and in consequence on their mechanical properties were investigated. The microstructure of the sintered materials have been characterized quantitatively prior to and after the creep experiments in order to discover the deformation mechanism. Graphene-oxide reinforced alumina composites were found to be more creep resistant than carbon nanofibers-reinforced alumina ones or monolithic alumina with the same grain size distribution. In all the cases, grain boundary sliding was identified as the deformation mechanism.     

Fine-Grained Silicon Nitride Ceramics Prepared from β-Powder

Fine β-powder with an average particle size of 0.28 μm was prepared by grinding and centrifugal sedimentation of sub-micrometer β-powder. Fine- and uniform-grained ceramics were fabricated from the powder by hot pressing. The average grain size of the ceramic was 0.21 μm. It was shown that this kind of microstructure was desirable for the matrix of in situ composite. It was also shown that the ceramics could be superplastically deformed at a temperature as low as 1500°C.     

Sintering kinetics and properties of NiFe2O4-based ceramics inert anodes doped with TiN nanoparticles

Sintering kinetics of NiFe₂O₄-based ceramics inert anodes for aluminum electrolysis doped 7 wt% TiN nanoparticles were conducted to investigate densification and grain growth behaviors. The linear shrinkage increased gradually with the increasing sintering temperature between 1000 and 1450°C, whereas the linear shrinkage rate exhibited a broad peak. The maximum linear shrinkage rate was obtained at 1189.4°C, and the highest densification rate was achieved at the relative density of 75.20%. Based on the pressureless sintering kinetics window, the sintering process was divided into the initial stage, the intermediate stage, and the final stage. The grain growth exponent reduced with increased sintering temperature, whereas the grain growth activation energy decreased by increasing sintering temperature and shortening dwelling time. The grain growth was mainly controlled by atomic diffusion. NiFe₂O₄-based ceramics possessed high-temperature semiconductor essential characteristics. The electrical conductivity of NiFe₂O₄-based ceramics first increased and then decreased with increasing sintering temperature, reached their maximum value (960°C) of 33.45 S/cm under 1300°C, mainly attributed to the relatively dense and uniform microstructure. The thermal shock resistance of NiFe₂O₄-based ceramic was improved by a stronger grain boundary bonding strength and lower coefficient of linear thermal expansion.     

Pb(Zr0.53Ti0.47)O3陶瓷超细粉体制备及分散研究

用化学共沉淀法合成了陶瓷Pb(Zr0.53Ti0.47)O3超细粉体,以聚乙二醇(PEG)为分散剂对所得粉体进行分散。通过X衍射、扫描电镜分析,研究了合成条件和煅烧温度对PZT粉体性能的影响。结果表明,随着煅烧温度的升高,PZT超细粉晶化度提高,一次晶粒尺寸增加,而二次粒子尺寸却减小。当分散剂分子量为10000,用量为1%时,得到分散良好的超细粉体。测定了用合成的粉体烧结的PZT陶瓷的压电性能。     

A review of two-step sintering for ceramics

The development of high-density ceramic materials with fine-grained microstructures has been studied to considerably improve their properties for high-performance applications. Many alternatives have been searched to refine their microstructure by changing their composition and/or processing. Among such alternatives, the densification of ceramic materials by sintering curve control is an effective, simple and economical microstructure refinement method. Thus, different thermal treatment techniques such as spark plasma sintering and microstructural forms of control such as the control of sintering conditions have been used to obtain nanostructured materials. One of the techniques widely used in recent years is two-step sintering. Two-step sintering (TSS) is a promising method used to obtain high-density bodies and smaller grain sizes. Two TSS methodologies are known: sintering with thermal pretreatment at a low sintering temperature, followed by a second stage at elevated temperature, and the more recent approach presented by Chen and Wang, which has been the most widely used. In addition to the sintering conditions (temperature, heating rate and sintering holding times) that must be suitable for each composition type, the starting materials, particle size and processing method may influence the obtained microstructure, especially the reduced grain size and increased densification. The current review of two-step sintering presents the effect of this technique on the grain density and sizes of different ceramic materials. The influence of the addition of doping agents and its effect on the mechanical properties in different systems is also presented in the current study.     

Influence of electric current on microstructure and electrical property of Al-doped ZnO ceramic consolidated by spark plasma sintering

Electrical current passing through SPS-ed Al-doped ZnO (AZO) ceramic in four conditions with same heating rate and dwell time has been conducted to investigate the influence of current on microstructure and electrical property. The applied current and thermal distribution of AZO ceramic are analyzed by finite element modeling. The local microstructure and electrical property of SPS-ed AZO in center and edge has been discussed in comparison. It is indicated that current density determines the thermal distribution and the coupling effects of current/thermal significantly influence the grain size, impurity phase ZnAl₂O₄ and dislocation defects. The increased current density contributes to the refinement of grain size and enhancement of impurity ZnAl₂O₄ which dramatically affects the resistivity. The elevated Hall mobility of SPS-ed AZO ceramic by the reduction of current density is mainly governed by weakening grain boundary scattering due to the enlarged grain size, which is confirmed to be the crucial factor on the variation of resistivity. The lowest resistivity of 6.1 × 10⁻⁴ Ω cm for SPS-ed AZO ceramic is achieved with extremely low current density passing through specimens.