板状氧化铝和电熔法板状刚玉的晶体形貌

ＡＬＣＯＡ板状氧化铝和电熔法板状刚玉系具有板片状晶体形貌的α－Ａｌ２Ｏ３多晶集合体──具有裂理面的聚片双晶     

不同烧结气氛下SiAlON结合刚玉材料的烧结行为和显微结构

以α-Al_2O_3微粉和Si_3N_4粉为主要原料,分别以Al粉、AlN粉、SiO_2微粉、Al粉 Ce_2O_3粉、Al粉 Si粉 Ce_2O_3粉作添加剂,在空气中裸烧(氧化气氛)和空气中埋炭(还原气氛)的条件下,分别进行1350℃、1450℃、1550℃、1600℃保温6h的热处理后,制备了SiAlON结合刚玉复相材料,并研究了烧结气氛、烧结温度和添加剂种类对试样烧结行为和显微结构的影响。结果表明添加稀土氧化物Ce_2O_3或少量SiO_2微粉能促进材料的烧结;在氧化气氛下,以SiO_2微粉为添加剂的试样的致密化程度随处理温度的升高而降低,而在埋炭还原气氛下,其致密化程度随温度的升高而提高;SEM观察还表明,含不同添加剂的试样在不同气氛中处理后的显微结构也不同。     

Sintering of NaCl powder: Mechanisms and first stage kinetics

Isothermal NaCl sintering experiments are conducted between 500 and 790 °C on powders 75, 100, 150 and 400 μm in average diameter. Along with literature data, the results are compared with theoretical predictions for the initial stage of sintering, to draw a general picture of the process over a wide spectrum of temperatures and particle sizes. Where densification is observed, the mechanism is grain boundary diffusion of material from boundary sources. As observed in previous work, there is a transition in dominance between this mechanism and the alternative, non-densifying, mechanism of evaporation–condensation. More detailed analysis shows that, after initial and effectively instantaneous plastic yielding driven by the surface energy, boundary diffusion will always dominate the initial stage of sintering for the conditions of this investigation, leading to some (possibly very small) degree of densification. For particles above about 150 μm at temperatures within 300 °C of the melting point of NaCl this mechanism will rapidly be displaced by evaporation–condensation, halting densification. An approximate equation is derived for the rate of sintering by evaporation–condensation in the presence of a finite-pressure inert atmosphere; the rate of this mechanism is found to be significantly reduced when sintering is carried out at pressures approaching atmospheric.     

Investigation of the intra-particle sintering kinetics of a mainly agglomerated alumina powder by using surface area reduction

An alumina precursor was prepared by the aluminium sulphate (0.20 M) and excess urea reaction in boiling aqueous solution. The precursor was calcined at 900 °C for 2 h and then δ-Al₂O₃ powder having volumetric agglomeration degree of 80% was obtained. Cylindrical compacts having diameter of 14 mm were prepared under 32 MPa by axial pressing using oleic acid as binder. Each compact was fired isothermally at various temperatures between 950 and 1400 °C. The firing time was changed from zero to 2 h. The fired compacts were examined by scanning electron microscopy (SEM) and nitrogen adsorption techniques. The specific surface areas (S/m² g^− 1) of the samples were calculated using the Brunauer, Emmett, and Teller (BET) procedure. The rate constant (k) and mechanism-characteristic parameter (n) were obtained for different temperatures between 950 °C and 1150 °C from the application of the neck-growth sintering rate (NGRM) model on the surface area reduction data. An Arrhenius equation and the parameter n for the sintering were found in the forms of k = (7.648 × 10⁶ h^− 1) exp (− 186,234 J mol^− 1 / RT) and n = 4.0 × 10^− 7 T³-1.7 × 10^− 3 T² + 2.3 T − 1030.8 respectively. The parameter n changes in the interval 0.61 <  n < 1.34 with rising temperature having maximum at about 1025 °C. Based on the SEM images and NGRM data, the intra-particle sintering was discussed.     

南非红柱石细粉的烧结行为和莫来石化研究

采用南非红柱石细粉(粒度≤0.088mm)为原料,粉体在200MPa下成型为36mm×10mm试样,选择煅烧温度为1200～1600℃(温度间隔50℃),研究了南非红柱石细粉的烧结行为。通过对试样烧后线变化率、体积密度、显气孔率及XRD分析数据进行处理,分析了红柱石分解和莫来石化反应的行为特征。结果表明:1200℃左右已有莫来石生成;1500℃红柱石分解完全。随着温度的升高,红柱石和莫来石两者的消长主要出现在1300～1400℃,而反应过程有如下先后顺序:材料膨胀,红柱石分解,莫来石生成。温度低于1400℃,莫来石化反应膨胀和基质烧结同时存在,出现局部膨胀和局部烧结排除气孔共存的现象,导致显气孔率的变化趋势与烧后线变化率及体积密度的变化趋势不对应。高于1450℃,液相促进烧结起主导作用,材料大幅度收缩。1400℃煅烧后试样的线收缩率最小。     

The microstructure,coefficient of thermal expansion and flexural strength of cordierite ceramics prepared from alumina with different particle sizes

Cordierite ceramics were produced from alumina with 5 and 0.65 μm particle sizes or AlOOH and talc, clays and feldspar, to determine the influence of the alumina particle size on the microstructure, coefficient of thermal expansion (CTE) and flexural strength (FS) of the ceramics. After sintering at 1300 °C the ceramics made from 5-μm-sized alumina consisted of cordierite, glass, quartz, mullite and alumina, and had the highest density, FS and CTE. The alumina grains act as inclusions, from which the trajectories of the cracks were deflected or terminated, which increases the FS and CTE. The ceramics from sub-micrometre-sized alumina or AlOOH contained a negligable amount and no alumina, respectively, together with other phases. This is reflected in the low CTE and FS. The cordierite ceramic with the lowest CTE of ∼2.0 × 10⁻⁶ K⁻¹ and a high FS of 100 MPa was prepared from the 0.65-μm-sized alumina particles.     

Mechanical modelling of microwave sintering and experimental validation on an alumina powder

Microwave sintering (MW) allows fast heating (≤30 min) and densification of ceramic materials, like alumina Al₂O₃. In order to predict the final material properties (density, size and grain size) the mechanical SOVS (Skorohold Olevsky Viscous Sintering) model is adapted and validated for conventional sintering of alumina. The model is implemented on ABAQUS with UMAT subroutine. Secondly, the SOVS model is modified for the microwave sintering by adapting the shear viscosity Arrhenius type law. Pre-exponential and exponential coefficients are modified for MW sintering. The calculated relative densities are compared to experimental results from conventional and microwave sintering and the relative difference remains under 3%. The coefficients identified for the MW sintering reveal a decrease in the shear viscosity by around 10 and an increase by up to 50 times in the grain boundaries diffusion coefficient.     

Effect of powder morphology on sintering kinetics,microstructure and mechanical properties of monazite ceramics

This work focuses on the effect of precursor morphology on the microstructural evolution of monazite-type lanthanum-europium phosphate ceramics during sintering, including grain growth rate, as well as correlations between microstructure, texture effects and their mechanical properties. Sintering kinetics of La_0.5Eu_0.5PO₄ powders with two different grain morphologies (needle-shaped and spherical) was studied by an in situ HT-ESEM method at 1340°C. La_0.5Eu_0.5PO₄ pellets with high density (99% of the theoretical density) were obtained for both precursor powders by hot pressing. Analysis of XRD data collected for the hot pressed pellets obtained from needle-shaped precursors revealed preferential orientation of the grains towards the (100) direction. Mechanical properties of the hot pressed pellets were studied by the Vickers indentation method. The dependence of microhardness and fracture toughness on microstructure and texture was investigated.     

The sintering kinetics of porcelain bodies made from waste glass and fly ash

Porcelain is a material produced from kaoline, quartz and potassium-feldspar. Recently, research of new materials, for example non-hazardous wastes, that are able to replace traditional fluxing agents without changing the process or quality of the final products has been realized. The aim of this work is to study the possibility of the use of glass powder waste and fly ash together for manufacturing porcelain. Instead of quartz, fly ash was used at the selected porcelain composition. The waste glass was added partially and fully in replacement of potassium-feldspar. Samples were fired in an electric furnace with a heating rate of 10 °C/min at 1100, 1150 and 1200 °C for a period of 1, 2, 3 and 5 h. The sintered samples were characterised by XRD (X-ray diffraction) and SEM (scanning electron microscopy). Sintering activation energies were determined based on the bulk density result. At 10, 15, 20 and 25 wt.% glass waste addition, the apparent activation energies were calculated to be 145, 113.5, 70.4 and 53.74 kJ/mol, respectively. It was found that the sintering activation energy decreased with increasing waste glass addition.     

Preparation and characterization of alumina/calcium-hexaluminate ceramic composites from ferrotitanium slag

Low-cost alumina/calcium-hexaluminate (Al₂O₃-CaAl₁₂O₁₉) ceramic composites were prepared using ferrotitanium slag in this paper. By making use of the TiO₂ and MgO originally existing in ferrotitanium slag, the sintering densification of Al₂O₃-CaAl₁₂O₁₉ composites was promoted. The results show that the optimum sintering temperature of the composites is 1500 ℃. The dominant sintering mechanism is the solid solution mechanism, i.e., Ti⁴⁺ and Mg²⁺ are dissolved in the CA₆ and Al₂O₃ lattices and generate numerous defects, which ultimately enhance the lattice diffusion coefficient and matter transport. Sintering densification improves the specific heat capacity, thermal conductivity, and thermal shock resistance of Al₂O₃-CaAl₁₂O₁₉ composites. Thermophysical properties analysis indicates that the composites can be potentially used for thermal storage and the slag-utilized ratio is about 60 wt.%. However, the layered cleavage of CA₆ limits further improvement of thermal shock resistance.     

Densification behavior and mechanical properties of nano-alumina ceramics prepared by Spark Plasma Sintering with pressure applied at different sintering stages

High densification and fine grain size are the key to achieve excellent mechanical properties of ceramic materials. Pressure-assisted sintering is an effective approach to achieve this goal. However, the pressure at different sintering stages has different effects on the densification behavior of nano-ceramics. In this work, it is found that adjusting the pressure applying regime during Spark Plasma Sintering of nano-alumina ceramics can effectively increase the densification rate and balance the relationship between the densification behaviors of particle coarsening, grain growth and vapor migration. When the pressure is applied at the beginning of the second sintering stage, the high densification and fine grain size microstructures can be both obtained at lower temperatures, leading to the best mechanical properties. This result is of great significance for the preparation of nano-ceramics with excellent mechanical properties.     

Determination of PVC powder drying kinetics at particle scale: Experimental study and modeling

An original experimental method is used to determine drying kinetic at particle scale. The particle scale kinetics was obtained by immersion of a small mass of wet polyvinyl chloride (PVC) particles (cake) in a batch dense fluidized bed containing inert hot particles (glass bead). The results are summarized here and prove clearly that the PVC drying is controlled by a competition between internal and external transfers. The drying kinetic was described by a particle scale model taking into account the convective–diffusive (mass transfer) and the convective–evaporative (heat transfer) phenomena. To validate this model with the experimental data, the experimental fluidized bed dryer is modeled following two different approaches: a perfect stirred reactor model and a 3D numerical simulation using the multiphase flow code NEPTUNE_CFD. The aim of this 3D simulation is to simulate the phenomena occurring, at local scale, in a dense fluidized bed dryer and to show the limitations of the perfect stirred reactor model.     

Sintering behaviour and characterisation of low-cost ceramic foams from coal gangue and waste quartz sand

New ceramic foams have been successfully synthesised with coal gangue and waste quartz sand, which supply a feasible way to recycle these hazardous solid wastes. An objective of this research was to investigate the sintering behaviour and effects of sintering conditions on the crystalline phase change, microstructure and main properties of final ceramic foams. Good correlations among porosity, thermal conductivity, water absorption, bulk density, mechanical properties were studied. Results indicated that increasing sintering temperature or time had similar effects on the physical–mechanical properties. Samples sintering at 1140°C for 1 hour exhibited the highest porosity (87.5%), lowest bulk density (0.39?×?10^?3?kg?m^?3), lowest thermal conductivity (0.085?W·(m?K)^?1), moderate water absorption (9.38%) and adequate flexible strength (2.4?MPa). Combined with excellent properties and low-cost characteristics, the new development for ceramic foams preparation will be widely used in building insulation materials for no-load bearing walls.     

燃烧法制备镁铝尖晶石粉体及其烧结活性研究

按Mg(NO3)2.6H2O与A l(NO3)3.9H2O的摩尔比为1∶2配料,分别添加20.120 g尿素、15.015 g尿素+4.500 g甘氨酸、7.987 g尿素+9.980 g甘氨酸为燃烧剂,采用燃烧合成法制备了镁铝尖晶石粉体,分析了镁铝尖晶石粉体的物相组成和显微结构,并用谢乐公式计算粉体的晶粒尺寸;然后,将镁铝尖晶石粉体成型为样块,在1 600℃保温3 h煅烧,计算块体的烧后永久线变化率,并检测烧后块体的吸水率、显气孔率和体积密度,分析其显微结构。结果表明:1)随着燃烧剂中甘氨酸含量的增加,制备的镁铝尖晶石粉体的晶粒尺寸逐渐减小,粒径分布逐渐均匀,但团聚现象逐渐明显。2)随着燃烧剂中甘氨酸含量的增加,由镁铝尖晶石粉体制成的块体在1 600℃保温3 h煅烧后的永久线收缩率逐渐增大,烧后块体的吸水率和显气孔率逐渐减小,体积密度逐渐增大;烧后块体中的颗粒粒径逐渐均匀,异常长大的晶粒逐渐减少。这表明,在燃烧剂中添加甘氨酸有利于提高燃烧合成法制备的镁铝尖晶石粉体的烧结活性。     

Sintering of WC-Ni nanocomposite powder: Experimental and artificial neural networks modeling study

The sintering behavior of WC-Ni nanocomposite powder was evaluated through experimental and statistical approaches to study the contribution of involving parameters of chemical composition (Ni wt. %) and sintering temperature on sinterability of system by assessing the resulted densification and microhardness. The experimental process was designed based on factorial experimental design for independent effective parameters of Ni percentage (12, 18 and 23 wt %), and sintering temperature (8 different values within 1350–1485 °C). The resulted products of experimental testing after compaction and sintering were analyzed by FESEM and EDX to image the microstructure and evaluate the chemical composition and elemental distribution. The density and microhardness were measured as well. An artificial neural network (ANN) was applied to describe the corresponding individual and mutual impacts on sintering. The ANN model was developed by feed-forward back propagation network including topology 2:5:2 and trainlm algorithm to model and predict density and microhardness. A great agreement was observed between the predicted values by the ANN model and the experimental data for density and microhardness (regression coefficients (R²) of 0.9983 and 0.9924 for target functions of relative density and microhardness, respectively). Results showed that the relative importance of operating parameters on target functions (relative density and microhardness) was found to be 62% and 38% for sintering temperature and Ni percentage, respectively. Also, ANN model exhibited relatively high predictive ability and accuracy in describing nonlinear behavior of the sintering of WC-Ni nanocomposite powder. The experimental results confirmed that the appropriate sintering temperature was influenced by Ni content. The optimum parameters were found to be 12 wt % Ni sintered at 1460 °C with the highest microhardness and relative density.     

Sintering characteristics of plasma-sprayed TBCs: Experimental analysis and an overall modelling

In this study, sintering behaviour of plasma-sprayed thermal barrier coatings (PS-TBCs) was investigated experimentally and theoretically. Results show that the sintering kinetics of PS-TBCs is highly stage-sensitive. The sintering proceeds significantly faster at initial short thermal exposure (<20 h), while it slows down dramatically at following long thermal exposure. A detailed examination on microstructural evolution of the PS-TBCs was carried out to understand their sintering behaviour. Results show that, different from the conventional sintering theory, the healing of 2D pores was dominantly responsible for the stage-sensitive sintering kinetics during thermal exposure. In brief, the sintering characteristics of the PS-TBCs are highly structure specific. In addition, a structural model was developed based on the structural characteristics of the PS-TBCs; and the model predicts a well consistent sintering behaviour with experiments. Finally, an outlook towards TBCs with higher performance was put forward.     

纳米结构脂质载体的制备及其聚集动力学研究

采用高压均质法制备包覆二苯酮-3（BP-3）和对甲氧基肉桂酸辛酯（OMC）两种紫外线吸收剂的纳米结构脂质载体（NLC）,并通过单因素实验探究了总油相含量、液体脂质用量、防晒剂含量、乳化剂种类及其用量对NLC粒径、ζ电位和包封率的影响,并优化了工艺配方。此外,还考察了电解质的种类、浓度以及乳化剂用量对NLC聚集行为的影响并建立了聚集动力学模型。结果表明,电解质的存在可以压缩胶体体系的双电层,导致ζ电位降低,加剧纳米粒之间的聚集结合。NLC样品在电解质NaCl和CaCl₂存在下的临界聚沉浓度分别为538.2和95.8 mmol·L^-1,且在CaCl₂溶液中的聚集速度比NaCl溶液中更快,适量的乳化剂对维持NLC体系的稳定性具有重要作用。     

管锥形填充床生物反应器水力特性及反应动力学分析

以NaCl为示踪剂,应用脉冲信号理论对管锥形填充床生物反应器内的水流流态进行流态分布实验,得出管锥形填料生物膜反应器流态是完全混合式;应用扩散理论以生物膜内的有机底物为去除对象,根据生物膜内微体积元的物料衡算式,对生物膜内有机底物的去除过程建立数学模型,分别得到在不同底物浓度条件下的三种数学模型;通过试验得出管锥形填充床反应器对污水中有机底物的去除符合一级反应动力学模型,由试验数据拟合其动力学模型为:lnC=-3.67Xt+337.04,回归显著性相关系数为:R2=0.8967。     

Autocatalytic curing kinetics of thermosetting polymers: A new model based on temperature dependent reaction orders

Herewith we discuss a new model for thermoset polymers that follow the autocatalytic curing kinetics. This model is proposed upon investigation of the crosslinking reaction of 2,2′-Bis(4-cyanatophenyl)iso-propylidene (BACy), under isothermal conditions over a range of temperatures between 180 °C and 260 °C without catalyst. BACy undergoes crossslinking via a trimerization mechanism of the nitrile groups following an autocatalytic kinetics rather than an nth order kinetics. Comparing with other autocatalytic kinetics models, the new model takes into account that the reaction orders of the curing reactions in the polymers are temperature dependent variables rather than constants. The new model provides excellent agreement with the experimental data in a wide range of conversions and reaction temperatures.     

Preparation and antibacterial properties of titanium-doped ZnO from different zinc salts

To research the relationship of micro-structures and antibacterial properties of the titanium-doped ZnO powders and probe their antibacterial mechanism, titanium-doped ZnO powders with different shapes and sizes were prepared from different zinc salts by alcohothermal method. The ZnO powders were characterized by X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), ultraviolet-visible spectroscopy (UV-vis), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and selected area electron diffraction (SAED), and the antibacterial activities of titanium-doped ZnO powders on Escherichia coli and Staphylococcus aureus were evaluated. Furthermore, the tested strains were characterized by SEM, and the electrical conductance variation trend of the bacterial suspension was characterized. The results indicate that the morphologies of the powders are different due to preparation from different zinc salts. The XRD results manifest that the samples synthesized from zinc acetate, zinc nitrate, and zinc chloride are zincite ZnO, and the sample synthesized from zinc sulfate is the mixture of ZnO, ZnTiO₃, and ZnSO₄ · 3Zn (OH)₂ crystal. UV-vis spectra show that the absorption edges of the titanium-doped ZnO powders are red shifted to more than 400 nm which are prepared from zinc acetate, zinc nitrate, and zinc chloride. The antibacterial activity of titanium-doped ZnO powders synthesized from zinc chloride is optimal, and its minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) are lower than 0.25 g L⁻¹. Likewise, when the bacteria are treated by ZnO powders synthesized from zinc chloride, the bacterial cells are damaged most seriously, and the electrical conductance increment of bacterial suspension is slightly high. It can be inferred that the antibacterial properties of the titanium-doped ZnO powders are relevant to the microstructure, particle size, and the crystal. The powders can damage the cell walls; thus, the electrolyte is leaked from cells.