硅质耐火泥施工性能的探讨

探讨了硅质耐火泥施工性能的具体要求,根据现有的国家技术标准进行样品试验,对原料的粒度配级和施工性能赋予剂上进行研究。最终得出结论,合理的粒度级配是焦炉用硅质火泥获得优良的施工性能的基础。必须添加水溶性有机物和无机盐组合添加剂,才能使火泥获得良好的施工性能。     

A Combined Experimental and Theoretical Approach to Study Temperature and Moisture Dynamic Characteristics of Intermittent Paddy Rice Drying

Intermittent drying of paddy rice is fully investigated both theoretically and experimentally. A model is developed to describe simultaneous heat and mass transfer for the drying stages and mass transfer for the tempering ones. The model is considered for both cylindrical and spherical geometries. The model excels in considering non-constant paddy rice and air physical properties as well as surface vaporization and convection. The consequent equations are numerically solved with finite-difference method of line using implicit Runge–Kutta. Furthermore, a set of experiments is conducted in a laboratory-scale fluidized bed dryer to estimate the moisture diffusivity of rice and evaluate the effects of different parameters. Two correlations for moisture diffusivity are derived for each geometry based on the experimental results. It is noteworthy that the geometry choice leads to significantly different moisture diffusivities. As a result, the diffusivity values obtained for spherical presentation is 2.64 times greater than that of cylinder. Moreover, the cylindrical model fits the experimental results more precisely, especially for tempering stage (AARD_cyl = 1.03%; AARD_sph = 1.53%). Model results reveal that thermal equilibrium is quickly reached within the first 2 min. Air velocity shows no influential effect on drying upon establishment of fluidized condition. In addition, drying rate is drastically improved after applying the tempering stage. A definition for tempering stage efficiency is also proposed which shows that 3 h tempering will be 80% efficient for the studied case. Rising temperature significantly improves the drying rate, while it does not contribute much in the tempering efficiency.     

Grain size effect on electrical properties of Mn-modified 0.67BiFeO3–0.33BaTiO3 lead-free piezoelectric ceramics

To investigate how grain size affects the dielectric, ferroelectric, and piezoelectric properties of Mn-modified 0.67BiFeO₃–0.33BaTiO₃ ceramics, we prepared samples with a wide variety of grain sizes from 4.1 μm to 0.59 μm via a conventional solid-state process that use the normal and the two-step sintering methods. Small-signal dielectric measurements show that all the samples exhibit a relaxor-like behavior and that grain size has little influence on the room-temperature dielectric permittivity. For grain sizes below 2 μm, the remanent polarization P_r and piezoelectric coefficient d₃₃ decrease with the grain size, whereas they remain almost constant near P_r = 27 μC/cm² and d₃₃ = 70 pC/N in samples with grain sizes exceeding 2 μm. The mechanism underlying the observed grain size effect is discussed in terms of the electric-field-induced formation of macroscopic ferroelectric domains.     

Microstructure of Ba1−xLaxTiO3−δ ceramics sintered by Spark Plasma Sintering

Nano-sized Ba_1−xLa_xTiO₃ (0.00 ≤ x ≤ 0.14) powders were prepared by a coprecipitation method and calcined at 850 °C in air. The corresponding ceramics were obtained by Spark Plasma Sintering (SPS) at 1050 °C. These ceramics are oxygen deficient and are marked as Ba_1−xLa_xTiO_3−δ. Both powders and ceramics were characterized by X-ray diffraction (XRD) and Transmission Electron Microscopy (TEM). The effect of lanthanum concentration on the densification behavior, on the structure and the microstructure of the oxides was investigated. Average grain sizes are comprised between 54 (3) nm and 27 (2) nm for powders, and 330 (11) nm and 36 (1) nm for ceramics according to the La-doping level. Powders crystallize in the cubic (or pseudo-cubic) perovskite phase. The structure of ceramics consists in a mixture of cubic (or pseudo-cubic) and tetragonal perovskite type phases. As the lanthanum content increases, the tetragonality of the samples decreases, as well as the grain size.     

Texture development and dielectric relaxor behavior of 0.80Na0.5Bi0.5TiO3–0.20K0.5Bi0.5TiO3 ceramics templated by plate-like NaNbO3 particles

Plate-like NaNbO₃ particles were used as templates to fabricate grain-oriented 0.96(0.8Na_0.5Bi_0.5TiO₃–0.2 K_0.5Bi_0.5TiO₃)–0.04NaNbO₃ (NKBT) ceramics. The effects of the sintering temperature and the soaking time on the grain orientation and the microstructure of the textured NKBT ceramics were investigated, and the dielectric relaxor behavior is discussed. The results show that textured ceramics were successfully obtained with orientation factor more than 0.8. The textured ceramics have a microstructure with strip-like grains aligning in the direction parallel to the casting plane. The degree of grain orientation increases initially, then decreases with increasing sintering temperature, and increases continuously with increasing soaking time. The textured NKBT ceramics shows obvious dielectric relaxor characteristics which can be well explained by microdomain–macrodomain transition theory with calculating criterion K. The results show that formation of texture is beneficial to microdomain–macrodomain transition, which lead to weaken relaxor behavior and raise the dielectric constant at T_tr.     

Dehydration by Successive Pressure Drops for Drying Paddy Rice Treated by Instant Controlled Pressure Drop

The aim of the present work was to study the application of the dehydration by successive pressure drops (DDS) process to different varieties of paddy rice initially treated by the instantaneous controlled pressure drop (DIC), which is a high-temperature, short time postharvest treatment (moisture content about 20–25% dry basis) and takes less than 30 s. DIC paddy rice is dried by DDS in a relatively short time and a high milling quality is obtained (12–13% db in about 2 h). The final rice is white, easily distinguishing it from the standard parboiled one. Without a tempering period, DDS rice has less than 3% of broken grains, with a high organoleptic quality (preservation of the natural aroma with a firm and non-sticky texture), and the cooking time of both brown (whole) and white DIC rice is very short (15 and 7 min, respectively).     

Soybean Drying by Two-Dimensional Spouted Bed

Abstract

Urease activity, cracking, and breakage are important factors in considering the quality of raw soybean for feed meal industries. A two-dimensional spouted bed dryer was investigated to determine its capability for thermally inactivating the urease enzyme and maintaining its other qualities. The experimental results have shown that the drying kinetics of soybean in a two-dimensional spouted bed dryer are of the form described in the thin layer drying. The expression for the model parameter in Newton's law of cooling equation accounting for the moisture contents and inlet air temperatures was developed. The initial moisture content and inlet air temperature conditions cause cracks in the kernels. The strong collision between kernels and deflector because of high superficial velocity leads to high percentage of broken soybeans in the spout region. However, the velocity of 15.9 m/s can reduce the breakage below 5%. The inactivation of urease at low-to-moderate moisture content is suitably described by the first order kinetics. The modified Monod equation is applied when the moisture content is higher than 26% dry basis due to the inhibitory effect of water content on the inactivation rate. To complete urease inactivation and maintain protein quality, the temperatures of 150°C should be used.     

The effect of rare earth oxides on the pressureless liquid phase sintering of α-SiC

Silicon carbide (SiC) ceramics have been fabricated by pressureless liquid phase sintering with Al₂O₃ and rare-earth oxides (Lu₂O₃, Er₂O₃ and CeO₂) as sintering additives. The effect was investigated of the different types of rare earth oxides on the mechanical property, thermal conductivity and microstructure of pressureless liquid phase sintered SiC ceramics. The room temperature mechanical properties of the ceramics were affected by the type of rare earth oxides. The high temperature performances of the ceramics were influenced by the triple junction grain boundary phases. With well crystallized triple junction grain boundary phase, the SiC ceramic with Al₂O₃–Lu₂O₃ as sintering additive showed good high temperature (1300 °C) performance. With clean SiC grain boundary, the SiC ceramic with Al₂O₃–CeO₂ as sintering additive showed good room temperature thermal conductivity. By using appropriate rare earth oxide, targeted tailoring of the demanding properties of pressureless liquid phase sintered SiC ceramics can be achieved.     

SPS densification and microstructure of ZrB2 composites derived from sol–gel ZrC coating

A technique for densifying ultra high temperature ceramic composites while minimising grain growth is reported. As-purchased ZrB₂ powder was treated with a zirconia-carbon sol–gel coating. Carbothermal reduction at 1450 °C produced 100–200 nm crystalline ZrC particles attached on the surface of ZrB₂ powders. The densification behaviour of the sol–gel coated powder was compared with both the as-purchased ZrB₂ and a compositionally similar ZrB₂–ZrC mixture. All three samples were densified by spark plasma sintering (SPS). The ZrB₂ reference sample was slow to densify until 1800 °C and was not fully dense even at 2000 °C, while the sol–gel modified ZrB₂ powder completed densification by 1800 °C. The process was studied by ram displacement data, gas evolution, SEM, and XRD. The sol–gel coated nanoparticles on the ZrB₂ powder played a number of important roles in sintering, facilitating superior densification by carbothermal reduction, nanoparticle coalescence and solid-state diffusion, and controlling grain growth and pore removal by Zener pinning. The sol–gel surface modification is a promising technique to develop ultra-high temperature ceramic composites with high density and minimum grain growth.     

连续铸挤Al-5Ti-1B合金线的组织与晶粒细化效果

采用连续铸挤成形Al-5Ti-1B合金线,采用光学显微镜和扫描电镜分析了Al-5Ti-1B合金线的显微组织和晶粒细化效果,并与A1-5Ti-1B合金锭进行了比较.结果表明,连续铸挤 Al-5Ti-1B合金线中TiAl3 相呈细小圆块状,分布均匀,TiB2粒子弥散分布于铅基体;而Al-5Ti-1B合金锭中TiA13相呈粗大板条状,TiB2 粒子偏聚于铝晶界呈粗大团块状.Al-5Ti-1B合金线对纯铝的晶粒细化效果优于Al-5Ti-1B合金锭,纯铝加入A1-5Ti-1B合金线5 min后,晶粒尺寸达到最小.