轻烧氧化镁的微观特征及其活性

活性是轻烧氧化镁的主要应用特性，它与轻烧氧化镁的微观结构密切相关。将新疆某粗晶菱镁矿粉在700～1 100℃煅烧制得轻烧氧化镁，对不同温度轻烧氧化镁的晶胞参数、晶粒尺寸、结晶度、比表面积、孔径分布、比孔容、平均孔径、活性和显微结构等进行表征，利用TG结合非等温反应动力学理论研究轻烧氧化镁的微粒结构，分析该微粒结构随温度变化的机理及其对轻烧氧化镁活性的影响。结果表明：从700℃至1 100℃，轻烧氧化镁晶胞参数a0缩小并趋于稳定，晶粒尺寸逐渐增大，比表面积逐渐减小，但其活性变化并不遵从这一规律，在800℃时活性最高；粗晶菱镁矿粉的热分解受相界面上的化学反应所控制，分解产物—轻烧氧化镁“假象”微粒由氧化镁微晶和贯通的气孔网络结构组成；随煅烧温度升高，氧化镁微晶的烧结影响轻烧氧化镁“假象”微粒结构，进而影响其活性；800℃轻烧试样晶粒尺寸较小，结晶度较低，比孔容较大，且由于晶格调整使其平均孔径最大，大的孔径有利于溶液进入微粒内部，促进反应进行，因此其活性最高。     

ZrO_2-Y_2O_3超细粉烧结动力学的研究

通过对ZrO_2-Y_2O_3纳米级粉末烧结动力学过程的研究,分析了陶瓷的致密化机理。实验结果表明:超细粉末在烧结初期,晶界扩散起主要作用。在存在团聚体的粉末中,由于团聚体与基体间界面应力的相互作用,致使烧结体的密度降低,影响了陶瓷的显微结构,从而限制了在烧结后期获得细晶粒的陶瓷体。对于无团聚体的粉末,通过烧结过程中晶粒生长过程的控制,可获得相对密度达98.5%(1250℃保温2h),且晶粒尺寸仅为160nm的Y-TZP陶瓷体。     

煅烧菱镁矿在氯化铵乙二醇溶液中的浸取动力学

系统地研究了菱镁矿的煅烧条件以及煅烧粉在氯化铵乙二醇溶液中的浸取动力学. 菱镁矿在750℃下煅烧2 h左右能使MgCO3分解完全而CaCO3不分解,过烧会使晶粒长大,从而降低浸取速率. 在消除外扩散影响的前提下,浸取过程在反应物(NH4Cl与MgO)以化学计量比反应、氯化铵的初始浓度为0~1.23 mol/L时符合未反应收缩核模型(Unreacted shrinking core model),且浓度为1.23 mol/L时浸取速率达到最大;浸取速率受表面化学反应控制,活化能为44.74 kJ/mol.     

轻烧氧化镁活性测定方法的研究

用柠檬酸法和三种不同水化法测定和研究了氧化镁的活性,研究了氧化镁活性与煅烧温度、煅烧时间之间的关系,结果表明,当煅烧温度为750℃时,由菱镁矿煅烧所得的氧化镁活性最高,在菱镁矿分解完全的情况下,煅烧时间以短为宜。本研究为氧化镁生产工艺控制提供了理论依据。     

纳米3Y-TZP粉体的制备及烧结曲线研究

采用共沉淀法制备纳米3Y-TZP粉体,并利用DSC-TG,XRD和TEM等测试方法对3Y-TZP的物理化学性能进行表征。通过研究纳米3Y-TZP粉体的烧结曲线,分析3Y-TZP素坯在烧结过程的致密化和显微结构。结果表明:在600℃下煅烧2h后,可获得晶粒尺寸为13nm,晶形发育良好、团聚较少的纳米3Y-TZP粉体。在烧结过程中,不同烧结阶段中的扩散机制不同,在烧结后期晶粒尺寸发生显著长大。     

菱镁矿制备活性氧化镁过程中超声作用研究

铵浸法用菱镁矿制备氧化镁工艺过程为：将菱镁矿煅烧粉碎后，与硫酸铵或氯化铵溶液反应，得到硫酸镁或氯化镁溶液，然后同碳酸铵或碳酸钠溶液反应，得到碱式碳酸镁，最后煅烧成氧化镁。该法的优点是不用除杂即可得到高纯度的氧化镁，如能解决其浸出率偏低、能耗较高的缺点，该法仍不失为一种较好的方法。为解决铵浸法存在的问题，研究了超声波在以菱镁矿、硫酸铵和碳酸铵为原料制备氧化镁过程中的应用。研究结果表明：超声波可提高菱镁矿中镁的利用率，并使生成的活性氧化镁有较高的比表面积和活性。实验步骤：1）矿样预处理。取适量矿样在高温箱式电阻炉中，     

团聚氧化镁粉料压块的烧结机理与动力学模型

团聚粉料压块具有由高烧结性粉粒所构成的双层堆积结构。在烧结的第一阶段，一级颗粒快速烧结导致二级颗粒的收缩与重排并增加大气孔。在烧结的第二阶段，一级与二级颗粒的烧结同时进行，但压块的烧结是被一级颗粒的烧结所控制。某些中期烧结模型与试验结果吻合。在烧结的第三阶段中，压块的致密和颗粒长大同时进行，其特点是大量的小晶粒晶界存在，气孔有很高的配位数。提出一个方程式，它与团聚氧化镁试块烧结试验结果相一致。     

菱镁矿活化烧结研究

研究了菱镁矿(M-C)及其衍生物 M-Ⅰ(轻烧)、M-H(轻烧-水化)、M-Ⅱ(轻烧-水化-脱水)和M-CF(加 2％ Fe_2O_3)在初、中、后期的烧结行为。推断了 M-Ⅰ、M-Ⅱ和M-H的初期烧结机理,计算了活化能;分析了中期阶段 M-Ⅰ和M-Ⅱ的致密化行为,确定了液相形成温度;讨论了后期阶段的气孔率变化以及整个烧结过程的致密化和晶粒长大特征。 实验结果证明:(1)M-Ⅰ、M-Ⅱ和M-H比M-C和M-CF 易烧结。说明二步煅烧比一步煅烧优越,轻烧和轻烧后水化是降低烧结温度的有效措施。Fe_2O_3 的加入对菱镁矿的烧结有一定的促进作用。(2)二步煅烧法的主要特征是低中温(由生坯到1400℃)致密化进行迅速。显气孔率下降份额占整个下降的十之八、九,固相烧结和1350～1400℃形成的初始液相起了重要作用。高温时属液相烧结,主要是晶粒长大,密度的提高不显著。M-Ⅰ、M-Ⅱ和M-H在1600℃已烧结,M-C和M-CF未烧结。     

氯化镁添加剂对菱镁矿轻烧粉末性质的影响

本工作研究了加氯化镁(MgCl_2·6H_2O)的浮选菱镁矿轻烧粉末的形貌及性质,结果表明,加入氯化镁添加剂可损坏菱镁矿母盐假象,改善粉末性质,促进MgO烧结。主要是氯化镁的加入促进了MgO晶粒长大,改变了母盐假象中的微晶的表面特性,使之较容易被一般的成型压力所破坏。加氯化镁的粉末一般都有着较高的烧结体积密度。     

纳米氧化镁制备工艺的研究

以六水硝酸镁和氨水为原料,合成氢氧化镁,研究了制备纳米氧化镁的煅烧及球磨工艺,并用X-射线扫描仪(XRD)、扫描电子显微镜(SEM)进行表征.结果表明:分步煅烧制备的纳米氧化镁比一步煅烧的晶粒尺寸小、团聚少,制备出晶粒尺寸为24.1 nm纳米氧化镁.     

Shape Sensitivity of Initial Sintering Equations

Initial sintering equations are shown to depend as much on possible particle shape variations in powder systems as on the mechanism of material transport. Also, competing mechanisms, such as surface diffusion, may alter the generally assumed relations between neck growth and shrinkage and so may affect the sintering kinetics. When shrinkage occurs by volume diffusion, both effects may be overcome by measuring the size of the interparticle boundary as well as the shrinkage during sintering. When grain boundary diffusion is the mechanism of shrinkage, both the cross-sectional area and the radius of curvature of the neck must be measured during sintering. The usefulness of simultaneous measurements of neck growth and shrinkage is demonstrated with literature data for copper.     

Effect of MgO Solute on Microstructure Development in Al2O3

The effect of MgO as a solid-solution additive in the sintering of Al₂O₃ was studied. The separate effects of the additive on densification and grain growth were assessed. Magnesia was found to increase the densification rate during sintering by a factor of 3 through a raising of the diffusion rate. The grain-size dependence of the densification rate indicated control primarily by grain-boundary diffusion. Magnesia also increased the grain growth rate during sintering by a factor of 2.5. The dependence of the grain growth rate on density and grain size suggested a mechanism of surface-diffusion-controlled pore drag. It was argued, therefore, that MgO enhanced grain growth by raising the surface diffusion coefficient. The effect of MgO on the densification rate/grain growth rate ratio was, therefore, found to be minimal and, consequently, MgO did not have a significant effect on the grain size/density trajectory during sintering. The role of MgO in the sintering of alumina was attributed mainly to its ability to lower the grain-boundary mobility.     

Modification of Sintering Kinetics by Solute Segregation in Al2O3

The segregation of MgO solute at grain boundaries in Al₂O₃ controls the grain-growth kinetics during sintering. In the initial stages of sintering where grain growth can be neglected, the MgO solute causes a decreased sintering rate by reducing the surface energy and/or the diffusion coefficient of the rate-determining species. During the later stages of sintering, the segregated solute decreases the grain-growth rate which dominates the sintering kinetics.     

Effect of sintering atmosphere on microstructures and properties of synthesized microporous MgO refractory raw material

Effects of sintering atmosphere on the microstructure and strength of magnesite were investigated using magnesite powder as raw material through X-ray diffraction, scanning electron microscopy, mercury porosimetry measurement, and so on. The results showed that the sintering atmosphere strongly affected the sintering behavior of magnesite. The specimens sintered in the reducing atmosphere had more and finer micro-sized pores inside the MgO particles compared with that in the oxidizing atmosphere at the same sintering temperature. Besides, MgO refractory raw material containing porous MgO microparticles with core–shell structure was obtained through the carbothermal reduction of MgO microparticles and subsequent oxidation of Mg vapor at the surface of MgO particles at 1500°C in the reducing atmosphere. At the reducing atmosphere and 1500°C, the microporous MgO refractory raw material with the core–shell structure of external dense and internal porous had an apparent porosity of 22.1% and a compressive strength of 51.6 MPa.     

Role of Solute Segregation at Grain Boundaries During Final–Stage Sintering of Alumina

The addition of minor amounts of MgO or NiO to Al₂O₃ inhibits grain growth during sintering and allows the sintering process to proceed to theoretical density by maintaining a high diffusion flux of vacancies from the pores to the grain boundaries. The inhibition of grain growth is accomplished by the segregation of solute at the grain boundaries, causing a decrease in the grain–boundary mobility. The segregation of MgO or NiO at the grain boundaries can be inferred from the results of the microhardness studies presented and is substantiated by autoradiographic experiments and also by lattice parameter determinations as a function of grain size.     

Effect of Calcination Temperature on Properties of Eggshell Ni/MgO–Al2O3 Catalyst for Partial Oxidation of Methane to Syngas

The effect of calcination temperature on the properties of eggshell Ni/MgO–Al₂O₃ catalysts was studied. Catalyst deactivation was also investigated. It is found that higher calcination temperature contributes to lower surface area, wider pore, and larger Ni crystallites. Small Ni crystallites and large pores favor the catalyst performance. Catalyst deactivation is due to the formation of NiO–MgO solid solution and/or NiAl₂O₄, phase transformation, and sintering.     

纯B4C和掺碳B4C的烧结机制

研究了中位粒径为0．42μm的纯B4C和掺碳B4C的烧结致密化过程。根据烧结温度和保温时间对线收缩率的影响。得出了它们的烧结动力学方程;由特征指数n值对比研究了它们的烧结致密机制。纯B4C的烧结致密机制为体扩散和晶界扩散,而掺碳B4C的烧结机制主要为晶界扩散,因此,掺碳对B4C起到了活化烧结的作用,在2160℃烧结45min,掺碳B4C烧结后相对密度大于90％,掺入的碳除了固溶于B4C晶格中之外,其它均以游离石墨形式存在,不形成新相。掺碳还导致B4C晶粒尺寸大大减小。     

Microstructural evolution during sintering in MgO powders precipitated from sea water under induced agglomeration conditions

Green compacts pressed by means of uniaxial compaction with Magnesia (MgO) powders precipitated from sea water and calcined at different temperatures were sintered under H₂ atmosphere at 1700 °C. The calcination, carried out between 900 and 1200 °C had a great influence in the final density and the microstructure. The densification of MgO agglomerated powders seems to be predictably related to grain growth and thus coarsening kinetics. At calcination temperatures higher than 900 °C, the volume of large pores was increased notably suggesting that the inhibited grain growth adversely affected the thermodynamics of pore sintering. Relative densities between 74 and 98% of theoretical density were reached in compacts obtained at different compaction pressures. The microstructural differences were examined by Scanning Electron Microscopy (SEM).     

Non-isothermal kinetic study of high-grade magnesite thermal decomposition and morphological evolution of MgO

Thermal decomposition was the prerequisite and basis for the utilization of magnesite resources. However, the calcination of magnesite was usually accompanied by high energy consumption, which not only caused serious waste of magnesite resources, but also restricted its high value-added utilization. Therefore, calcination conditions were the key to controlling thermal decomposition process of magnesite. The kinetics of high-grade magnesite thermal decomposition was elaborated by non-isothermal thermogravimetric analysis, and meanwhile the effect of heating rate on the magnesite thermal decomposition reaction and morphology of MgO particles were characterized. Both Doyle and Gorbatchev approximate functions were used to simulate the magnesite thermal decomposition process, where the experimental data (correlation coefficient) fitted by the latter could obtain more acceptable kinetic parameters of the magnesite thermal decomposition. The good linear relationship between the activation energy and the pre-finger factor allowed for a kinetic compensation of the thermal decomposition of magnesite. Furthermore, higher heating rate induced the formation of terraced grains, grain network group, cubic grains, and spherical grains for the samples sintered at 1200°C. The heating rate largely affected the magnesite particle morphology, grain size distribution and activity, and also provided important technical indicators in the actual production of magnesia refractory raw materials.