煅烧温度对溶胶-凝胶合成BCZT纳米粉体的影响

针对传统固相反应法制备粉体存在的成分偏离和引入杂质等问题,采用溶胶-凝胶方法合成了0.5Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3) TiO3(BCZT)无铅纳米粉体,主要研究了煅烧温度对其结构和形貌的影响.TG-DSC、XRD、SEM、TEM、FTIR等测试结果表明:在900～1100℃的温度范围内,溶胶-凝胶合成的BCZT粉体表现为单一的立方相结构;随着煅烧温度的升高,粉体的结晶性逐渐增强,颗粒尺寸增大;适宜的粉体煅烧温度为900,与传统固相反应法相比可降低约400℃;在此煅烧温度下得到的BCZT粉体粒径均匀,尺寸约50 ～ 60 nm,结晶良好,成分均匀.     

共沉淀法制备Ba(Fe0.5 Nb0.5)O3纳米粉体

以廉价的Nb2O5为铌源,采用共沉淀法制备了纳米Ba(Fe0.5Nb0.5)O3粉体.用XRD、TG-DSC、FT-IR、SEM等测试手段分析了烧结温度、保温时间、体系温度、pH值等对前驱体粉体的物相、形貌及晶粒大小的影响,并对粉体的磁学性能进行了表征.结果表明:合成温度在950℃,体系温度在30℃和50℃均可获得纯相立方钙钛矿结构的Ba(Fe0.5 Nb0.5)O3粉体,保温时间和pH值对粉体相结构变化无影响.在体系温度为30℃,pH=10,前驱体粉体在950℃煅烧2h后获得颗粒尺寸约30 nm的Ba(Fe05Nb0.5)O3粉体,且粉体具有弱的铁磁性.     

Ca2+掺杂γ-La2S3多晶的制备

本文以气固反应硫化得到的γ-La2S3粉体为原料,采用热压烧结的方法制备出了γ-La2S3多晶体片.研究了不同Ca2+掺杂量对得到粉体相结构的影响,并分析了烧结温度、保温时间对多晶体γ-La2S3红外透过率的影响.结果表明:掺入碱土金属离子Ca2+有利于低温获得稳定的高温型γ-La2S3相,在Ca/La物质的量比为1:5～1:15时能得到纯相的γ-La2S3粉体.在烧结温度为1100 ℃,保温时间为1 h时制备出的γ-La2S3多晶体片,晶粒细小均匀,无明显气孔,在10～14 μm波段的最大红外透过率约为40;.     

固相合成YAG微晶粉体的XRD研究

以高纯Al_2O_3和Y_2O_3超微粉为原料,通过高能球磨和固相反应法制备了Y_3Al_5O_(12)(YAG)微晶;采用X射线衍射法(XRD)研究了球磨时间、煅烧温度、保温时间等对合成YAG微晶相组成的影响.结果表明:原料粉体的粒度随研磨时间延长而降低;随煅烧温度的提高,产物的物相由Y_4Al_2O_9和YAlO_5 逐渐向YAG相转变;延长保温时间有利于Y~(3+)和Al~(3+)的扩散以及Al_2O_3和Y_2O_3的固相反应.当Al_2O_3和Y_2O_3的摩尔比为5 ∶ 3,混合粉体经过15 h的机械球磨并于1300 ℃煅烧40 min即可得到单一物相的YAG微晶粉末.     

Si_3N_4粒度对逆反应烧结制备Si_3N_4/SiC复合材料性能的影响

研究了不同粒度的Si3N4粉体对制备的逆反应烧结Si3N4-SiC复合材料性能的影响。结果表明:同一升温制度下,Si3N4粒度从0.074 mm减小到0.045 mm,Si3N4-SiC复合材料体积密度增大,显气孔率变低,抗折强度降低。随着Si3N4粒度减小,材料中的Si3N4含量降低;中温保温从900℃升到1200℃,Si3N4含量升高;高温保温温度从1350℃到1500℃,材料内部氧化程度增高,Si3N4含量降低,SiO2含量升高;Si3N4-SiC复合材料玻璃相增多,玻璃相中有SiO2晶体析出,使得孔隙被生成的玻璃相和发育的SiO2晶粒填充,材料的气孔减少。     

非水解和水解溶胶-凝胶法合成MgAl2O4粉体对比研究

分别采用非水解和水解溶胶-凝胶法制备MgAl2O4粉体,利用DTA-TG、XRD和FE-SEM等研究了凝胶煅烧过程中的相变化、粉体形貌及其烧结性能.研究表明,水解凝胶经900℃煅烧0.5h后可形成大量镁铝尖晶石晶相,但仍有少量的方镁石相存在,非水解凝胶经900℃煅烧0.5h后只有镁铝尖晶石晶相.采用水解和非水解法所合成MgAl2O4粉体的平均粒径分别为90nm和50 nm,其经1600℃烧结3h后晶粒尺寸分别为1～2 μm和0.5 ～1μm,晶粒多呈八面体形状,相比之下,采用水解法时MgAl2O4晶体发育较好,出现了明显的生长台阶.     

MgTiO3对Mg2SiO4-SiC复相材料烧结性能的影响

在采用传统固相法预合成Mg2SiO4和MgTiO3粉体的基础上,在H2气氛下常压烧结制备了Mg2SiO4-MgTiO3-SiC复相材料。研究了Mg2SiO4和MgTiO3的最佳原料配比及MgTiO3的添加量、烧结温度、保温时间对复相材料烧结性能和相组成的影响。结果表明:Mg2SiO4粉的最佳Mg/Si物质的量比为2.02,MgTiO3粉的最佳Mg/Ti物质的量比为1,Mg2SiO4-MgTiO3-SiC复相材料的相组成为Mg2SiO4、MgTiO3和6H-SiC;MgTiO3的加入可促进材料烧结,最佳保温时间为1 h。     

偏磷酸钙玻璃陶瓷的结晶化研究

以Ca(H2PO4)2·H2O和H3PO4为原料,按0.45的Ca/P物质的量比制备了偏磷酸钙玻璃,在300～700 ℃对玻璃进行热处理,获得了偏磷酸钙玻璃陶瓷.利用XRD、FT-IR等研究了玻璃结晶化过程中的结构变化,利用SEM观察了玻璃陶瓷的微观形貌.结果表明:玻璃陶瓷中的结晶相为纯的β-Ca(PO3)2;玻璃结晶过程伴随温度提高逐渐发生,在400 ℃保温3 h已经形成了β-Ca(PO3)2晶体相;温度提高,玻璃陶瓷红外光谱的精细结构明显;随结晶化温度升高和保温时间延长,玻璃陶瓷中结晶相形貌由纤维状变为晶界明显紧密排布的短棒状晶粒.     

硫酸钾熔盐法微硅粉制备莫来石晶须研究

以微硅粉和Al2(SO4)3为反应物料,硫酸钾为熔盐介质,分别在800℃、900℃、1000℃和1100℃下煅烧并保温1h,溶解、过滤、烘干后得到白色疏松状莫来石粉体.利用XRD和SEM对合成粉体进行物相分析和显微形貌观察.结果表明:1000℃为合成莫来石最佳温度,合成的莫来石晶须直径为0.1 ～0.8 μm,长度为4～5 μm.同时对莫来石晶须的形成机理进行了探讨.     

pH值和烧结温度对溶胶-凝胶方法制备堇青石粉体性能的影响

以正硅酸乙酯、镁盐和铝盐为原料,通过溶胶-凝胶法合成堇青石粉体,通过TGA-DSC、XRD等测试手段,讨论了反应体系pH值和烧结温度对产物晶相和性能的影响.结果表明:体系pH值为2的试样,在900℃时,出现尖晶石相和假蓝宝石相,升温到1000℃的时候,开始有μ-堇青石出现,在1250℃煅烧2h几乎全部转化为α-堇青石,体系pH为7的试样,在900℃时,有μ-堇青石形成,经1250℃煅烧后,β-堇青石相增多,μ-堇青石相和镁铝尖晶石相消失,合成粉体的平均晶粒尺寸随pH值增加呈先减小后增加的趋势,在不同温度煅烧下的压片试样,体系pH值为7的试样收缩最大.     

Science for the small and the tall,the young and the old

Public engagement becomes increasingly important for scientists. One reason is the demand of the taxpayer to know what her or his money is being spent on, and why. The other one is that in a world that increasingly relies on technology, student engagement even at a very young age becomes a target to assure the needed supply of well-educated and especially motivated scientists for the decades to come. And it falls on the older generation of current researchers to leave the comfort of their lab once in a while, to awaken the interest for science among the population. Many people may know that there is a ‘liquid crystal’ in their mobile (cell) phone display, but when prompted, no one really knows what that liquid crystal actually is, let alone how the display they use many times every day, actually works in principle. It is part of our job to change this. In this contribution Valentina Domenici and Ingo Dierking would like to report on two recent Science Festival events in which they took part, one held in Genoa, Italy, and the other in Manchester, UK.     

On the role of the porous shell of the solid core of the earth in the anomalous heat and mass flow to the mantle

The model of the pressure-induced first-order phase transition of a metal melt to the metallicglass state considers a thermodynamically nonequilibrium porous near-surface shell of the solid core of the Earth, which contacts cyclonic vortices in the liquid core. Anomalous flows of heat and light-material mass to the mantle from the solid core at these contact points are calculated. These anomalous flows are shown to be comparable with the observed ones under the assumption of a rapid increase in the melt viscosity at pressures of 1–10 Mbar, which is characteristic of a solid core. In this case, the porous layer permeability may be very low.     

On the dependence of the surface tension at the crystal-melt interface on the impurity concentration

The stationary task of impurity diffusion in a melt has been solved within a two-dimensional crystallization model in a second-order approximation with respect to the amplitude of deviation from a smooth crystallization front. The dependence of the surface tension Γ at the interface on the impurity concentration C is taken into account in the form Γ = Γ₀ + ζ _d C, where Γ₀ and ζ _d are constants. The variational method is used to obtain the condition for the transition from a smooth crystallization front to a cellular one. It is shown that calculated cell sizes are in agreement with the experimental data in the literature only when the parameter ζ _d ≠ 0. For binary systems with distribution coefficients k < 1 and k > 1, ζ _d should be positive and negative, respectively.

     

On the inhomogeneity of the transition surface layer of the solid core of the earth

Different geophysical data and conclusions of theoretical models, which can give information about the behavior of the solid and liquid cores of the Earth as well as about the existence of a transition layer as a temperature-hysteresis region at a relatively weak first-order phase transition, are compared. It is concluded that liquid inclusions inevitably exist in this region; these inclusions are involved (due to the complex convective processes occurring in the liquid core) in the transport of light materials from some areas of the solid-core surface. The porosity and permeability of the transition layer determine the seismic acoustic inhomogeneities in these areas, which contact the convective flows in the liquid core. In particular, this explains the well-known ??east-west?? effect. Obviously, the model of the crystalline core is not the only possible alternative for a model of a core with a metallic glasslike structure.     

A new example for the applicability of the relationship between the particle size and the kinetic of the crystal growth

Experiments have been made with PbCrO₄ precipitating system in a batch type tank. In the experiments the speeds of stirring and the residence times were changed. The particle size distribution of precipitates formed has been determined. The rate determining mechanism of the crystal growth were evaluated by two different methods (NIELSEN method and method elaborated in our institute). The two methods used have indicated the same informations about the rate determining processes. It also has been stated that the coagulation does not disturb the selection of the suitable growing mechanism.     

On the entropy difference between the vitreous and the crystalline state

The paper deals with the entropy difference between frozen-in phases and their equilibrium counterparts. First, the nature of data compiled in thermochemical data collections are briefly reviewed, comprising data for non-equilibrium phases. Then, experimental evidence from earlier literature is compiled showing that the conventional entropy of a frozen-in phase at zero Kelvin assumes a non-zero residual value S(0). Based on calorimetric data from multiple sources, the same evidence is elaborated for diopside glass, yielding S_glass(0) = 24.8 ± 3 J/(mol K), a value reproducing a result publishes earlier. The zero Kelvin enthalpy of this glass is H_glass(0) = 81±8 kJ/mol. For S_glass(0), a structural interpretation in terms of silicate chain mixing is proposed, yielding a lower threshold for S_glass(0). From the point of view of statistical mechanics, non-zero residual entropies of frozen-in phases can be derived from ensemble averages, however, not from time averages.     

On the slope of the growing pyramids

By means of some theoretical models the present work shows that pyramids of different slopes can grow on a crystal face at constant supersaturation due to the interaction between steps, arising from close spaced screw dislocations. Pairs of adjacent screw dislocations of opposite sign, as well as other groups of closely spaced screw dislocations, can operate as more active sources of growing steps than the single screw dislocations. Both independent and dependent step propagation velocities have been considered in the present discussion.     

On the Influence of the Crystallization Conditions on the Microstructure of the Directionally Solidified Eutectic of the LiF – LiYF4 System

This paper particularly deals with the transitions in degree and type of ordering of a given microstructure. As a model system the eutectic of the LiF – LiYF₄ system was used for studies of transitions from ‘anomal’ microtextures with non-cooperative production of seeds of both phases to the fibrous microtexture. It was shown that even if the matrix phase LiYF₄ contains a relative high volume ratio (36%) of LiF it is possible to create highly ordered fibrous microtextures with pronounced hexagonal ordering of particular LiF fibres. It was also shown that methods of image processing could be used for sensitive evaluation of small changes in degree of ordering of the microstructure. These procedures make possible a deeper understanding of the influence of the solidification conditions on the microstructure of the directionally solidified eutectic and creates possibilities for optimalisation of the technological process.