Crystallization study of (TiO2, ZrO2)-rich SiO2-Al2O3-CaO glasses Part I Preparation and characterization of zirconolite-based glass-ceramics

Nuclear power reactors generate long-lived radionuclides such as minor actinides (Np, Am, Cm) which are mainly responsible for the long term radiotoxicity of high level nuclear wastes obtained after reprocessing of nuclear spent fuel. Specific highly durable matrices such as glass-ceramics appear as good candidates for the immobilization of minor actinides. This work concerns the synthesis and the characterization of zirconolite (CaZrTi₂O₇) based glass-ceramics prepared by controlled devitrification of (TiO₂, ZrO₂)-rich SiO₂-Al₂O₃-CaO parent glasses for which neodymium was selected to simulate the radioactive trivalent minor actinides. The present study reports the effect of increasing TiO₂, ZrO₂ and CaO amounts in glass composition on the structure and the composition of the zirconolite crystals (formed as the only crystalline phase in the bulk of the glass), on their nucleation rate I(Z) and on the volume proportion of crystalline phase V of the glass-ceramics. It appears that I(Z) and V strongly increase when the parent glass composition changes. Neodymium electron spin resonance (ESR) shows that the total amount of Nd³⁺ ions incorporated in the zirconolite phase increases with TiO₂, ZrO₂ and CaO amounts in parent glass composition.     

TiO_2掺杂对SiO_2-Al_2O_3-MgO系玻璃结构和力学性能的影响

采用传统熔体冷却法制备TiO_2掺杂量为0~1.8wt%的TiO_2/SiO_2-Al_2O_3-MgO系玻璃,探讨了不同TiO_2质量分数对玻璃体积密度、弯曲强度、压缩强度、压缩模量和结构稳定性的影响规律。结果发现:当TiO_2含量小于1.5wt%时,TiO_2/SiO_2-Al_2O_3-MgO系玻璃的光学带隙随着TiO_2含量的增加而减小、玻璃结构更加稳定,其体积密度、弯曲强度、压缩强度以及压缩模量均随着TiO_2含量的增加而上升;当TiO_2含量超过1.5wt%后,该玻璃体系的结构稳定性和力学性能均随着TiO_2含量增加而下降;当TiO_2的质量分数为1.5wt%时,玻璃的光学带隙达到最小值为3.75eV,各项力学性能达到最优,其弯曲强度为110.36 MPa、压缩强度为240.18 MPa、压缩模量为115.03GPa。适量TiO_2的掺杂,减少了玻璃网络结构中非桥氧的数量,使孤立的岛状网络单元重新聚合,从而显著提高了玻璃的结构稳定性和力学性能;但过量的TiO_2迫使TiO_2/SiO_2-Al_2O_3-MgO系玻璃结构中的桥氧键断裂生成非桥氧,由此显著降低了其结构稳定性和力学性能。     

退火工艺对SiO2-Al2 O3-MgO-F系玻璃陶瓷分相和析晶的影响

以SiO2、Al2O3、MgO和AlF3为主要原料,采用高温熔融法,在一定温度下将原料熔化,然后将熔体浇铸到预热的模具中,经不同退火工艺得到基础玻璃。将此玻璃快速加热到析晶温度,进行相同的析晶处理,得到玻璃陶瓷样品。应用TEM、SEM和XRD等技术,系统研究了退火工艺对SiO2-Al2O3-MgO-F系玻璃陶瓷分相和析晶的影响。结果表明：随着退火温度的提高,玻璃的结构逐渐得到松弛,促进了Mg^2 和F^-离子的扩散,玻璃内分相程度逐渐增大,其分相形貌由液滴状→连通状→团簇状过渡;玻璃的分相程度对高温析晶时晶体的析出形态也有一定影响,与低温退火的玻璃相比较,高温退火时析出的云母晶体更加均匀;随着退火时间的延长,析出的晶体变得越来越细小,晶体间的交错度和晶体的均匀程度逐渐增大。     

Crystallization of CaO−Al2O3−SiO2 and CaO−MO−Al2O3−SiO2 (M=Mg,Zn) glasses

A range of CaO−Al₂O₃−SiO₂ glasses have been prepared by fusion of pure starting materials in platinum crucibles. Compositions containing large amounts on network formers, Al₂O₃ and SiO₂, are difficult to crystallize. If the amount of network former is reduced, glasses will self-nucleate and crystallize more readily, but the products of crystallization tend to react with water. This conflict has been partly resolved by adding MgO and ZnO and tailoring compositions so as to produce a phase, variously designated “Q” or “pleochroite”, ideally Ca₂₀Al_32-2v MgvSivO₆₈, withv close to 4. Pleochroite crystallizes with a typically fibrous morphology. Preliminary experiments on fragments and melt-cast glass rods indicate that these compositions can be heat treated without deformation to yield highly crystalline, transparent ceramics.     

Formation of crystalline phases in (SiO2–CaO–Fe2O3)–TiO2 based glasses

Glass ceramics were obtained by annealing of the SiO₂–CaO–Fe₂O₃ system, and the devitrifying effect of adding TiO₂ was compared with Fe₂O₃. In spite of its higher field strength, TiO₂ was unable to accelerate the devitrifying kinetics as could be seen in the TTT diagrams between 825 and 1000 °C. In both kinds of composition, a strong increase in electric conductivity was observed by SEM as crystalline phases appeared in heat-treated glass, which corresponded to a redistribution of iron oxide in the matrix material. XRD showed the presence of wollastonite and haematite, as well as amorphous silica, and energies of activation (E_a) were calculated with respective values of 210 and 450 kJ/mol for wollastonite and haematite.     

SiO2-Al2O3-MgO-K2O-(F,Cr2O3)玻璃陶瓷的析晶

为了研究形核剂氟化物、氧化铬及其复合作用对SiO2-Al2O3-MgO-K2O系玻璃陶瓷形核和析晶的影响,采用高温熔融法制备了不同氟化物和氧化铬含量的基础玻璃,并对该基础玻璃进行了差热分析、显微分析和X射线衍射分析.结果表明:在所研究的玻璃体系中,氟化物或氧化铬的加入,都不同程度地削弱了玻璃体的结构,降低了玻璃的转变温度;当基础玻璃只含有氟化物时,玻璃在表面析出云母晶体,内部发生分相现象,而玻璃中仅含有氧化铬时,玻璃中发生整体析晶;没有氟化物的存在,不能析出云母晶体;当同时加入两种元素的物质后,对玻璃体析晶的影响不是简单的加合,复合的作用更为突出,进一步降低了玻璃的转变温度和析晶温度,整体均匀析出云母晶体.     

Optical absorption of nickel (II) in Na2O-NaCl-B2O3 and Na2O-NaBr-B2O3 glasses

The optical absorption spectra of nickel(II) have been studied in Na₂O-NaX-B₂O₃ glasses X = Cl or Br); the absorption bands have been interpreted in terms of Ligand Field Theory. In halide-free B₂O₃-rich glasses nickel(II) is octahedral, and halide substitution is accompanied by the formation of a tetrahedral nickel(II)-oxide-halide complex. In alkali-rich glasses nickel(II) is square planar and/or tetrahedral and no halide substitution can be observed. It is suggested that the halide ions are unable to substitute for oxygen in the predominantly covalent nickel(II)-oxygen bonds of the complex formed in basic glasses.     

α-Al2O3基片(0001)表面及其对ZnO吸附位置研究

建立了一种2×2α-Al2O3 (0001)基片表面吸附ZnO模型,在周期边界条件下的k空间中,采用基于密度泛函理论的局域密度近似平面波超软赝势法,对α-Al2O3 (0001)基片表面结构及其ZnO分子在表面最初吸附生长位置进行了计算研究.由于较大的表面弛豫,使得氧原子全部暴露于基片最外表面,明显地表现出O原子电子表面态;驰豫后的表面能对ZnO分子产生较强的化学吸附,表面电子结构将发生明显的变化,其表面最优吸附生长点的方位正好偏离α-Al2O3 (0001)表面氧六角对称30°;并计算了这些吸附生长点处Zn与表面O的结合能.     

Correlation of electrical properties and internal friction in mixed conduction glasses containing ionic and electronic conduction (1) Fe2O3-Na2O-P2O5 glasses

The electrical properties and internal friction in (40–x)Fe₂O₃·xNa₂0.60P₂O₅ glasses were measured. Two or three peak on internal friction were observed in the temperature range of –100 to 300° C at a frequency of about 1 Hz. The peak area of internal friction could be explained quantitatively by the additivity law of diffusion of Na⁺ ion and hopping of electrons which are carriers similar to those of dielectric loss. Activation energy, peak temperature of dielectric loss and internal friction showed almost the same value. Both relaxation phenomena have the same mechanism which is due to the diffusion of Na⁺ion and the hopping of electrons between Fe²⁺ Fe³⁺. The high-temperature peak is assumed to result from the interaction between protons or alkali ions and non-bridging oxygen.     

Enhancing energy storage density of (Ba, Sr)TiO3 ceramic particles by coating with Al2O3 and SiO2

The dielectric and energy storage properties of Ba_0.4Sr_0.6TiO₃ particles coated by Al₂O₃ and SiO₂ oxide were investigated. Results showed that the dielectric properties and energy storage density were improved apparently. The improvement of the energy storage density can be ascribed to two factors: one was that the breakdown strength was notably improved by the decrease of the porosity and defects; the other was the reduction of the grain size. Furthermore, the optimized composition showed the maximum energy storage density of 5.09 J/cm³, whereas the energy storage density of uncoated Ba_0.4Sr_0.6TiO₃ was only 0.24 J/cm³.     

Role of zirconium in nucleation and crystallization of a (SiO2, Al2O3, MgO,ZnO) glass

Journal of Materials Science Letters -     

Microhardness study of (1-x-y)(B2O3)-x(Li2O)-y(MCI2), (M=Cd, Zn) glasses

Results of microhardness measurements on (1-x-y)(B₂O₃)-x(Li₂O)-y(MCI₂), (M=Cd, Zn) glasses, in the applied load range 25–500 g, are presented. The microhardness was found to decrease with increase in load up to 50 g, then it increased and finally attained a practically constant value with increase in load. The effects of composition of the glasses on microhardness are discussed.     

Effect of transition elements (Ti,V, Cr,Mn, Co,Ni) oh thermal stability of Fe-B metallic glasses Part II: Crystallization temperature

The thermal stability against crystallization of iron-boron-based metallic glasses is investigated. The effect of small additions of transition elements on the crystallization temperature is shown, and a steric factor associated with the different substitutional atoms is pointed out to explain the results.Some analogies with the effect of substitutional elements on the primary recrystallization of iron-based cold-yorked solid solutions are suggested.     

Effect of 1 wt% CoO addition on dielectric and microstructural behaviour of (SrO·TiO2)-(2SiO2·B2O3) glass and glass ceramic

Glass of the nominal composition 64 wt%(SrO·TiO₂)·35 wt%(2SiO₂·B₂O₃)-1 wt%(CoO) was prepared. The glass samples were subjected to heat treatment at 900 and 950 C. The phase progression in these glass ceramics from X-ray diffraction studies shows the formation of Sr₂B₂O₅ as primary crystalline phase followed by rutile (TiO₂), Sr₃Ti₂O₇, SrB₂Si₂O₈ and Sr₃B₂SiO₈ as secondary phases. The first DTA exothermic peak of glass corresponds to the crystallization of Sr₂B₂O₅, rutile and Sr₃Ti₂O₇ phase while second crystallization peak may be assigned to the formation of SrB₂Si₂O₈ and Sr₃B₂SiO₈ phases. From microstructure studies we find that strontium borate grows with larger grain size whereas the other phases like Sr₃Ti₂O₇, TiO₂ appear smaller in size. Cobalt oxide content in the strontium titanate borosilicate glass ceramic gives the thermal stability to dielectric behaviour and decreases the dielectric loss. Paper presented at the poster session of MRSI AGM VI, Kharagpur, 1995     

Crystallization of AlPo4-5 from a system-1.5 (C3H7)3N.1.0Al2O3.1.0 P2O5. 40.0 H2O : Characterization of the products of crystallization

Crystallization of AlPO₄-5 from a gel of composition 1.5 Pr₃N (tripropylamine)· 1.0 Al₂O₃· 1.0 P₂O₅· 40.0 H₂O has been investigated by carrying out the hydrothermal synthesis of the crystalline aluminophosphate in a closed stainless steel pressure bomb at 423 K for different durations of the crystallization (0–48 h). The solid products obtained during the course of crystallization have been characterized thoroughly for their crystallinity (as AlPO₄-5), crystal size and morphology, N₂-sorption capacity (at 78 K and relative pressure of 0.3), thermal analysis, site energy distribution by TPD of pyridine, acidity by chemisorption of pyridine at 673 K and also for their catalytic activity in cumene cracking and o-xylene isomerization reactions at 673 K. The aluminophosphate formed at the different crystallization periods differ from each other largely in these properties. The optimum crystallization period for the synthesis of AlPO₄-5 is about 24 h.