Internal residual stresses in glass-ceramics: A review

Internal residual stresses arise in glass-ceramics upon cooling down from the crystallization temperature. These stresses are due to the thermal expansion and the elastic mismatch between the crystalline and glassy phases. Therefore, the mechanical properties of glass-ceramics are likely to depend not only on their composition and microstructure but also on the type (tension or compression) and magnitude of these residual stresses. In this work, we critically review the most commonly used theoretical models concerning residual stresses in glass-ceramics and glass-matrix composites, taking into consideration the effects of crystallized volume fraction, crystal shape and thermal expansion anisotropy. We also discuss most of the reported measurements of residual stresses in these dual-phase materials using different techniques, such as X-ray diffraction, nuclear magnetic resonance, Raman and fluorescence spectroscopy, and indentation. The available models and experimental results regarding spontaneous microcracking due to residual stresses are also discussed. Finally, guidelines for future work are suggested.     

On the phase diagram of polymorphic ethanol: Thermodynamic and structural studies

It is well known that ethanol exhibits a very interesting polymorphism presenting different solid phases: a fully-ordered (monoclinic) crystal, a (bcc) plastic crystal, which by quenching becomes an orientationally-disordered crystal with glassy properties (hence sometimes named ‘glassy crystal’), and the ordinary amorphous glass. We have carried out calorimetric, X-ray diffraction, and Brillouin-scattering experiments above liquid-nitrogen temperatures and have found several new features that shed more light on the rich and interesting phase diagram of ethanol. Firstly, we have identified up to four different varieties of the monoclinic crystalline phase depending on the thermal history. We also present new specific-heat data of these glassy and crystalline phases below the glass transition temperature up to the melting temperature. Furthermore, we have unexpectedly found that the amorphous phase can also be obtained by the unusual route of a very slow cooling of the liquid in some particular experimental set-ups, evidencing the heterogeneous character of the crystallization kinetics of these molecular glass-formers.     

Effect of devitrification on ion motion in lithium-disilicate glass

Li-disilicate glass-ceramics consist of microcrystallites imbedded in the glassy Li₂O · 2SiO₂ matrix where the number and size of the crystallites depend on the devitrification heat treatment. To assess ion motion in these model glass-ceramics, we have measured the temperature dependence of the dc conductivity, σ_dc, and the ⁷Li nuclear spin relaxation (NSR) rate, 1/T₁, in samples with various crystalline fraction, c, ranging from c = 0 (pure glass) to c = 1 (fully devitrified polycrystalline ceramic). The Cole–Cole presentation of the complex impedance shows two separate arcs caused by the remarkable difference of the ionic motion in the glassy and crystalline phase. These two arcs correspond to a bi-exponential decay of the ⁷Li nuclear spin magnetization where the resulting two NSR rates are induced by the ionic motion in the two phases. Thus the NSR and σ_dc data provide a comprehensive picture of the ionic motion in the glassy and crystalline phases. In particular, the ionic motion is the fastest in the glass; then at lower values of c we observe a metastable crystalline phase with ionic motion much greater than in the stable (LS₂) crystalline phase existing at large c-values.     

Crystallization kinetics of 1Na2O · 2CaO · 3SiO2 · glass monitored by electrical conductivity measurements

We investigated the kinetics of crystal nucleation, growth, and overall crystallization of a glass with composition close to the stoichiometric 1Na₂O · 2CaO · 3SiO₂. The nucleation and subsequent growth of sodium-rich crystals in this glass decreases the sodium content in the glassy matrix, drastically hindering further nucleation and growth. Compositional changes of the crystals and glassy matrix at different stages of the crystallization process were determined by EDS. These compositional variations were also monitored by electrical conductivity measurements, carried out by impedance spectroscopy, in glassy, partially, and fully crystallized samples. The electrical conductivity of both crystalline and glassy phases decreases with the increase of the crystallized volume fraction. Starting at a crystallized volume fraction of about 0.5, the crystalline phase dominates the electrical conductivity of the sample. This behavior was corroborated by an analysis of the activation energy for conduction. We show that electrical conductivity is highly sensitive and can indicate compositional shifts, changes in the spatial distribution of mobile ions in the glassy matrix. Conductivity measurements are thus a powerful tool for the investigation of complex heterogeneous systems, such as partially crystallized glasses and glass-ceramics.     

云母/莫来石可加工微晶玻璃的制备与析晶

通过调整SiO2-MgO-Al2O3-K2O-F基础玻璃的组成和热处理条件,制备了云母/莫来石复合可加工微晶玻璃.应用扫描电子显微镜和X射线衍射技术研究了可加工微晶玻璃的析晶特征.结果表明:添加3.0;ZnO(质量分数)的基础玻璃析出了锌尖晶石相,而锌尖晶石的析出抑制莫来石相的形成,没有得到云母/莫来石复合材料.当玻璃中添加3.0;的V2O5后,在试样中同时析出莫来石和云母晶体,但没有形成莫来石/云母复合的组织.含V2O5 8.0;的玻璃在等温析晶中,从表面析出莫来石和粗大枝状的云母晶体,云母间相互交错程度较低,只有在随炉升温的情况下,云母晶体以莫来石相为核心异质生长,形成均匀分布的云母/莫来石复合微晶玻璃材料.     

烧结法制备高炉渣CMAS系统微晶玻璃的研究

利用高炉渣及其它辅助原料制备基础玻璃,采用一步烧结法制备主晶相为辉石的CaO-MgO-Al2O3-SiO2(CMAS)微晶玻璃.综合运用DSC,XRD以及场发射扫描电子显微镜等测试手段,分析热处理制度对高炉渣CMAS微晶玻璃的析晶行为及性能的影响.结果表明:随着热处理温度上升,微晶玻璃的主晶相均为辉石,次晶相均为长石,晶相析出量增加,微晶玻璃的体积密度及抗折强度均呈现先增后减趋势.随着热处理时间增加,微晶玻璃的体积密度及抗折强度均呈现下降趋势.当热处理温度为1020 ℃,晶化时间30 min时,样品的机械性能最好,体积密度为2.690 g·cm-3,抗折强度为67.00 MPa.     

Leaching behavior of nuclear waste glass heterogeneities

The results of a six-month burial experiment in granite are discussed. An alkali borosilicate simulated nuclear waste glass was buried in 3 m boreholes at the 345 m level in the Stripa mine. Some glass specimens containing crystallites exhibit preferential attack of the interface between crystalline and glassy phases. The crystalline phase, identified as spinel solid solution, exhibits better chemical resistance than the glassy phase. Results obtained from homogeneous and heterogeneous glasses (i.e., those containing a crystalline phase) are compared.     

高炉熔渣微晶玻璃的结构与性能研究

高温熔渣具有大量显热与渣体.采用熔融法制备微晶玻璃可以更好地利用其热和渣,达到高效利用的目的.通过高温条件下混熔的方式制备性能稳定的基础玻璃.利用差示扫描量热仪(DSC)确定基础玻璃的热处理工艺制度.结合高分辨透射电子显微镜(TEM)、X射线衍射分析(XRD)、场发射扫描式电子显微镜(FE-SEM)对微晶玻璃的结构进行了研究.研究表明:基础玻璃中氟含量的增加,有利于促进微晶玻璃成核和晶体长大,降低微晶玻璃的形核结晶温度.在低温热处理得到微晶玻璃的主晶相为萤石,高温热处理得到的微晶玻璃析出了霞石和萤石两个微晶相.高炉渣微晶玻璃中,微晶相的出现可有效提高微晶玻璃的力学性能,试样的显微硬度最高可达585.68 MPa,抗折强度最高可达126.21 MPa.     

EAM calculations of the thermodynamics of amorphous copper

The structure and thermodynamics of pure amorphous Cu is studied with molecular dynamics, Monte Carlo and quasi-harmonic calculations using the embedded atom method. The pair distribution function, atomic volume, thermal expansion, enthalpy, entropy, and Gibbs free energy are calculated for the cyrstalline, liquid and amorphous phases. The glass transition temperature of the amorphous phase is found to be approximately 400 K ( 0.3 T_m). The free energy difference between the amorphous and supercooled liquid phases was determined to be at most 0.01 eV per atom. The free energy of the liquid and crystalline phases in our calculation is found to agree within 0.4% of experiment over the temperature range 400–2200 K. The free energy of the glass is found to be fairly well described by the simple Turnbull model. This is the first atomic-level calculation of the free energy of a metallic glass.     

制备NZP玻璃陶瓷的替代路线(玻璃反应烧结)中环境和粒度的影响

NZP材料因其具有可调节的低热膨胀性在在热冲击方面有潜在应用,而且NZP材料还具有良好的化学和热稳定性,还经常被作为陶瓷核废料中放射性核元素的封装和固定的候选材料.NZP型晶体结构允许使用大量的离子替代,使其产生了热膨胀的可调性以及离子导电性.然而,NZP在整块材料中制作起来非常困难,因为其过程需要保证高温和很长的烧结时间.通过对磷酸盐玻璃使用玻璃反应烧结工艺,在一种低温替代路线中获得了含有钨(IV)、锡(IV)的NZP结晶相.通过使用微波加热以及传统的熔炼和铸造技术,制备了一种含有适量NaPO3-Sn(II)O-W(VI)O3化合物的玻璃.粉碎后,玻璃粉末压于室温下冷却.将绿色的弹丸在烧结反应温度下分别经历不同的时间段进行固化.差热(DTA)实验中,结果显示不同的参数影响着NZP相的性能.在特定条件如玻璃的初始位置小于100 μm的玻璃颗粒度及在大气环境下固化,能够得到一种含有单一晶相的玻璃陶瓷,且晶相具有NZP型晶体结构.烧结过程中需要有氧的存在,但氧分压的提高并没有使NZP相的含量有所改善.此外,温度要严格控制高于710 ℃,保证获得的NZP玻璃陶瓷中不含有任何次晶相.     

Effect of heat-treatment condition on crystallization behavior and thermal expansion coefficient of Li2O-ZnO-Al2O3-SiO2-P2O5 glass-ceramics

Utilizing P₂O₅ as nucleation agent, a Li₂O-ZnO-Al₂O₃-SiO₂ glass was prepared by conventional melt quenching technique and subsequently converted to glass-ceramics with different crystal phases. During the processing, two-step heat-treatments including nucleation and crystallization were adopted. The effects of heat-treatment on the crystal type, the microstructure and the thermal expansion behavior of the glass-ceramics were studied by means of differential scanning calorimetry, X-ray powder diffraction analysis, scanning electron microscopy and thermal expansion coefficient tests. It was shown that the crystallization of occurred after the glass was treated at 580 °C. As the temperature increased from 580 °C to 630 °C, cristobalite and were identified as main and second crystal phases, respectively, in the glass-ceramic. An increase in the temperature to 700 °C, the β-quartz solid solution in the glass-ceramic accompanied by a decrease in cristobalite content. The transformation from to γ₀-Li₂ZnSiO₄ took place from 700 °C to 750 °C. The resulting crystallization phases in the glass-ceramics obtained at the temperature higher than 750 °C were β-quartz solid solution and γ₀-Li₂ZnSiO₄. The glass-ceramics containing or β-quartz solid solution crystal phase possessed a microstructure formed by the development of dendritic crystals. The thermal expansion coefficient of the glass-ceramics varied from 36.7 to 123.8 × 10⁻⁷ °C⁻¹ in the temperature range of 20-400 °C, this precise value is dependent on the type and the proportion of the crystalline phases presented.     

Effect of frit size on sintering, crystallization and electrical properties of wollastonite glass-ceramics

Wollastonite glass-ceramics were prepared through pressureless sintering. The sinterability of the prepared samples of the glassy powder in the system (SiO₂-CaO-Na₂O-Fe₂O₃-WO₃) was investigated in the temperature range 720-900 °C and soaking time of 180 min. The influence of the increase in the glass powder particle size on the sinterability and dielectric properties of the glass-ceramic samples was studied.The sintered specimens were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD) methods. XRD analysis revealed that wollastonite was the main crystalline phase in the sintered glass-ceramics. Dielectric properties such as dielectric constant and dielectric loss were measured via a network analyzer at 10 GHz.It was observed that the increase of the glassy powder particle size improved the sinterability and dielectric properties of the glass-ceramic specimens. Wollastonite glass-ceramics with 16 μm particle size had maximum constant and minimum loss (ε_r = 10.10 and tan δ = 0.005) compared with the other glass-ceramics.     

A new class of infrared transmitting glass-ceramics based on controlled nucleation and growth of alkali halide in a sulphide based glass matrix

Oxides-based glass-ceramics have been intensively studied and while they exhibit exceptional thermo-mechanical properties, their transparency in the infrared is limited to the 3 μm region. In this paper we describe a new type of glass-ceramics which are transparent up to 11 μm and based on the controlled nucleation and crystallization of cesium chloride sub-micron particles inside a Ge-Sb-S glass matrix. The evolution of the optical transmission versus annealing time and temperature has been investigated. Observations under scanning electronic microscopy as well as X-ray diffraction indicate that the crystalline phase is a primitive cubic cell with a parameter slightly inferior to that of pure CsCl and that the grain sizes are about 100 nm. A preliminary test on fracture propagation shows a much better resistance of glass-ceramics to cracks than the corresponding pure glass matrix.     

Phase Transition Studies in N(p-n-Heptyloxy Benzylidene)p-Toluidine, 70.1, Across I-N,N-SA and SA-SB Phases

The results of density and ultrasonic velocity variation with temperature in N(p-n-heptyloxy benzylidene)p-toluidine, HYBT, 70.1 exhibiting nematic, smectic-A and crystalline smectic-B phases, are presented. The isotropic-nematic, nematic-smectic-A phase transitions are found to be first order while the smectic-A crystal smectic-B transition is weak first order. The thermal expansion coefficient α computed from the molar volume data is presented. The molar sound velocity and molar compressibility of HYBT computed from the density and ultrasonic velocity data in isotropic phase are compared with the values of the other compounds in a homologous series. They are found to be in good agreement with theory.     

Thermal expansion of amorphous metals

The thermal expansion of three commercial amorphous metals has been measured from room temperature to above the crystallization temperature. The results clearly demonstrated the glassy nature of these materials and show the reduction in volume which results from crystallization. The glass transformation temperature is more evident here than on the usual differential scanning calorimeter curves. The activation energies associated with the glass transformation and crystallization are derived from data taken as a function of heating rate. The measured temperatures for the glass transformation and crystallization from the thermal expansion measurements are the same as those obtained from DSC measurements at the same heating rate.     

Sol–gel synthesis and characterization of barium (magnesium) aluminosilicate glass sealants for solid oxide fuel cells

Solid oxide fuel cells (SOFC) correspond to efficient energy conversion systems coupled with low emissions of pollutants. In the aim to fabricate high temperature planar SOFC, glass and glass-ceramic sealants are developed to associate several criteria and properties : high thermal expansion (11.0 to 12.0 ? 10^? 6 K^? 1), high electrical resistance > 2 kΩ/cm², good thermochemical compatibility with the other active materials of the fuel cell, and stability under H₂ and H₂O atmospheres at an operation temperature of 800 °C for a long time. According to these requirements, new BAS (BaO–Al₂O₃–SiO₂) and BMAS (BaO–MgO–Al₂O₃–SiO₂) glass-ceramic sealants have been developed by sol–gel route which is a non-conventional process for such applications. By this soft chemistry process, we anticipate a decrease in the glasses processing temperature due to a better homogeneity between cationic precursors in the mixture and a more important reactivity of materials. Experimental results in terms of thermomechanical properties, thermal expansion coefficient, crystalline phase content, and microstructure were discussed. In particular, the influence of the %BaO on the thermomechanical properties of glass-ceramics was described. Changes in properties of glass-ceramics were closely related to the microstructure. The influence of MgO on glass processing temperatures, on the structure and on the microstructure is evaluated in order to confirm that these glass-ceramics are promising candidates to SOFC applications. So, after performing a systematic investigation to the various systems, the properties of suitable glass were proposed.     

Leaching of some lithium silicate glasses and glass-ceramics by HCl

The leaching of some binary and ternary lithium silicate glasses and their respective glass-ceramics by HCl is reported.The leaching rate of lithium silicate glasses gradually decreases with the decrease of the percentage of Li₂O or by the introduction of small amounts of a third component, e.g. Al₂O₃, MgO, ZnO or B₂O₃. With a further increase in the proportions of B₂O₃ or ZnO the rate of leaching increases. The rate of leaching is also substantially modified by the conversion of glasses into glasses-ceramics.The results obtained are discussed in terms of the effects of the different ions on the rate of the interdifussion of the lithium and hydrogen ions in the glass and the leached layer, the phase separation developed in the glass, the type and concentration of crystalline phases developed in glass-ceramics and the composition of the residual glass phase.     

Structure,chemical durability and crystallization behavior of incinerator-based glassy systems

Vitrification by melting is being proposed as a convenient method to solidify different kinds of silicate and other oxide-based inorganic wastes. Incinerator bottom and fly ashes have been mixed with glass cullet, feldspar and clay by-products as melting fluxing agents. Washing, drying, and grinding pre-treatments followed by melting at 1450 °C lead to the formation of glasses and glass-ceramics, depending on the starting materials composition and thermal treatment. The obtained glasses have been studied by SEM, chemical durability tests in aqueous and alkaline environment, leaching test (UNI 10802), and by differential thermal analysis. The glass-ceramics morphology was investigated by XRD and SEM. The results were explained by the structure of the glasses caused by the presence of different amount of modifiers in the glassy lattice. The obtained glasses show good chemical resistance, in particular in alkaline environment and thermal characterization highlighted that the materials are also suitable to obtain glass-ceramics.     

Low-temperature properties of glassy and crystalline states of n-butanol

We present low-temperature measurements of the specific heat and the thermal conductivity for the three solid phases of n-butanol, namely glass, crystal and “glacial” phases. Whereas crystal and glass ones are found to exhibit the expected thermal behavior for crystalline and non-crystalline solids, respectively (i.e. Debye theory for crystals, and universal low-temperature properties with a boson peak and a concomitant plateau in the thermal conductivity for glasses), the so-called “glacial phase” behaves as a very defective crystal, confirming earlier work that identifies it as a mixed phase of nanocrystallites and a disordered matrix. We have also measured longitudinal and transverse sound velocities at low temperatures for the glass phase. The elastic Debye coefficient and Debye temperature of the glass determined from these measurements are found to agree very well with the calorimetric ones obtained from a SPM analysis of the specific heat.     

Acoustic properties of aspirin in its various phases and transformation stages studied by Brillouin scattering

The acoustic behaviors of aspirin were studied in its crystal, liquid, glass states as well as transformation stages between them. The Brillouin doublet of the longitudinal acoustic (LA) mode propagating to the [100] direction of aspirin crystals showed a splitting in a temperature range of 393-413 K below the melting temperature, which indicated a softening and distortion of the layered aspirin crystal in its premelting stage. One LA mode was observed in the liquid and glassy phases of aspirin, consistent with Brillouin selection rule for isotropic materials. The acoustic damping in the glassy aspirin was systematically higher than that in the crystal aspirin, which was in contrast to ibuprofen materials. These suggested different activity of remnant local dynamics that could couple to the acoustic waves, persistent in the glassy state of these two representative pharmaceuticals. In the supercooled liquid phase, the Brillouin spectrum displayed a new spectral feature characterized by a high-frequency cutoff and a distributed low-frequency contribution other than the Brillouin peak of the amorphous aspirin. This could be ascribed to the formation of crystalline grains in the glassy matrix due to cold crystallization.