New Sintered Li2O–Al2O3–SiO2 Ultra‐Low Expansion Glass‐Ceramic

We developed a new Li₂O–Al₂O₃–SiO₂ (LAS) ultra‐low expansion glass‐ceramic by nonisothermal sintering with concurrent crystallization. The optimum sintering conditions were 30°C/min with a maximum temperature of 1000°C. The best sintered material reached 98% of the theoretical density of the parent glass and has an extremely low linear thermal expansion coefficient (0.02 × 10^?6/°C) in the temperature range of 40°C–500°C, which is even lower than that of the commercial glass‐ceramic Ceran^® that is produced by the traditional ceramization method. The sintered glass‐ceramic presents a four‐point bending strength of 92 ± 15 MPa, which is similar to that of Ceran^® (98 ± 6 MPa), in spite of the 2% porosity. It is white opaque and does not have significant infrared transmission. The maximum use temperature is 600°C. It could thus be used on modern inductively heated cooktops.     

Enhanced power factor of textured Al‐doped‐ZnO ceramics by field‐assisted deforming

Field‐assisted deforming method has been used to prepare c‐axis textured Al‐doped‐ZnO Ceramics (AZO) ceramics. In such cases, AZO ceramics with different degree of texture can be controlled efficiently. As a consequence, the electrical conductivity has been significantly enhanced for AZO ceramic with high degree of texture. The electrical conductivity for highly textured AZO is as high as 29.5 S·m^?1, 6 times higher than random orientation AZO ceramics. The enhanced electrical conductivity leads to a higher power factor of 5.3×10^?4 W·m^?1·K^?2 at 750 K, a 60.6% improvement over the random orientation AZO ceramic.     

Effects of Nd Content on Structure and Chemical Durability of Zirconolite–Barium Borosilicate Glass‐Ceramics

The effects of Nd₂O₃ content (0–12 wt %) on crystalline phases, microstructure, and chemical durability of barium borosilicate glass‐ceramics belonging to SiO₂–B₂O₃–Na₂O–BaO–CaO–TiO₂–ZrO₂–Nd₂O₃ system were studied. The results show that the glass‐ceramics with 2–6 wt% of Nd₂O₃ possess mainly zirconolite and titanite phases along with a small amount baddeleyite phase in the bulk. Calcium titanate appears when the Nd₂O₃ content increases to 8 wt%, and the amount of quadrate calcium titanate crystals increases with further increasing content of Nd₂O₃. For the glass‐ceramics with 6 wt% Nd₂O₃ (Nd‐6), Nd elements homogeneously distribute in zirconolite, titanite, and residual glass phases. There is a strong enrichment of Nd in calcium titanate crystals for the sample with 10 wt% Nd₂O₃. The viscosity of Nd‐6 glass is about 49 dPa·s at 1150°C. Moreover, Nd‐6 glass‐ceramics show the lower normalized leaching rates of B (LR_B), Ca (LR_Ca), and Nd (LR_Nd) when compared to that of the sample with 8 wt% Nd₂O₃. After 42 days, LR_B, LR_Ca, and LR_Nd of the Nd‐6 glass‐ceramics are about 6.8 × 10^?3, 1.6 × 10^?3, and 4.4 × 10^?6 g·m^?2·d^?1, respectively.     

Sealing Glass‐Ceramics with Near Linear Thermal Strain,Part I: Process Development and Phase Identification

A widely adopted approach to form matched seals in metals having high coefficient of thermal expansion (CTE), e.g. stainless steel, is the use of high CTE glass‐ceramics. With the nucleation and growth of Cristobalite as the main high‐expansion crystalline phase, the CTE of recrystallizable lithium silicate Li₂O–SiO₂–Al₂O₃–K₂O–B₂O₃–P₂O₅–ZnO glass‐ceramic can approach 18 ppm/°C, matching closely to the 18 ppm/°C–20 ppm/°C CTE of 304L stainless steel. However, a large volume change induced by the α‐β inversion between the low‐ and high‐ Cristobalite, a 1^st order displacive phase transition, results in a nonlinear step‐like change in the thermal strain of glass‐ceramics. The sudden change in the thermal strain causes a substantial transient mismatch between the glass‐ceramic and stainless steel. In this study, we developed new thermal profiles based on the SiO₂ phase diagram to crystallize both Quartz and Cristobalite as high expansion crystalline phases in the glass‐ceramics. A key step in the thermal profile is the rapid cooling of glass‐ceramic from the peak sealing temperature to suppress crystallization of Cristobalite. The rapid cooling of the glass‐ceramic to an initial lower hold temperature is conducive to Quartz crystallization. After Quartz formation, a subsequent crystallization of Cristobalite is performed at a higher hold temperature. Quantitative X‐ray diffraction analysis of a series of quenched glass‐ceramic samples clearly revealed the sequence of crystallization in the new thermal profile. The coexistence of two significantly reduced volume changes, one at ~220°C from Cristobalite inversion and the other at ~470°C from Quartz inversion, greatly improves the linearity of the thermal strains of the glass‐ceramics, and is expected to improve the thermal strain match between glass‐ceramics and stainless steel over the sealing cycle.     

Low‐Temperature Sintering of β‐Spodumene Ceramics Using Li2O–GeO2 as a Sintering Additive

Low‐temperature sintering of β‐spodumene ceramics with low coefficient of thermal expansion (CTE) was attained using Li₂O–GeO₂ sintering additive. Single‐phase β‐spodumene ceramics could be synthesized by heat treatment at 1000°C using highly pure and fine amorphous silica, α‐alumina, and lithium carbonate powders mixture via the solid‐state reaction route. The mixture was calcined at 950°C, finely pulverized, compacted, and finally sintered with or without the sintering additive at 800°C–1400°C for 2 h. The relative density reached 98% for the sample sintered with 3 mass% Li₂O–GeO₂ additive at 1000°C. Its Young's modulus was 167 GPa and flexural strength was 115 MPa. Its CTE (from R.T. to 800°C) was 0.7 × 10^?6 K^?1 and dielectric constant was 6.8 with loss tangent of 0.9% at 5 MHz. These properties were excellent or comparative compared with those previously reported for the samples sintered at around 1300°C–1400°C via melt‐quenching routes. As a result, β‐spodumene ceramics with single phase and sufficient properties were obtained at about 300°C lower sintering temperature by adding Li₂O–GeO₂ sintering additive via the conventional solid‐state reaction route. These results suggest that β‐spodumene ceramics sintered with Li₂O–GeO₂ sintering additive has a potential use as LTCC for multichip modules.     

Glass‐Ceramics in the System BaO–SrO–ZnO–SiO2 with Adjustable Coefficients of Thermal Expansion

Glasses from the system BaO–SrO–ZnO–SiO₂ with different Ba/Sr ratios were characterized regarding crystallization behavior as well as the thermal expansion of almost fully crystallized glasses. Depending on the SrO concentration, different crystalline phases precipitate from the glasses. Those with low SrO concentrations precipitate crystals with the structure of low‐temperature BaZn₂Si₂O₇ as one of the major phases. Higher SrO concentrations cause the formation of Ba_1?xSr_xZn₂Si₂O₇ solid solutions with the structure of high‐temperature BaZn₂Si₂O₇. Both, the low‐ as well as the high‐temperature phase exhibit very different thermal expansion behaviors ranging from a very high coefficient of thermal expansion in the case of the low‐temperature phase to a very low coefficient of thermal expansion in the case of the high‐temperature phase. The glass‐ceramics with the highest and that with the lowest coefficient of thermal expansion measured between 100°C and 800°C show a difference of 7.9 × 10^?6 K^?1, which is caused solely by a substitution of BaO with SrO. In contrast, the maximum variation in the thermal expansion of the glasses was only 1.5 × 10^?6 K^?1. The microstructure of sintered and afterward crystallized glass powders was analyzed via scanning electron microscopy and showed crack‐free samples with low porosity.     

Effect of P2O5 on the Nonisothermal Sinter‐Crystallization Process of a Lithium Aluminum Silicate Glass

Dense (~98.5%), lithium aluminum silicate glass‐ceramics were obtained via the sinter‐crystallization of glass particle compacts at relatively low temperatures, that is, 790–875°C. The effect of P₂O₅ on the glass‐ceramics' sinter‐crystallization behavior was evaluated. We found that P₂O₅ does not modify the surface crystallization mechanism but instead delays the crystallization kinetics, which facilitates viscous flow sintering. Our glass‐ceramics had virgilite (Li_xAl_xSi_3‐xO₆; 0.5 < x < 1), a crystal size <1 μm, and a linear thermal expansion coefficient of 2.1 × 10^?6°C^?1 in the temperature range 40–500°C. The overall heat treatment to obtain these GCs was quite short, at ~25 min.     

Ultra‐Low Fire Glass‐Free Li3FeMo3O12 Microwave Dielectric Ceramics

A new ultra‐low fire glass‐free microwave dielectric material Li₃FeMo₃O₁₂ was investigated for the first time. Single phase ceramics were obtained by the conventional solid‐state route after sintering at 540°C–600°C. The atomic packing fraction, FWHM of the A_g oxygen‐octahedron stretching Raman mode and Qf values of samples sintered at different temperatures correlated well with each other. The sample with a Lower Raman shift showed a higher dielectric constant. Interestingly, the system also showed a distinct adjustable temperature coefficient of resonant frequency (from ?84× 10^?6/°C to 25 × 10^?6/°C).     

A Novel Glass‐Ceramic with Ultra‐Low Sintering Temperature for LTCC Application

In this paper, the phase compositions and the dielectric properties of 3ZnO–2B₂O₃ glass‐ceramic prepared by solid‐state method were investigated. The X‐ray diffraction patterns show that all sintered samples consist of Zn₃B₂O₆ and α‐Zn(BO₂)₂. The dielectric properties changed significantly with the sintering temperature. After sintering at 650°C for 30 min, the glass‐ceramic exhibits optimum dielectric properties: a dielectric constant of 7.5 and a dielectric loss of 0.6 × 10^?3 at 10 MHz. The chemical compatibility with Ag electrode under the co‐fired process illustrates a potential application in low temperature co‐fired ceramic field for the glass‐ceramic.     

Ultra High Energy‐Storage Density in the Barium Potassium Niobate‐Based Glass‐Ceramics for Energy‐Storage Applications

The barium potassium niobate‐based glass‐ceramics with high energy‐storage density, high discharge efficiency, and fast discharge speed have been prepared. It was found that dielectric breakdown strength decreases when the crystallization temperature increases. Glass‐ceramics have high energy‐storage density up to 14.58 ± 1.14 J/cm³ with high breakdown strength of 2382 ± 92 kV/cm. Discharge energy density and discharge efficiency of glass‐ceramic capacitor were achieved through a pulse charge–discharge circuit. The reduction of discharge efficiency with the increase of crystallization temperature is mainly caused by interfacial polarization.     

Kinetics of the thermal dissociation of ZnO exposed to concentrated solar irradiation using a solar‐driven thermogravimeter in the 1800–2100 K range

The two‐step H₂O‐splitting thermochemical cycle based on the Zn/ZnO redox reactions is considered for solar H₂ production, comprising the endothermal dissociation of ZnO followed by the exothermal hydrolysis of Zn. A solar‐driven thermogravimeter, in which a packed‐bed of ZnO particles is directly exposed to concentrated solar radiation at a peak solar concentration ratio of 2400 suns while its weight loss is continuously monitored, was applied to measure the thermal dissociation rate in a set‐up closely approximating the heat and mass transfer characteristics of solar reactors. Isothermal thermogravimetric runs were performed in the range 1834–2109 K and fitted to a zero‐order Arrhenius rate law with apparent activation energy 361 ± 53 kJ mol^?1 K^?1 and frequency factor 14.03 × 10⁶ ± 2.73 × 10⁶ kg m^?2 s^?1. Application of L‘vov’s kinetic expression for solid decomposition along with a convective mass transport correlation yielded kinetic parameters in close agreement with those derived from experimental data. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Sealing glass‐ceramics with near‐linear thermal strain,part III: Stress modeling of strain and strain rate matched glass‐ceramic to metal seals

Thermal mechanical stresses of glass‐ceramic to stainless steel (GCtSS) seals are analyzed using finite element modeling over a temperature cycle from a set temperature (T_set) 500°C to ?55°C, and then back to 600°C. Two glass‐ceramics having an identical coefficient of thermal expansion (CTE) at ~16 ppm/°C but very different linearity of thermal strains, designated as near‐linear NL16 and step‐like SL16, were formed from the same parent glass using different crystallization processes. Stress modeling reveals much higher plastic strain in the stainless steel using SL16 glass‐ceramic when the GCtSS seal cools from T_set. Upon heating tensile stresses start to develop at the GC‐SS interface before the temperature reaches T_set. On the other hand, the much lower plastic deformation in stainless steel accumulated during cooling using NL16 glass‐ceramic allows for radially compressive stress at the GC‐SS interface to remain present when the seal is heated back to T_set. The qualitative stress comparison suggests that with a better match of thermal strain rate to that of stainless steel, the NL16 glass‐ceramic not only improves the hermeticity of the GCtSS seals, but would also improve the reliability of the seals exposed to high‐temperature and/or high‐pressure abnormal environments.     

Residence time distribution in a single‐phase rotor–stator spinning disk reactor

A reactor model for the single‐phase rotor–stator spinning disk reactor based on residence time distribution measurements is described. For the experimental validation of the model, the axial clearance between the rotor and both stators is varied from 1.0 × 10^?3 to 3.0 × 10^?3 m, the rotational disk speed is varied from 50 to 2000 RPM, and the volumetric flow rate is varied from 7.5 × 10^?6 to 22.5 × 10^?6 m³ s^?1. Tracer injection experiments show that the residence time distribution can be described by a plug flow model in combination with 2–3 ideally stirred tanks‐in‐series. The resulting reactor model is explained with the effect of turbulence, the formation of Von Kármán and Bödewadt boundary layers, and the effect of the volumetric flow rate. © 2013 American Institute of Chemical Engineers AIChE J, 59: 2686–2693, 2013     

Influence of fluorine source and TiO2 on glass ceramics based on red clay and magnesite

Abstract

Within the Fe₂O₃–BaO–Al₂O₃–SiO₂ glass system, the influence of different fluorine source and TiO₂ on the crystallisation behaviour and thermal expansion characteristics has been investigated. These aluminosilicate glasses, with different nucleation catalysts (TiO₂, AlF₃ and NH₄F), were successfully prepared from Saudi Arabia raw materials (red clay and magnesite). Thermal behaviour, crystalline phases, microstructures, and thermal expansion coefficient were studied by DTA, XRD, SEM techniques, as well as a dilatometer. Spinel, hexacelsian, monocelsian, kinoshitalite, and Ba-osumilite were developed in glass ceramic samples. Spinel was the early phase formed in all glasses; however, with increasing temperature, hexacelsian and kinoshitalite were developed in fluorine free glasses and fluorine containing glasses respectively. Ba-osumilite was developed and hexacelsian was converted into monocelsian during lengthy heat treatment at 1150°C. A bulk crystalline microstructure was obtained; however, a microscale structure was observed in fluorine free samples and a homogeneous nanoscale microstructure was developed in fluorine containing samples. The coefficient of thermal expansion (CTE) varied according to the phases and the heat treatment parameters. Development of spinel, hexacelsian and kinoshitalite in glass ceramic samples gave CTE values between 86·44 × 10^?7 and 52·53 × 10^?7°C^?1, whereas crystallisation of Ba-osumilite and monocelsian gave CTE values between 51·29 × 10^?7 and ?7·35 × 10^?7°C^?1.     

A study of compressive strength between zirconia framework and veneering ceramic as a function of thermal expansion coefficient using Shell–Nielsen test method

This study investigated the adhesion between zirconia framework and four veneering ceramic (VC) materials with varying coefficients of thermal expansions (CTE). Zirconia rods (N?=?40) (ICE Zirkon) (diameter: 4 mm, height: 20 mm) were milled and sintered. After firing, the zirconia rods were air-abraded and cleaned. They were randomly assigned to receive four VCs (n?=?10/group), namely (a) Vita VM9 (VZ; 9–9.2?×?10^?6? K^?1), (b) Cerabien ZR (CZ; 9.1?×?10^?6 K^?1), (c) Matchmaker ZR (MM; 9.4?×?10^?6?K^?1), and (d) Ice Zirconia Ceramic (IZ; 9.6?×?10^?6?K^?1). The VCs were then fired onto zirconia rods (height: 2 mm, thickness: 2 mm) circumferentially and were thermocycled for 6000 times (5/55 °C, dwell time: 30?s). Specimens were loaded from the top of the zirconia rods (0.5 mm/min) in a universal testing machine until debonding. Shell–Nielsen bond strength values were calculated (MPa). Failure types were evaluated under SEM. The data were statistically analyzed (one-way ANOVA, Tukey’s; α?=?0.05). Weibull distribution values including the Weibull modulus (m) (0.05) was calculated. The highest mean bond strength (MPa) was obtained for CZ (42.08?±?4.08), followed by VZ (41.77?±?4.92), MM (40.7?±?3.64), and IZ (40.05?±?5.78). While mean bond strength for VZ, MM, and IZ were not significantly different (p?>?0.05), CZ was significantly higher than that of IZ (p?<?0.05). The lowest shape value was for VZ (m?=?16.94) and the highest for MM (m?=?20.16). Mainly, adhesive failures followed by mixed failures were observed. VCs with a greater mismatch of CTE with the zirconia framework exhibited similar Shell–Nielsen bond strength to those with fewer mismatches. CTE mismatch did not affect the results of CZ (9.1?×?10^?6 K^?1) and IZ (9.6?×?10^?6 K^?1).     

Synthesis,characterization, and thermo‐optical properties of azobenzene polyurethane containing chiral units

An optically active levoazobenzene polyurethane (PU) was synthesized and was based on the chromophore 4‐(4′‐nitrophenylazo) phenylamine, the chiral reagent L (?)‐tartaric acid, and toluene diisocyanate. The chemical structure and thermal properties were characterized by ultraviolet–visible spectroscopy, Fourier transform infrared spectroscopy, ¹H‐NMR spectroscopy, and differential scanning calorimetry. The PU had high number‐ and weight‐average molecular weights up to 52 300, a large glass‐transition temperature of 235.7°C, and an optical rotation of ?18.06°, The optical parameters, including the refractive index (n) and thermo‐optic coefficient (dn/dT); the dielectric constant (?) and its variation with temperature; and the thermal volume expansion coefficient and its variation with temperature of PU were obtained. The dn/dT and ? values for the polymer were in the range ?4.1200 to 3.6257 × 10^?4 °C^?1 and 2.00 ± 0.11, respectively. The dn/dT values were one order of magnitude larger than those of inorganic glasses, such as zinc silicate glass (5.5 × 10^?6 °C^?1) and borosilicate glass (4.1 × 10^?6 °C^?1), and were larger than organic materials, such as polystyrene (?1.23 × 10^?4 °C^?1) and poly(methyl methacrylate) (?1.20 × 10^?4 °C^?1). The ? values were lower than that of alicyclic polyimide and semiaromatic polyimide. The obtained PU is expected to be useful for optical switching and optical waveguide areas. The conclusion has a little significance for the development of a new digital optical switch. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Thermal Properties of Rare‐Earth Monosilicates for EBC on Si‐Based Ceramic Composites

Rare‐earth (RE) monosilicates are promising candidates as environmental barrier coating (EBC) materials for ceramic matrix composites for aerospace applications. Five rare‐earth monosilicate materials have been investigated: Y₂SiO₅, Gd₂SiO₅, Er₂SiO₅, Yb₂SiO₅, and Lu₂SiO₅ produced from RE oxides and silica starting materials pressed and sintered at 1580°C under flowing air. Relative densities above 94% were obtained for all samples and ceramics were made containing 85–100 wt% of the RE monosilicate according to X‐ray diffraction (XRD) with RE disilicates as the second phase in the Gd, Yb, and Lu silicate systems. Microstructures were characterized using scanning electron microscopy and XRD, and thermal properties measured including specific heat, thermal expansion, and thermal diffusivity. For the first time, specific heat capacity values are reported for the monosilicates [0.45–0.69 J·(g·K)^?1]. Thermal expansion coefficients (TECs) of the dense samples ranged between 5.9 and 10.3 × 10^?6 K^?1 measured for 473 to 1473 K. All EBCs have low thermal conductivities [1.8 W·(m·K)^?1 or less] making them excellent EBC insulators.     

Influence of Crystallization on the Conversion of Sm3+→Sm2+ in SrO–Bi2O3–K2O–B2O3 Glass‐Ceramics

Sm³⁺‐doped glass 13SrO–2Bi₂O₃–5K₂O–80B₂O₃ was fabricated by the conventional melt‐quenching technique. The glass‐ceramics were obtained by heating the as‐prepared glasses in air atmosphere at selected temperatures 550°C, 600°C, 615°C, and 650°C, respectively. The luminescence spectra of both Sm³⁺ and Sm²⁺ were detected in the ceramic heated at 650°C where crystalline phase is formed. The as‐prepared glass and the ceramics heated at 550°C, 600°C, and 615°C show only the emission due to Sm³⁺. In the sample heated at 650°C in air atmosphere, however, part of Sm³⁺ ions was converted to Sm²⁺, giving rise to sharp emission lines which are characteristic of Sm²⁺ in crystalline state. It is suggested that Sm²⁺ ions are located at Sr²⁺ site in the ceramic while Sm³⁺ ions are located at Bi³⁺ sites. The Sm²⁺‐doped glass‐ceramic has a high optical stability because the fluorescence intensity decreases by only about 8% of its initial value upon excitation at 488 nm Ar⁺ laser.     

Lithium Disilicate Glass‐Ceramics by Heat Treatment of Lithium Metasilicate Glass‐Ceramics Obtained by Hot Pressing

Lithium disilicate glass‐ceramics are widely used as dental ceramics due to their machinability and translucency. In this study, lithium disilicate glass‐ceramic was fabricated through heat treatment of lithium metasilicate glass‐ceramics obtained by hot pressing of glass powder composed of SiO₂–Li₂O–P₂O₅–ZrO₂–Al₂O₃–K₂O–CeO₂ at low temperature. The crystalline phase, microstructure, and mechanical properties were investigated. The results indicated that lithium metasilicate glass‐ceramic with a relative density of higher than 99% was obtained after hot pressing, and glass‐ceramic with interlocked rod‐like Li₂Si₂O₅ crystals and good flexural strength (338 ± 20 MPa) was successfully obtained through heat treatment. The two‐step method was believed to be feasible in tailoring the microstructure and mechanical properties of lithium disilicate glass‐ceramics.     

Theoretical Prediction and Experimental Investigation on the Thermal and Mechanical Properties of Bulk β‐Yb2Si2O7

The thermal and mechanical properties of β‐Yb₂Si₂O₇ were investigated using a combination of first‐principles calculations and experimental investigations. Theoretically, anisotropic chemical bonding and elastic properties, weak interatomic (010) and (001) planes in the crystal structure, damage tolerance, and low thermal conductivity are predicted. Experimentally, preferred orientation, superior mechanical properties, and damage tolerant behavior for hot‐pressed bulk β‐Yb₂Si₂O₇ are approved. Slipping along the weakly bonded {010}, {001}, or {100} planes, grain delamination, buckling, and kinking of nanolaminated grains are identified as main mechanisms for damage tolerance. The anisotropic linear thermal expansion coefficients (CTEs) are: α_a = (3.57 ± 0.18) × 10^?6 K^?1, α_b = (2.49 ± 0.14) × 10⁶ K^?1, and α_c = (1.48 ± 0.22) × 10^?6 K^?1 (673–1273 K). A low thermal conductivity of ~2.1 W (m·K)^?1 at 1273 K has been confirmed. The unique combination of these properties endow it a potential candidate for thermal barrier coating (TBC)/environmental barrier coating of silicon‐based ceramics.