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.     

氮气气氛下铁铝尖晶石的合成

烧结法合成铁铝尖晶石需要控制适当的气氛,以保证氧化亚铁(FeO)处在其稳定存在温度与氧分压条件下.以往通过对不同氧分压下Fe2O3-Al2O3相图以及1 500℃时FeO-Fe2O3-Al2O3三元系统等温截面图的分析,说明氧分压越低越有利于铁铝尖晶石的合成,而且Fe3O4与FeAl2O4的固溶度随着氧分压降低以及铁铝...     

Effect of Impact Modifier Types on Mechanical Properties of Rubber‐Toughened Glass‐Fibre‐Reinforced­ Nylon 66

The effect of adding rubber on the properties of glass‐fibre‐filled nylon 66 was investigated in this study. Styrene‐Ethylene‐Butylene‐Styrene and Ethylene‐Propylene elastomers grafted with maleic anhydride (SEBS‐g‐MA and EP‐g‐MA, respectively) were used to toughen the nylon‐matrix composites. Impact strength and elongation at break were found to increase with increasing rubber content, but flexural strength, tensile strength and stiffness decreased; however, by adding moderate amounts of rubber to glass‐fibre‐reinforced nylon 66, a desirable balance between stiffness and toughness of the material may be obtained. For example, the addition of 10 wt.% of SEBS‐g‐MA to nylon 66 with 23.62 wt.% glass fibre loading resulted in 28.3% and 167% increase in tensile strength and impact strength of the composites, respectively, when compared to neat nylon 66. This suggests that combining both glass fibres and rubber with nylon 66 is a useful strategy to optimize and enhance the properties of nylon 66. The procedure may be used to recycle polyamides, in general, and to develop components for under‐the‐hood automotive applications, in particular.     

Heat Treatment of Irradiated Graphite in an Oxidizing Atmosphere

It was demonstrated that thermal treatment and decontamination methods can be used for the processing of irradiated nuclear graphite. Special features of the treatment of the irradiated graphite surface with an oxidizing agent were considered. A mathematical model of the interaction of a gaseous oxidizing agent with irradiated graphite was described with consideration for the release of stored Wigner energy. It was established that the qualitative and quantitative compositions of gaseous reaction products depend on thermal and gas-dynamic process conditions. The mathematical models and calculation algorithms proposed can be used for the process optimization of the heat treatment of irradiated graphite for the selective recovery of a number of radionuclides (mainly, ¹⁴C and ³⁶Cl), which are concentrated in a thin near-surface layer of graphite components.     

Copper‐Catalyzed Decarboxylative CN Triple Bond Formation: Direct Synthesis of Benzonitriles from Phenylacetic Acids Under O2 Atmosphere

A copper‐catalyzed reaction of phenylacetic acids with urea was found to afford benzonitriles under an oxygen atmosphere. This reaction proceeds smoothly by a sequence of decarboxylation, dioxygen activation, C H bond functionalization, and nitrile formation with urea as the nitrogen source. Molecular oxygen was found to play a crucial role in this transformation. This reaction represents a novel protocol for the formation of benzonitriles in an environmental friendly way and with good functional group tolerability.

     

Er3+ Ions‐Doped Germano‐Gallate Oxyfluoride Glass‐Ceramics Containing BaF2 Nanocrystals

Er³⁺ ions‐doped germano‐gallate oxyfluoride glass‐ceramic containing BaF₂ nanocrystals was prepared through conventional melt quenching and subsequent thermal treatment method. X‐ray diffraction patterns and transmission electron microscope images confirmed the formation of BaF₂ nanocrystals in glass‐ceramics. Preferential incorporation of Er³⁺ ions into the BaF₂ nanocrystals were confirmed by the absorption spectra and emission spectra, and enhanced upconversion emission and infrared emission were observed. Relatively high transmittance in the mid‐infrared region indicated great potential of this germano‐gallate oxyfluoride glass‐ceramics as host materials for the efficient mid‐infrared emission from rare‐earth ions.     

Measurement of Mechanical Strength of Glass‐to‐Metal Joints

Three different test geometries were used to apply shear loading to fracture glass‐to‐metal joints typical of seals intended for use in planar solid oxide fuel cells (SOFCs): asymmetric compression; symmetric compression; and four‐point asymmetric bending. The measured apparent shear strengths were found to differ by an order of magnitude depending on the test configuration employed. In particular, the apparent shear strength measured in the asymmetric compression test was very low. Conversely, the highest apparent shear strengths were measured using the symmetric compression test and the four‐point asymmetric bend test gave an intermediate result. It is shown, by finite element modelling, that these differences are caused by differences in the normal stresses transverse to the joint. The locus of failure was always along the glass/metal interface in all test geometries. It is concluded that mechanical test procedures used to characterize glass‐ceramic seals in SOFC stacks need to be selected and interpreted with great care.     

Microwave Dielectric Properties of Novel Glass‐Free Low‐Firing Li2CeO3 Ceramics

Novel glass–free low temperature firing microwave dielectric ceramics Li₂CeO₃ with high Q prepared through a conventional solid‐state reaction method had been investigated. All the specimens in this paper have sintering temperature lower than 750°C. XRD studies revealed single cubic phase. The microwave dielectric properties were correlated with the sintering conditions. At 720°C/4 h, Li₂CeO₃ ceramics possessed the excellent microwave dielectric properties of ε_r = 15.8, Q × f = 143 700 (GHz), and τ_f  = ?123 ppm/°C. Li₂CeO₃ ceramics could be excellent candidates for glass‐free low‐temperature co‐fired ceramics substrates.     

Luminescence of Eu‐Doped Transparent LuPO4 Glass‐Ceramics

Dual valence Eu‐doped transparent glass‐ceramics containing LuPO₄ nanocrystals were fabricated by melt‐quenching technique in air atmosphere. Their luminescent properties were systematically investigated by excitation, emission spectra, and decay lifetime measurements. The prominent Stark splitting, low forced electric‐dipole ⁵D₀ – ⁷F₂ transition and long decay lifetimes of Eu^{3 +} emission for glass‐ceramics reveal the incorporation of Eu^{3 +} into LuPO₄ nanocrystals. The enhanced Eu^{2 +} emission and reduction mechanism of Eu^{3 +} to Eu^{2 +} after crystallization are discussed briefly. Our results indicate that transparent LuPO₄ glass‐ceramics may find applications in photonics.     

Modeling of Reactive Diffusion in Glass‐Ceramics

A model of reactive ion‐exchange diffusion in glassy materials (glasses and glass‐ceramics) has been developed. The model can be used to simulate the processes of ion‐exchange‐induced decrystallization of glass‐ceramics and ion‐exchange‐induced glass crystallization. Analysis of the model in dimensionless form resulted in determining a set of dimensionless complexes comprising dimensional parameters of the ion exchange system (diffusivity, rate constant of grain dissolution, initial size and composition of the crystalline grains, etc.). Numerical values of the complexes enable one, not solving the problem, to predict the composition and structure of glass‐ceramic subsurface layers produced in a certain glassy material and under given ion‐exchange conditions. It was shown that the developed model correctly describes experimental data on ion‐exchange‐induced glass‐ceramics decrystallization. It appears that the model can be used in developing a new technology of optical glass‐ceramics for photonic applications.     

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.     

Structural and Kinetic Analysis of BaCl2 Nanocrystals in Fluorochlorozirconate Glass‐Ceramics

The presence of BaCl₂ nanocrystals and the crystallographic phase that they adopt controls the optical behavior of fluorochlorozirconate glass‐ceramics. We have used in situ X‐ray diffraction heating experiments and ex situ transmission electron microscopy to follow the BaCl₂ nanocrystal nucleation and growth processes as a function of heating rate and isothermal hold temperature. The BaCl₂ nanocrystals nucleate with the hexagonal crystal structure and grow as spherical particles to a size of ~10 to 20 nm. They then undergo a structural transformation to the orthorhombic phase and their shape changes to rounded disks, with diameters ranging from 150 to 250 nm, and thicknesses ranging from 80 to 120 nm. The change in size results from Ostwald ripening of the hexagonal BaCl₂ nanocrystals to form the orthorhombic BaCl₂ nanocrystals.     

Glass‐transition temperature–conversion relationship for an epoxy–hexahydro‐4‐methylphthalic anhydride system

The DGEBA–MHHPA epoxy system has found increasing applications in microelectronics packaging, making crucial the ability to understand and model the cure kinetics mechanism accurately. The present article reports on work done to elucidate an appropriate model, modified from the empirical DiBenedetto's equation, to relate the glass‐transition temperature (T_g) to the degree of conversion for a DGEBA–MHHPA epoxy system. This model employs the ratio of segmental mobility for crosslinked and uncrosslinked polymers, λ, to fit the model curve to the data obtained. A higher ratio value was shown to indicate a more consistent rate of increase of T_g in relation to the degree of conversion, while a lower value indicated that the rate of T_g increase was disproportionately higher at higher degrees of conversion. The best fit value of λ determined by regression analysis for the DGEBA–MHHPA epoxy system was 0.64, which appeared to be higher than for those previously obtained for other epoxy systems, which ranged from 0.43–0.58. The highest T_g value obtained experimentally, T_{g max}, was 146°C, which is significantly below the derived theoretical maximum T_g∞ value of 170. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 511–516, 2000     

Glass‐transition behavior of poly(methacrylonitrile)

The glass‐transition temperatures (T_g) of some polymers reported in the literature have always been a source of great uncertainty. The values reported for poly(methacrylonitrile) (PMAN, 100 and 120°C) are well above the value determined in this study (67°C). It is clearly shown by FTIR and DSC work that formation of cyclic structures during the drying of PMAN, even at low temperatures, is the main reason for the high T_g values observed. The contributions of naphthydrine type cyclic structures and intermolecular crosslinks in the increase of the T_g are determined over an aging temperature interval of 90–300°C. The combined effects of intra‐ and intermolecular linking cause an increase in the T_g from 67 to 116°C. The hardness measurements also confirm the value determined by DSC. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 1936–1943, 2001     

2D‐ and 3D Observation and Mechanism of Self‐Healing in Glass–Boron Composites

The self‐healing of a crack in a glass–boron composite has been observed by X‐ray nanotomography. It shows the occurrence of a healing effect within the bulk of the composite, despite of a limited oxygen access in the crack. This 3D tomographic observation offers new insights in the mechanism of healing, complementary to in situ high‐temperature environmental scanning electron microscopy. In addition, nano‐X‐ray fluorescence imaging, electron microprobe and solid‐state NMR gave evidence that the molten B₂O₃, produced by the oxidation of boron particles at 700°C, reacts with the glass matrix to form borosilicate compounds that also contribute to heal the crack. The high viscosity of B₂O₃ at 700°C leads to the formation of bridges between the walls of the crack, which limit oxygen diffusion. Thus, the B particle oxidation is not completed after a single healing cycle, meaning that several healing cycles can be obtained in a composite.     

Migration of Potassium in an Fe2O3/H‐ZSM‐5 Composite Catalyst

The migration of potassium in an iron/H‐ZSM‐5 bifunctional system was investigated by pressing K/Fe₂O₃ and H‐ZSM‐5 in a pellet using 2 t of pressure. These pellets were heated at 350 °C in air for a number of days. Migration of potassium was visualized using energy‐dispersive X‐ray profiling. The distribution of potassium in the Fe₂O₃ phase and the H‐ZSM‐5 phase was approximately constant, with a step change over the phase boundary. The step change varied as a function of the heating time. The amount of potassium migrated from the Fe₂O₃ phase to the H‐ZSM‐5 phase was quantified using NH₃‐TPD. It is shown that an equilibrium distribution between potassium in the Fe₂O₃ phase and the H‐ZSM‐5 phase is obtained after ca. 7 days of heating.     

Fabrication of Sr0.5Ba0.5Nb2O6 Nanocrystallite‐Precipitated Transparent Phosphate Glass‐Ceramics

Transparent (Sr_0.5Ba_0.5)Nb₂O₆ (SBN50) nanocrystallite‐precipitated phosphate glass‐ceramics were prepared by a conventional glass‐ceramic process. x(SrO–BaO–2Nb₂O₅) ? (100–4x)P₂O₅ (xSBNP) glasses with a refractive index of 1.9–2.0 exhibited high water resistance owing to the presence of Q⁰ and Q¹ phosphate units. Both bulk and surface crystallization of the SBN50 phase were observed in 20SBNP and 21SBNP glass‐ceramics. Although the nominal content of SBN50 crystals in the 21SBNP glass was larger than that in the 20SBNP glass, the latter exhibited better crystallinity of SBN50 and a higher number density of precipitated SBN nanocrystallites. By tuning the two‐step heat‐treatment and the chemical composition, transparent SBN50‐precipitated glass‐ceramics were successfully obtained. Given that no remarkable increase of the relative dielectric constants was observed after crystallization of the SBN50 nanocrystallites, it is postulated that the relative dielectric constant of the bulk is mainly governed by the amorphous phosphate region, and that the contribution of precipitation of the SBN50 nanocrystallites to the dielectric constant is not very significant in this system.     

Effects of Rubber Addition to an Epoxy Resin and Its Fiber Glass‐Reinforced Composite

Epoxy resins are considered as one of the most important class of thermosetting polymers and find extensive use in various fields. However, these materials are characterized by a relatively low toughness. In this respect, many efforts have been made to improve the toughness of cured epoxy resins. In this work, samples of epoxy resin diglycidyl ether of bisphenol‐A and fiber glass‐reinforced composite of this polymer with and without liquid carboxyl‐terminated butadiene acrylonitrile (CTBN) copolymer were prepared to assess the effect of CTBN rubber on the properties of polymeric and composite laminate specimens. The addition of CTBN into the polymeric specimens led to a decrease in the glass transition temperature, fracture stress (from 70.39 to 56.34 MPa), and tensile elasticity modulus (from about 3.51 to 2.65 GPa), accompanied by an increase in elongation (from 2.47 to 5.64%). However, the degradation temperature of the polymeric system was not modified. Infrared analysis evidenced the occurrence of chemical reaction between the two components, and scanning electron microscopy results suggested rubber particles deformation as the prevailing toughening mechanism. The rubber addition in the composite specimens, promoted an increase simultaneous in fracture stress and in elongation at fracture. The elasticity tensile modulus has not changed. This probably results from the increased deformation capacity of the matrix, which prevents its premature cracking, and better adhesion between fibers and matrix observed in the CTBN‐modified composite laminates. POLYM. COMPOS., 2012. © 2011 Society of Plastics Engineers     

Effect of Microwave Processing on the Crystallization and Energy Density of BaO‐Na2O‐Nb2O5‐SiO2‐B2O3 Glass‐Ceramics

Barium sodium niobate (BNN) glass‐ceramics were successfully synthesized through a controlled crystallization method, using both a conventional and a microwave hybrid heating process. The dielectric properties of glass‐ceramics devitrified at different temperatures and conditions were measured. It was found that the dielectric constant increased with higher crystallization temperature, from 750°C to 1000°C, and that growth of the crystalline phase above 900°C was essential to enhancing the relative permittivity and overall energy storage properties of the material. The highest energy storage was found for materials crystallized conventionally at 1000°C with a discharge energy density of 0.13 J/cm³ at a maximum field of 100 kV/cm. Rapid microwave heating was found to not give significant enhancement in dielectric properties, and coarsening of the ferroelectric crystals was found to be critical for higher energy storage.     

Thermophysical Properties of Multiphase Borosilicate Glass‐Ceramic Waste Forms

Multiphase borosilicate glass‐ceramics represent one candidate to contain radioactive nuclear waste separated from used nuclear fuel. In this work, the thermophysical properties from room temperature to 1273 K were investigated for four different borosilicate glass‐ceramic compositions containing waste loadings from 42 to 60 wt% to determine the sensitivity of these properties to waste loading, as‐fabricated microstructure, and potential evolutions in microstructure brought about by temperature transients. The thermal expansion, specific heat capacity, thermal diffusivity, and thermal conductivity are presented. The impact of increasing waste loading is shown to have a small but measurable effect on the thermophysical properties between the four compositions, contrasted to a much greater impact observed when transitioning from predominantly crystalline to amorphous systems. Thermal cycling below 1273 K was not found to measurably impact the thermophysical properties of the compositions investigated here.