Crystallization,mechanical, and optical properties of transparent,nanocrystalline gahnite glass‐ceramics

This paper describes the preparation of a transparent glass‐ceramic from the SiO₂‐K₂O‐ZnO‐Al₂O₃‐TiO₂ system containing a single crystalline phase, gahnite (ZnAl₂O₄). TiO₂ was used as a nucleating agent for the heat‐induced precipitation of gahnite crystals of 5‐10 nm. The evolution of the ZnAl₂O₄ spinel structure through the gradual formation of Al‐O bonds was examined by infrared spectroscopy. The dark brown color of the transparent precursor glass and glass‐ceramic was eliminated using CeO₂. The increase in transparency of the CeO₂‐doped glass and glass‐ceramics was demonstrated by UV‐visible absorption spectroscopy. EPR measurements confirmed the presence of Ce³⁺ ions, indicating that CeO₂ was effective in eliminating the brown color introduced by Ti³⁺ ions via oxidation to Ti⁺⁴. The hardness of the glass‐ceramic was 30% higher than that of the as‐prepared glasses. This work offers key guidelines to produce hard, transparent glass‐ceramics which may be potential candidates for a variety of technological applications, such as armor and display panels.     

Synthesis of Eu3+‐doped BaBi2Ta2O9 based glass‐ceramic nanocomposites: Optical and dielectric properties

In this paper we report for the first time synthesis of Eu³⁺‐doped transparent glass‐ceramics (TGC) with BaBi₂Ta₂O₉ (BBT) as the major crystal phase using the glass system SiO₂–K₂O–BaO–Bi₂O₃–Ta₂O₅ by melt quenching technique followed by controlled crystallization through ceramming heat treatment. DSC studies were conducted in order to determine a novel heat‐treatment protocol to attain transparent GCs by controlling crystal growth. The structural properties of the BBT GCs have been investigated using XRD, FE‐SEM, TEM and FTIR reflectance spectroscopy. Optical band gap energies of the glass‐ceramic samples were found to decrease with respect to the precursor glass. An increased intensity of emission along with increase in the average lifetime of Eu³⁺ was observed due to incorporation of Eu³⁺ ions into the low‐phonon energy BBT crystal site. The local field asymmetric ratios of all the samples were observed greater than unity. The dielectric constant (ε_r), dielectric loss, and dissipation factor values of both the base glass and ceramized samples were found to decrease with increase in frequency.     

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.     

Synthesis and characterization of ferrimagnetic glass‐ceramic frit from waste

The large amount of generated waste determines the importance of their valorization. Red mud is the residue of the Bayer process, which stored cumulative value raises 2.7 Bt. This paper describes an easy way to produce a ferrimagnetic glass‐ceramic frit, using bauxite residue, fly ash and glass cullet as raw materials. The synthesized frit consists of faceted and dendritic agglomerated crystals of magnetite and titanomagnetite embedded in a glass matrix, which exhibits a saturation magnetization (M_S) of 6.3 emu/g, a remanent magnetization (M_R) of 2.7 emu/g and a coercive field (H_C) of 347 Oe. Furthermore, it presents Vickers hardness value of H_V = 5.55 ± 0.16 GPa and fracture toughness value of K_IC = 1.64 ± 0.34 MPa·m^1/2.     

Remarkable piezoelectricity and stable high‐temperature dielectric properties of quenched BiFeO3–BaTiO3 ceramics

Effects of quenching process on dielectric, ferroelectric, and piezoelectric properties of 0.71BiFeO₃?0.29BaTiO₃ ceramics with Mn modification (BF–BT?xmol%Mn) were investigated. The dielectric, ferroelectric, and piezoelectric properties of BF–BT?xmol%Mn were improved by quenching, especially to the BF–BT?0.3 mol%Mn ceramics. The dielectric loss tanδ of quenched BF–BT?0.3 mol%Mn ceramics was only 0.28 at 500°C, which was half of the slow cooling one. Meanwhile, the remnant polarization P_r of quenched BF–BT?0.3 mol%Mn ceramics increased to 21 μC/cm². It was notable that the piezoelectric constant d₃₃ of quenched BF–BT?0.3 mol%Mn ceramics reached up to 191 pC/N, while the T_C was 530°C, showing excellent compatible properties. The BF–BT?xmol%Mn system ceramics showed to obey the Rayleigh law within suitable field regions. The Rayleigh law results indicated that the extrinsic contributions to the dielectric and piezoelectric responses of quenched BF–BT?xmol%Mn ceramics were larger than the unquenched ceramics. These results presented that the quenched BF–BT?xmol%Mn ceramics were promising candidates for high‐temperature piezoelectric devices.     

Second harmonic generation in germanotellurite bulk glass‐ceramics

In this study, we have investigated the use of silver cation as nucleating agent in germanotellurite glass matrix of compositions (100?x) [70TeO₂–10GeO₂–10Nb₂O₅–10K₂O]–xAg₂O (x=0‐6 mol%), in order to promote bulk crystallization. Density measurements, differential scanning calorimetry, X‐ray diffraction, UV‐Vis, and Raman spectroscopies have been performed to study the crystallization process. We have observed bulk crystallization of a unique noncentrosymmetric phase, K[Nb_1/3Te_2/3]₂O_4.8, which has been investigated for its second‐order optical activity. Transparent to translucent glass‐ceramics have been successfully tailored under thermal treatment and second harmonic generation signals were recorded on the glass‐ceramic samples as a function of their synthesis procedure. It is suggested that the second‐order optical properties observed are strongly related to the organization of crystallites within phase‐separated domains.     

Antiferroelectric‐ferroelectric phase transition in lead‐free AgNbO3 ceramics for energy storage applications

The high‐energy storage density reported in lead‐free AgNbO₃ ceramics makes it a fascinating material for energy storage applications. The phase transition process of AgNbO₃ ceramics plays an important role in its properties and dominates the temperature and electric field dependent behavior. In this work, the phase transition behavior of AgNbO₃ ceramics was investigated by polarization hysteresis and dielectric tunability measurements. It is revealed that the ferrielectric (FIE) phase at room temperature possesses both ferroelectric (FE)‐like and antiferroelectric (AFE)‐like dielectric responses prior to the critical AFE‐FE transition point. A recoverable energy storage density of 2 J/cm³ was achieved at 150 kV/cm due to the AFE‐FE transition. Based on a modified Laudau phenomenological theory, the stabilities among the AFE, FE and FIE phases are discussed, laying a foundation for further optimization of the dielectric properties of AgNbO₃.     

Enhanced magnetic and dielectric properties of Ti‐doped YFeO3 ceramics

Polycrystalline YFeO₃ (YFO) and YFe_1?(4/3)xTi_xO₃(YFTO) ceramics were prepared using the powder synthesized from the sol‐gel route. X‐ray diffraction analyses of the polycrystalline ceramics revealed the crystallization of the phase in orthorhombic crystal structure associated with the space group Pnma. The magnetization versus magnetic field hysteresis loops were obtained at room temperature for YFO and YFTO ceramics. The magnetic property changes from weak ferromagnetic in YFO to ferromagnetic in YFTO ceramics. The dielectric constant recorded at room temperature for YFTO ceramics was six times higher than that of YFO, whereas the dielectric loss gets reduced to 0.06 from 0.3 for YFO at 1 kHz. Impedance spectroscopy study carried out on YFO and YFTO ceramics confirmed the existence of non‐Debye‐type relaxation. Observed single semicircle in Z′ vs ?Z′′ plot established the incidence of intrinsic (bulk) effect and ruled out any grain boundary or electrode effects. The mechanism for the dielectric relaxation and electrical conduction process observed in YFO and YFTO ceramics was discussed by invoking electric modulus formalisms. Activation energy obtained by ac conductivity study suggested that the conduction process in YFO was linked up with the existence of the polaron and oxygen vacancies, whereas only oxygen vacancies contribute to the conduction process in YFTO ceramics.     

High dielectric constant and high‐Q in microwave ceramics of SrTiO3 co‐doped with aluminum and niobium

Al/Nb co‐doped SrTiO₃ microwave ceramics with the composition of SrTi_1–x(Al_0.5Nb_0.5)_xO₃ (x = 0.03, 0.05, 0.1, and 0.15) have been synthesized via a standard solid‐state reaction method. The substitution of (Al_0.5Nb_0.5)⁴⁺ in B‐site inhibits the reduction in Ti⁴⁺ ions and the growth of grain size, then the transport of mobile charge carriers is limited, and thus the Q value is improved. For the SrTi_0.9(Al_0.5Nb_0.5)_0.1O₃ ceramics, in addition to their high dielectric constant (ε_r ~185), they exhibit correspondingly a high Qf value (~ 9077 GHz) at 2.9 GHz, making the microwave ceramics suitable for myriad device miniaturization and high‐performance wireless communication.     

Development of brannerite glass‐ceramics for the immobilization of actinide‐rich radioactive wastes

Brannerite‐based glass‐ceramics have been developed as potential waste forms for the immobilization of actinide‐rich radioactive wastes. For the first time, the formation of brannerite phases in glass has been demonstrated using uranium (U) and plutonium (Pu) with additions of gadolinium and hafnium as neutron absorbers. Both XRD and SEM‐EDS confirm that brannerite is the dominating phase with compositions close to Y_0.5U_0.5Ti₂O₆, Gd_0.2Pu_0.3U_0.5Ti₂O₆, and Gd_0.1Hf_0.1Pu_0.2U_0.6Ti₂O₆ internally crystallized in the glass. TEM SAED and Raman spectroscopy reveal the typical structure and vibration modes for brannerite. In addition, the presence of U⁵⁺ species as designed in the formulations has been confirmed by diffuse reflectance spectroscopy. More importantly, the U and Pu were partitioned exclusively in the ceramic phases with no detectable actinide in the glass.     

Understanding the structural origin of crystalline phase transformations in nepheline (NaAlSiO4)‐based glass‐ceramics

Nepheline (Na₆K₂Al₈Si₈O₃₂) is a rock‐forming tectosilicate mineral which is by far the most abundant of the feldspathoids. The crystallization in nepheline‐based glass‐ceramics proceeds through several polymorphic transformations — mainly orthorhombic, hexagonal, cubic — depending on their thermochemistry. However, the fundamental science governing these transformations is poorly understood. In this article, an attempt has been made to elucidate the structural drivers controlling these polymorphic transformations in nepheline‐based glass‐ceramics. Accordingly, two different sets of glasses (meta‐aluminous and per‐alkaline) have been designed in the system Na₂O–CaO–Al₂O₃–SiO₂ in the crystallization field of nepheline and synthesized by the melt‐quench technique. The detailed structural analysis of glasses has been performed by ²⁹Si, ²⁷Al, and ²³Na magic‐angle spinning — nuclear magnetic resonance (MAS NMR), and multiple‐quantum MAS NMR spectroscopy, while the crystalline phase transformations in these glasses have been studied under isothermal and non‐isothermal conditions using differential scanning calorimetry (DSC), X‐ray diffraction (XRD), and MQMAS NMR. Results indicate that the sequence of polymorphic phase transformations in these glass‐ceramics is dictated by the compositional chemistry of the parent glasses and the local environments of different species in the glass structure; for example, the sodium environment in glasses became highly ordered with decreasing Na₂O/CaO ratio, thus favoring the formation of hexagonal nepheline, while the cubic polymorph was the stable phase in SiO₂–poor glass‐ceramics with (Na₂O+CaO)/Al₂O₃ > 1. The structural origins of these crystalline phase transformations have been discussed in the paper.     

Microwave dielectric properties of SnO‐SnF2‐P2O5 glass and its composite with alumina for ULTCC applications

Ultralow‐temperature sinterable alumina‐45SnF₂:25SnO:30P₂O₅ glass (Al₂O₃‐SSP glass) composite has been developed for microelectronic applications. The 45SnF₂:25SnO:30P₂O₅ glass prepared by melt quenching from 450°C has a low T_g of about 93°C. The SSP glass has ε_r and tanδ of 20 and 0.007, respectively, at 1 MHz. In the microwave frequency range, it has ε_r=16 and Q_u × f=990 GHz with τ_f=?290 ppm/°C at 6.2 GHz with coefficient of thermal expansion (CTE) value of 17.8 ppm/°C. A 30 wt.% Al₂O₃ ‐ 70 wt.% SSP composite was prepared by sintering at different temperatures from 150°C to 400°C. The crystalline phases and dielectric properties vary with sintering temperature. The alumina‐SSP composite sintered at 200°C has ε_r=5.41 with a tanδ of 0.01 (1 MHz) and at microwave frequencies it has ε_r=5.20 at 11 GHz with Q_u × f=5500 GHz with temperature coefficient of resonant frequency (τ_f)=?18 ppm/°C. The CTE and room‐temperature thermal conductivity of the composite sintered at 200°C are 8.7 ppm/°C and 0.47 W/m/K, respectively. The new composite has a low sintering temperature and is a possible candidate for ultralow‐temperature cofired ceramics applications.     

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.     

High‐Q microwave ceramics of Li2TiO3 co‐doped with magnesium and niobium

In order to solve the problems of acceptor/donor individual doping in Li₂TiO₃ system and clarify the superiority mechanism of co‐doping for improving the Q value, Mg + Nb co‐doped Li₂TiO₃ have been designed and sintered at a medium temperature of 1260°C. The effects of each Mg/Nb ion on structure, morphology, grain‐boundary resistance and microwave dielectric properties are investigated. The substitution of (Mg_1/3Nb_2/3)⁴⁺ inhibits not only the diffusion of Li⁺ and reduction in Ti⁴⁺, but also the formation of microcracks in ceramics, which promotes the enhancement of Q value. The experiments reveal that Q × f value of Li₂TiO₃ ceramics co‐doped with magnesium and niobium is 113 774 GHz (at 8.573 GHz), which is increased by 113% compared with the pure Li₂TiO₃ ceramics. And the co‐doped ceramics have an appropriate dielectric constant of 19.01 and a near‐zero resonance frequency temperature coefficient of 13.38 ppm/°C. These results offer a scientific basis for co‐doping in Li₂TiO₃ system, and the outstanding performance of (Mg + Nb) co‐doped ceramics provides a solid foundation for widespread applications of microwave substrates, resonators, filters and patch antennas in modern wireless communication equipments.     

Improved dielectric properties in A′‐site nickel‐doped CaCu3Ti4O12 ceramics

The improved dielectric properties and voltage‐current nonlinearity of nickel‐doped CaCu₃Ti₄O₁₂ (CCNTO) ceramics prepared by solid‐state reaction were investigated. The approach of A′‐site Ni doping resulted in improved dielectric properties in the CaCu₃Ti₄O₁₂ (CCTO) system, with a dielectric constant ε′≈1.51×10⁵ and dielectric loss tanδ≈0.051 found for the sample with a Ni doping of 20% (CCNTO20) at room temperature and 1 kHz. The X‐ray photoelectron spectroscopy (XPS) analysis of the CCTO and the specimen with a Ni doping of 25% (CCNTO25) verified the co‐existence of Cu⁺/Cu²⁺ and Ti³⁺/Ti⁴⁺. A steady increase in ε′(f) and a slight increase in α observed upon initial Ni doping were ascribed to a more Cu‐rich phase in the intergranular phase caused by the Ni substitution in the grains. The low‐frequency relaxation leading to a distinct enhancement in ε′(f) beginning with CCNTO25 was confirmed to be a Maxwell‐Wagner‐type relaxation strongly affected by the Ni‐related phase with the formation of a core‐shell structure. The decrease of the dielectric loss was associated with the promoted densification of CCNTO and the increase of Cu vacancies, due to Ni doping on the Cu sites. In addition, the Ni dopant had a certain effect on tuning the current‐voltage characteristics of the CCTO ceramics. The present A′‐site Ni doping experiments demonstrate the extrinsic effect underlying the giant dielectric constant and provides a promising approach for developing practical applications.     

Significantly enhanced energy‐storage density in the strontium barium niobate‐based/titanate‐based glass‐ceramics

Two‐step crystallization process was employed to improve microstructure and energy‐storage density of the strontium barium niobate‐based/titanate‐based glass‐ceramics. By using two‐step crystallization process, the optimum nucleation temperature was obtained to improve dielectric breakdown strength. Compared to the breakdown strength by one‐step crystallization process, the breakdown strength by two‐step crystallization process is increased about 1.89 times with the optimum nucleation temperature. Energy‐storage density of 7.73 ± 0.26 J/cm³ is significantly improved by two‐step crystallization process and is about 2.9 times of 2.63 ± 0.17 J/cm³ by one‐step crystallization process. This result is attributed to the homogeneous nucleation improving the microstructures of glass‐ceramics. Identification and quantification of crystalline phases by using Rietveld refinement reveals the difference of dielectric constants for one‐step and two‐step crystallization processes.     

Decisive role of mixed‐valence structure in colossal dielectric constant of LaFeO3

The role of mixed‐valence structure in colossal dielectric constant (CDC) behavior has been investigated in LaFeO₃ ceramics by tuning the ratio of Fe²⁺/Fe³⁺ through substituting Al for Fe. The ratio of Fe²⁺/Fe³⁺ is decreased gradually from 1.0 to 0.0 by increasing the concentration of Al³⁺. Two clear‐cut correlations have been found: (i) the relationship between the CDC behavior and the ratio of Fe²⁺/Fe³⁺ follows an exponential function and (ii) the activation energy of the polaron relaxation is proportional to , where is the intrinsic dielectric constant. These findings underscore the role of the mixed‐valence structure in CDC behavior and suggest that adjusting the mixed‐valence structure through doping/alloying can be a promising strategy to achieve superior CDC behavior in transition‐metal oxides.     

Co-optimization of microwave dielectric properties in Ba(Mg1/3Ta2/3)O3 complex perovskite ceramics through ordered domain engineering

It is an important subject to improve the temperature coefficient of resonant frequency (τ_f) and thermal conductivity (κ) of microwave dielectric ceramics without reducing the Qf value. Ordered domain engineering was applied to realize the previous objectives in Ba(Mg_1/3Ta_2/3)O₃ ceramics. With the increasing ordering degree from 0.835 to 0.897, the optimized Qf value was obtained. Meanwhile, near zero τ_f from 11.9 to 5.6 ppm °C⁻¹ was achieved, together with increased κ from 5.5 to 7.6 W m⁻¹ K⁻¹, and enhanced dielectric strength from 801 to 921 kV cm⁻¹. The noticeable ordered domain structure with large ordered domains (∼100 nm) and low-energy domain boundaries was revealed in Ba(Mg_1/3Ta_2/3)O₃. The consequent weakened phonon scattering rises the thermal conductivity. The increased bond covalency and oxygen distortion in ceramics with higher ordering degree were suggested as a cause of enlarged bandgap, which enhanced the dielectric strength. The reduced τ_f is dominated by the less “rattling” space of the cations in the ordered state by inducing more positive τ_ε. The reduced τ_f, optimized thermal conductivity, and Qf value in the present work indicate that the ordered domain engineering could open up a new direction for the optimization of microwave dielectric ceramics.     

Silicon oxycarbide glasses and glass‐ceramics: “All‐Rounder” materials for advanced structural and functional applications

Silicon oxycarbides can be considered as being carbon‐containing silicates consisting of glass networks in which oxygen and carbon share bonds with silicon. The carbon‐for‐oxygen substitution in silicate glass networks has been shown to induce significant changes in the network connectivity and consequently strong improvements in the properties of the silicate glass network. For instance, SiOC glasses exhibit Young's moduli, hardness values, glass transition, and crystallization temperatures which are superior to those of vitreous silica. Moreover, the silicon oxycarbide glass network exhibits unique structural features such as reduced mass fractal dimension and nano‐heterogeneity, which significantly affect and/or dictate its properties and behavior. In the present Review, a consideration of the current state of the art concerning the synthesis, processing, and various structural and functional properties of silicon‐oxycarbide‐based glasses and glass‐ceramics is done. Thus, the synthesis of silicon oxycarbides starting from macromolecular precursors such as polysiloxanes or alkoxysilanes‐based sol‐gel systems as well as current advances related to their processing will be critically reviewed. In addition, various structural and functional properties of silicon oxycarbides are presented. Specific emphasis will be put on the intimate correlation between the molecular architecture of the precursors and the structural features and properties of the resulting silicon oxycarbides.     

Self‐limited crystallization‐mediated removal of hydroxyl in glass

The effective removal of hydroxyl groups (OH) is receiving the attention of scientists interested in developing high‐performance photonic glass. Previous approaches rely on stringent control of the various drying techniques which meet with limited success in silicate glass obtained by the sol‐gel method. Here, we present a novel in situ strategy to remove structural OH groups, based on the self‐limited nanocrystallization‐triggered local chemical reaction between OH and F^? in the glassy phase. The experimental data revealed that a more than 100‐fold increase in the emission intensity can be realized. Moreover, the mechanism was discussed and it can be attributed to the effective removal of structural OH with especially strong binding energy. The results suggest an innovative avenue for the development of photonic glasses with efficient luminescence, excellent optical transmission, and improved reliability.