Preparation of infrared transparent YAG-based glass-ceramics with nano-sized crystallites

Transparent YAG-based glass-ceramics were prepared by a novel method called amorphous sintering followed by controlled crystallization (ASCC) from the compositions of 62.5Al₂O₃–(37.5 ? x)Y₂O₃–xLa₂O₃ (in molar ratio, x = 5, 7, 10 and 20). The stability of the YAG glass was improved by the incorporation of La₂O₃, which increased the activation energy for crystallization. With 10 mol% La₂O₃, bulk YAG glass was prepared by hot-pressing and showed an infrared transmittance of 66%. The YAG glass was converted into glass-ceramics by post annealing at 875 °C for 5 h for controlled crystallization. The obtained glass-ceramic sample showed a crystallite size of 20–50 nm and an infrared transmittance of 60%. With increasing annealing time, the crystallites grew up quickly, resulting in a significant decrease in transparency. In the hot-pressed glass, nano-sized YAG nuclei (～5 nm) were found, which were probably responsible for the crystallization behavior observed at temperatures (e.g. 875 °C) below the onset crystallization temperature.     

Selective excitation in transparent oxyfluoride glass-ceramics doped with Nd3+

Transparent oxyfluoride nano-glass-ceramics have been prepared by melting-quenching and doped with five different Nd³⁺ concentrations (0.1–2 mol%) to obtain the most efficient ⁴F_3/2 → ⁴I_11/2,13/2 emission. It was observed by differential thermal analysis (DTA) that the addition of Nd³⁺ does not affect the crystallization mechanism which corresponds to a diffusion-controlled volumetric process that starts from a constant number of nuclei. Nevertheless, the presence of the dopant affects the kinetics due to the progressive increase of T_g on increasing the Nd³⁺ content. LaF₃ crystals with a size between 9 and 12 nm are obtained after heat treatments at T_g + 20–80 °C as confirmed by X-ray diffraction (XRD) and high resolution transmission electron microscopy (HR-TEM). Energy dispersive X-ray (EDX) analysis shows the incorporation of Nd³⁺ ions into the LaF₃ nano-crystals. Judd-Ofelt analysis from the absorption spectra further demonstrate the incorporation of Nd³⁺ ions into the fluoride phase and the most relevant parameters such as radiative lifetime and stimulated emission cross-section are calculated. A detailed optical characterisation clearly shows that Nd³⁺ ions in the glass-ceramics are incorporated in both crystalline and amorphous phases. Low temperature site-selective emission and excitation spectra, together with the different lifetime values of the ⁴F_3/2 state depending on the excitation and emission wavelengths, allow emission from Nd³⁺ ions in the LaF₃ nanocrystals to be identified and correlated with the structural properties. As the Nd³⁺ concentration is increased beyond 0.1 mol%, a stronger quenching of lifetime is observed for Nd³⁺ ions residing in LaF₃ crystals than for those dispersed in the glass matrix. This strong concentration quenching is explained by the much higher concentration of Nd³⁺ ions in the crystalline phase with respect to that in the glass matrix.     

Optical and mechanical properties of amorphous bulk and eutectic ceramics in the HfO2–Al2O3–Y2O3 system

Bulk glasses containing HfO₂ nano-crystallites of 20–50 nm were prepared by hot-pressing of HfO₂–Al₂O₃–Y₂O₃ glass microspheres at 915 °C for 10 min. By annealing at temperatures below 1200 °C, the bulk glasses were converted into transparent glass-ceramics with HfO₂ nano-crystallites of 100–200 nm, which showed the maximum transmittance of ～70% in the infrared region. An increase of annealing temperature (>1300 °C) resulted in opaque YAG/HfO₂/Al₂O₃ eutectic ceramics. The eutectic ceramics contained fine Al₂O₃ crystallites and showed a high hardness of 19.8 GPa. The fracture toughness of the eutectic ceramics increased with increasing annealing temperature, and reached the maximum of 4.0 MPa m^1/2.     

Low firing and microwave dielectric properties of BaTi4O9 with B2O3–ZnO–La2O3 glass addition

A high-Q low firing ceramics material was fabricated from a composite of 80 wt% of BaTi₄O₉ powder and 20 wt% of B₂O₃–ZnO–La₂O₃ glass flux. The sample sintered at 900 °C for 3 h had the maximum bulk density. B₂O₃–ZnO–La₂O₃ glass was found to act as the sintering aid. The material sintered at 900 °C for 3 h consists of BaTi₄O₉, LaBO₃, an unidentified crystal phase and residual glass phase, and possesses excellent microwave dielectric properties: permittivity k ≈ 27, quality factor Q × f ≈ 20,000 GHz, temperature coefficient of resonant frequency τ_f ≈ 6.5 ppm/°C.     

Growth and optical properties of LiF/LaF3 eutectic crystals

Neutron imaging devices employing a scintillator can be used in various fields, and eutectic crystals can be suitable for the imaging with a fine position resolution of a few hundred micrometers. Since LiF and LaF₃ have different refractive indexes of 1.41 and 1.64 at 300 nm, respectively, the eutectic crystal is expected to behave as a scintillator with light guiding properties. Thus, the optical properties of Ce-doped LiF/LaF₃ eutectic crystal grown by micro-pulling down method were investigated. The light output of LiF/Ce:LaF₃ eutectic crystal was relatively small. The emission peaks at 300 nm originating from Ce³⁺ of 5d–4f transition were observed under excitation by UV photons and 5.5 MeV alpha rays. Moreover, the photo-luminescence decay time of Ce-doped LiF/LaF₃ eutectic crystal was estimated to be 17 ± 1 ns.     

Structural,optical, and spectroscopic properties of Er3+-doped TeO2–ZnO–ZnF2 glass-ceramics

Transparent fluorotellurite glass-ceramics have been obtained by heat treatment of precursor Er-doped TeO₂–ZnO–ZnF₂ glasses. ErF₃ nanocrystals nucleated in the glass-ceramics have a typical size of 45 ± 10 nm. Based on the Judd-Ofelt theory, the main radiative parameters for the ⁴I_13/2 → ⁴I_15/2 transition have been obtained. The split of the absorption and emission bands and the reduction of the Ω₂ parameter, as compared to the glass, confirm the presence of Er³⁺ ions in a crystalline environment in glass-ceramic samples. The analysis of the ⁴I_13/2 decays suggests that a fraction of Er³⁺ ions remains in a glass environment while the rest forms nanocrystals. For the glass-ceramics, intense red and green upconversion emissions were observed with an enhancement of the ⁴F_9/2 → ⁴I_15/2 red one compared to the glass sample. The temporal evolution of the red emission together with the excitation upconversion spectra suggests that energy transfer processes are responsible for the enhancement of the red emission.     

Dielectric properties of new glass-ceramics for LTCC applied to microwave or millimeter-wave frequencies

A new glass-ceramics material called new-glass-ceramics (NGC) that consists of MgAl₂O₄ crystals (spinel) and highly crystallized Li–Mg–Zn–B–Si–O glass has been developed for microwave or millimeter-wave frequency applications. NGC can be sintered at temperatures below 1000 °C and co-fired with internal copper electrodes that have high electrical conductivity. Its dielectric constant is 7.4 and its Q value is higher than 2000 at 24 GHz. NGC specimens were investigated using X-ray diffraction analysis (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray spectrometry (EDX). NGC mainly consists of MgAl₂O₄, Mg₃B₂O₆, and Li₂MgSiO₄ crystal phases, which have high Q values. Using NGC, we could make band pass filter (BPF) with the size 3.2 mm × 2.5 mm × 1.3 mm for fixed wireless access (F.W.A.) system at 26 GHz. This BPF can be mounted on circuit board by solder and shows good characteristics.     

Influence of the sintering temperature on the structural feature and bioactivity of sol–gel derived SiO2–CaO–P2O5 bioglass

A sol–gel method was utilized to synthesize the gel with the composition of 58 mol% SiO₂–38 mol% CaO–4 mol% P₂O₅. The thermal properties were studied using thermogravimetric and differential thermal analysis (TG/DTA). Then the gels were sintered at 700, 900, 1000 and 1200 °C. The structure features were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM), in addition in vitro assays were carried out in simulated body fluid (SBF). The results revealed that at sintering temperature above 900 °C, crystallization occurred and glass-ceramics with pseudowollastonite and wollastonite were formed. Furthermore with the increase of sintering temperature, the amount of pseudowollastonite decreased while that of wollastonite increased. In vitro tests indicated that the crystallization did not inhibit the samples bioactivity. After soaking in SBF, the formation of apatite was confirmed on glass and glass-ceramics surface, and the bioactivity of the glass-ceramics was based on the formed pseudowollastonite and wollastonite.     

Investigation on crystallization and influence of Nd3+ doping of transparent oxyfluoride glass-ceramics

Nd³⁺ doped transparent oxyfluoride glass-ceramics containing LaF₃ nano-crystals were prepared by melt quenching. Differential thermal analysis (DTA) showed that the addition of NdF₃ decreased the LaF₃ crystallization temperature. The kinetics studies indicated that the crystallization was a diffusion-controlled growth process with zero nucleation rate for the whole process. LaF₃ crystallization activation energy and crystalline size as the function of NdF₃ doping lever exhibit the closely related evolutions. For the crystallized samples, transmission electron microscopy (TEM), X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDS) and absorption spectroscopy experiments evidenced the homogeneous distribution of spherical LaF₃ nano-crystals (10–20 nm) with the incorporation of Nd³⁺ ions in the lattice among the oxide glassy matrix, which is important for obtaining desirable luminescent performance of the material.     

Self-crystallized Ba2LaF7:Nd3+/Eu3+ glass ceramics for optical thermometry

Eu³⁺/Nd³⁺-codoped Ba₂LaF₇ transparent bulk glass ceramics were successfully fabricated by glass self-crystallization. The structure and morphology of the sample were investigated by X-ray diffraction, transmission electron microscopy (TEM), high-resolution TEM, and selected area electron diffraction. The fluorescence intensity ratios of Nd³⁺ emission at 800 nm to the Eu³⁺ emission at 699 nm (⁵D₀ → ⁷F₄) were measured under 578.3 nm laser excitation in a wide temperature range from 290 to 740 K. A relatively good temperature sensing performance was obtained with a maximum relative sensitivity of 1.02% K⁻¹ at 420 K. Both the emission peaks for temperature sensing were located in the optical window of biological tissue, which is favorable for biomedical applications. The results indicate that Ba₂LaF₇:Nd³⁺/Eu³⁺ glass ceramics have a potential application as temperature probes.     

Sintering behavior of La2O3–B2O3–TiO2 glass powders prepared by spray pyrolysis for low temperature co-fired ceramics

Fine-sized La₂O₃–B₂O₃–TiO₂ glass powders with spherical shape were directly prepared by spray pyrolysis at a temperature of 1500 °C. The optimum flow rate of the carrier gas to prepare the glass powders with dense inner structure and fine size by complete melting was 10 L/min. The ratio of La/Ti was identified to be 2.06:1, which was close to the original starting ratio of La/Ti in mixture of the spray solution. The T_g and T_c of the powders were 614 and 718 °C. The crystal structures within the powders were observed from the sintered disc at 630 °C. The mean sizes of the powders changed from 0.24 to 0.71 μm when the concentrations of the spray solution were changed from 0.025 to 0.5 M. The BET surface areas of the powders changed from 4.4 to 1.6 m²/g. The grain sizes of the sintered discs increased with increasing the sintering temperatures. The main crystal structure of the sintered discs was LaBO₃.     

Viscosity,fragility and structure of Na2O–CaO–Al2O3–SiO2 glasses of increasing Al/Si ratio

The research and development of a new float glass with higher content of Al₂O₃ is essential for the commercial flat glass. The study on the workability and kinetic fragility of Na₂O–CaO–Al₂O₃–SiO₂ glass melts with different Al/Si ratios has been linked with the structure. The viscosities as a function of temperature for glass melts were derived on the basis of Vogel–Fulcher–Tamman (VFT) equation. Some characteristic temperatures and four characteristic temperature intervals of forming process in tin bath were estimated. The results showed that: adding 12 wt% Al₂O₃ substitute for SiO₂, the melting point (Tm) increased about 35 K, entire temperature interval in tin bath narrowed down about 20 K, the shortening of workability was mainly reflected in the viscosity range of 10^5.75–10¹⁰ Pa s, the fragility index m increased by 15%. It reveals an inverse correlation between the workability and the fragility. The structural changes on the tetrahedron structural unit Qⁿ (n=1, 2, 3, 4) were obtained by using Raman spectroscopy. Our analysis indicates that: the number of NBO reducing and a more polymerized structure with adding Al/Si ratios are responsible for the increase of viscosity; the tetrahedral distortion, a decrease of Q³/Q² in the Qⁿ species, is responsible for the increase of fragility.     

Self-organized nano-crystallisation of BaF2 from Na2O/K2O/BaF2/Al2O3/SiO2 glasses

In glasses with the compositions (100 ? x)(2Na₂O·16K₂O·8Al₂O₃·74SiO₂)xBaF₂ (with x = 0 to 6), the glass transition temperature decreases with increasing BaF₂-concentration. Samples with x = 6 were thermally treated at temperatures in the range from 500 to 600 °C for 5–160 h. This leads to the crystallisation of BaF₂. The quantity of crystalline BaF₂ increases with increasing time of thermal treatment, while the mean crystallite size remains constant within the limits of error. The glass transformation temperature of partially crystallised samples increases with increasing crystallisation time and approaches a value equal to the temperature, at which the samples were treated. This is explained by the formation of a highly viscous layer enriched in SiO₂ which is formed during crystallisation. This layer acts as a diffusion barrier and hinders further crystal growth.     

Low temperature firing of Co2Y–NiCuZn ferrite composites

Hexagonal structure magnetoplumbite ferrites have revealed a higher dispersion frequency than that of nickel ferrites because of the magnetoplumbite's magnetic anisotropy. The magnetoplumbite ferrite densification temperature always exceeds 1000 °C and the initial low temperature firing permeability of magnetoplumbite ferrites with added glass is too low (μ_i = 2–4). Therefore, it is desirable to develop a material that has a higher permeability at above 300 MHz and can be densified at temperatures below 900 °C. The Bi₂O₃–B₂O₃–ZnO–SiO₂ (BBSZ) glass addition effects on the densification and magnetic properties of Co₂Y–NiCuZn ferrite composites with various Co₂Y/NiCuZn ferrite ratios were investigated. The densification of Co₂Y–NiCuZn ferrite composites was enhanced by the addition of glass at low sintering temperatures (<900 °C) due to the liquid phase sintering. Co₂Y–NiCuZn ferrite composites with 4 wt% BBSZ glass sintered at 900 °C show a relative density above 90%, a high-initial-permeability of 5–6, a quality factor of above 30 in the 200–300 MHz frequency and a resonance frequency above 1 GHz, which can be used in high frequency multilayer chip inductors.     

Synthesis and crystal structures of n = 3 Ruddlesden–Popper phase CaSr3Mn3−xFexO10 (x = 1.5, 1.0)

The triple layer Ruddlesden–Popper phases (n = 3 R–P phase) CaSr₃Mn_3−xFe_xO₁₀ (x = 1.5, 1.0) were stabilized by solid state reaction in air atmosphere and their crystal structures were refined with neutron diffraction data obtained at room temperature and at 5 K by means of Rietveld method. Both phases adopt space group I4/mmm and revealed no magnetic reflection at low temperature. Magnetic susceptibility data of CaSr₃Mn_1.5Fe_1.5O₁₀ and CaSr₃Mn₂FeO₁₀ compound showed spin glass transition at 25 and 12 K, respectively.     

Influence of Li2O–B2O3–SiO2 glass on the sintering behavior and microwave dielectric properties of BaO–0.15ZnO–4TiO2 ceramics

This paper reports the investigation of the performance of Li₂O–B₂O₃–SiO₂ (LBS) glass as a sintering aid to lower the sintering temperature of BaO–0.15ZnO–4TiO₂ (BZT) ceramics, as well as the detailed study on the sintering behavior, phase evolution, microstructure and microwave dielectric properties of the resulting BZT ceramics. The addition of LBS glass significantly lowers the sintering temperature of the BZT ceramics from 1150 °C to 875–925 °C. Small amount of LBS glass promotes the densification of BZT ceramic and improves the dielectric properties. However, excessive LBS addition leads to the precipitation of glass phase and growth of abnormal grain, deteriorating the dielectric properties of the BZT ceramic. The BZT ceramic with 5 wt% LBS addition sintered at 900 °C shows excellent microwave dielectric properties: ε_r=27.88, Q×f=14,795 GHz.     

An ab initio study of ZrB2–SiC interface strength as a function of temperature: Correlating phononic and electronic thermal contributions

This work focuses on understanding correlations between thermal conduction and mechanical strength in a model high temperature material interface. Analyses examine single crystal ZrB₂, single crystal SiC, and a 〈0 0 0 1〉–〈1 1 1〉 ZrB₂–SiC interface using a framework based on Car Parrinello molecular dynamics (CPMD) ab initio simulation method from 500 K to 2500 K. Analyses indicate that the strength reduction with increase in temperature is strongly correlated to phonon and electron thermal diffusivity change. With increase in temperature, phonon thermal diffusivity increases in the case of ZrB₂ and reduces in the cases of SiC as well as the interface. Electron contribution to thermal diffusivity increases with temperature increase in the case of interface. Examination of change in thermal properties at different mechanical strain levels reveals that the mechanisms of strength and thermal property change with increase in temperature may be similar to the mechanisms responsible for property change with change in applied strain.     

Glasses for laser joining of zirconia ceramics

Laser joining of ZrO₂ ceramics using glasses and glass ceramics as sealing components requires optimized systems. The ternary systems SiO₂–BaO–B₂O₃ and BaO–SrO–SiO₂ were selected as a basis for development of suitable glass compositions for the laser joining process. Additives such as CaO, TiO₂, Al₂O₃, and MgO were used to control the crystallization processes and hence the thermal expansion coefficients during glass synthesis. The glass viscosity, the strength of the ceramic-glass-ceramic joint, and the joint tightness are other important glass properties which were optimized for the laser process. For glass G018-345, this yielded strengths of up to 225 MPa (Weibull modulus of m = 8.6) and He leak rates of up to 4.3 × 10⁻⁵ mbar l s⁻¹. Because of the varying viscosities obtained, the optimized glass systems could be used selectively in a temperature range of 700–900 °C.     

High temperature properties of multiferroic BiFeO3–DyFeO3–BaTiO3 solid solutions

Dielectric and magnetic properties of the xBiFeO₃–yDyFeO₃–zBaTiO₃ solid solution ceramics at high temperature range of RT ∼600 °C have been characterized. For the more detailed understandings of the multiferroic property, the relation between the crystal structure transition, magnetic transition, dielectric transition with increasing temperature have been analyzed. Residual magnetization M_r under the low and high applied magnetic fields (H = 20 Oe, 8 kOe) and the dielectric properties, ɛ_r and tan δ, with varying measuring frequency and temperature have been characterized using the vibrating sample magnetometer and LCR meter, respectively. The neutron diffraction data has been collected at the temperature range of RT ∼800 °C. The low DyFeO₃ concentration samples (y = 0, 0.025) show the magnetic transitions at temperature range of 410–430 °C, while the high DyFeO₃ samples (y ≥ 0.05) show the additional transition at 250–290 °C. The magnetic transition at 410–430 °C corresponds to the crystal structural transition to the tetragonal P4mm from the rhombohedral R-3c, at which the BiFeO₃ and the DyFeO₃ samples lose their antiferromagnetic ordering.     

Sintering behaviour of a glass obtained from MSWI ash

The sintering behaviour of a glass obtained by Municipal Solid Waste Incinerator (MSWI) bottom ash (WG) was investigated and compared with a Na₂O–MgO–CaO–SiO₂ composition (CG). The sintering activation energy, E_sin, and the energy of viscous flow, E_η, were evaluated by dilatomeric measurements at different heating rates. The formation of crystalline phases was evaluated by Differential Thermal Analysis (DTA) and X-Ray Diffraction (XRD), and observed by Scanning Electron Microscopy (SEM) and Transition Electron Microscopy (TEM). In CG, the sintering started at ≈10¹³ dPa s viscosity and E_sin (245 kJ/mol) remains constant in the measured range of shrinkage, up to 9%. In WG the densification started at ≈10¹¹ dPa s, E_sin resulted to be 395 kJ/mol up to 5% shrinkage, 420 kJ/mol at 8% and 485 kJ/mol at 10% shrinkage. The sintering rate decreased due to the beginning of the pyroxene formation and the densification stopped in the temperature range 1073–1123 K after formation of 5 ± 3% and 13 ± 3% crystal phase, at 5 and 20 K/min, respectively. Higher densification and improved mechanical properties were obtained by applying the fast heating rate, i.e. 20 K/min.