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.     

Characterization of Li–Zn–Fe crystalline phases in low temperature ceramic glaze

Ceramic glaze containing Li₂O and ZnO was prepared at a low firing temperature of 1100 °C. Addition of 0–30 wt.% iron oxide content developed brown color with a metallic sparkling effect from crystallization after soaking at 980–1080 °C. Using XRD, SEM/EDS and Raman microscopy the crystalline phases were determined as lithium zinc ferrite (Li_xZn_1?2xFe_2+xO₄ where x = 0.05–0.20), hematite (α-Fe₂O₃) and anorthite (CaAl₂Si₂O₈). The most preferable metallic sparkling effect was caused by the lithium zinc ferrite phase obtained from the glaze containing 10 wt.% of iron oxide. Thermal analysis by STA after heat treatment indicated that crystallization temperature of lithium zinc ferrite and the effective soaking temperature depended on the iron oxide content in the glaze. The influence of excessive iron oxide content on the crystallization behavior of lithium zinc ferrite, anorthite and hematite phases is discussed.     

Non-isothermal crystallization kinetics and microwave properties of Bi0.75Y2.25Fe5O12 prepared by coprecipitation

The non-isothermal crystallization kinetics of Bi_0.75Y_2.25Fe₅O₁₂ powder prepared by coprecipitation has been investigated. The activation energy of crystallization was calculated by differential scanning calorimetry (DSC) at different heating rates. Analysis of non-isothermal DSC data presented values of 650 kJ/mol and 2.48 for the activation energy of crystallization and the Avrami exponent, respectively. This Avrami exponent value was consistent with surface and internal crystallizations occurring simultaneouly. The complex permittivity and permeability of Bi_0.75Y_2.25Fe₅O₁₂ were measured at X-band (8–12 GHz) by cavity perturbation technique. For dielectric properties, the real part of permittivity, ?′, was obtained in the range of 13.18–15.28. For magnetic properties, the real part of permeability, μ′, was obtained in the range of 12.21–20.98.     

Sol–emulsion–gel synthesis of cordierite ceramics for high-frequency multilayer chip inductors

Nano-Cordierite powders used for high frequency chip inductors (MLCIs) were prepared by sol–emulsion–gel method. Effects of precursor concentration and [H₂O]/[Si] molar ratio on this material were studied. The sol–emulsion–gel processing of Mg₂Al₄Si₅O₁₈ as well as its dielectric property were investigated. Owing to the better packing efficiency and therefore higher surface energy of the freestanding nano-powder, the pressed pellets made by cordierite powder showed 98.6% theoretical density at 900 °C for 2 h. The additive Bi₂O₃ was utilized to promote the crystallization or transformation to α-cordierite and sintering. The sol–emulsion–gel-derived cordierite ceramics have low dielectric constant (ε=3.0～4.0; 18 GHz) and low dielectric loss (tgδ<0.001; 18 GHz) and can be co-fired with high conductivity metals such as Au, Ag/Pd internal electrode at low temperature (900 °C), suggesting that it was an ideal dielectric material for high-frequency multilayer chip inductors.     

Determination of crystal growth rate for porcine insulin crystallization with CO2 as a volatile acidifying agent

Crystallization is controlled by two steps that determine the quality and the final size of the product, nucleation and growth, which are functions of supersaturation. Recently, Hirata et al. [1] crystallized insulin using CO₂ as a volatile acid to impose supersaturation on the system. The objective of the present work was to determine the growth kinetics of insulin crystallization in 50 mM NaHCO₃ solution with 0.4 mM ZnCl₂ in a CO₂ atmosphere at 15 °C, adjusting the parameters of the equation G = k_g × S^g to the experimental data. The solubility of insulin in the NaHCO₃/CO₂/ZnCl₂ system at 15 °C was determined as a function of pH in the range of 6.30–7.34. The crystal growth data allowed determination of the growth order “g” (g = 2.9). Although protein crystallization has some features that differ from the crystallization of less complex molecules, the apparent growth kinetics of insulin were successfully analyzed here with the same empirical methods used for small molecules, which can easily be scaled up for industrial applications to achieve specific size and purity, the goals of industrial crystallization. The method used in this work is a useful tool for describing and simplifying optimization of industrial protein crystallization processes.     

Anisotropy oxidation of textured ZrB2–MoSi2 ceramics

Oxidation behavior of hot forged textured ZrB₂–20 vol% MoSi₂ ceramics with platelet ZrB₂ grains was investigated at 1500 °C for exposure time from 0.5 to 12 h. Compared to untextured ceramics, the textured ceramics showed obvious anisotropic oxidation behavior and the surface normal to the hot forging pressure demonstrated better oxidation resistance. Such improvement in the oxidation resistance is primarily considered as a higher intrinsic ZrB₂ atomic density on the orientated {0 0 l} planes in the textured ceramics. It is expectable that the anisotropic textured ZrB₂–MoSi₂ ceramics can offer better oxidation resistance when a certain surface with higher oxidation resistance is exposed to air at elevated temperature.     

Diphasic aluminosilicate gels with two stage mullitization in temperature range of 1200–1300 °C

The crystallization of mullite in amorphous diphasic gel aged for 6 months has been studied using non-isothermal differential scanning calorimetry (DSC) and powder X-ray diffraction with Rietveld structure refinement analysis. The diphasic premullite gels undergo structural changes by aging even when they are calcined at 700 °C. These changes imply segregation of the sample to Al₂O₃-rich and SiO₂-rich regions. From the Al₂O₃-rich region crystallizes poorly defined AlSi spinel at 977 °C followed by two-step mullite crystallization in the temperature interval of 1200–1300 °C. Two overlapped exothermic peaks on DSC scan of aged gel were observed; the first at 1233 °C and the second at 1261 °C. The former is attributed to mullite crystallization by transformation of AlSi spinel, by which excess alumina occurs, which in the second step of mullitization reacts with amorphous SiO₂-rich phase. The activation energy for mullite crystallization in the first step was E_a=935±14 kJ mol⁻¹ and the Avrami exponent n=2.5. The values E_a=1119±25 kJ mol⁻¹ and n=1.2 were obtained for mullite formation in the second step. If amorphous SiO₂-rich phase is extracted from the sample, the value E_a=805±26 kJ mol⁻¹ is obtained. Mullite crystallizing from AlSi spinel (when SiO₂-rich phase has been extracted) differentiates compositionally from that formed by both reactions. Smaller unit cell parameters and higher amount of oxygen vacancies are incorporated into tetrahedral positions of mullite structure, as was determined by Rietveld structure refinement method.     

Induction of ferroelectricity in nanoscale ZrO2 thin films on Pt electrode without post-annealing

Large stable ferroelectricity in nanoscale undoped zirconia (ZrO₂) thin films prepared without post-annealing has been demonstrated for the first time. Remanent polarizations up to 12 μC cm⁻² were obtained in the as-deposited ZrO₂ thin films prepared by remote plasma atomic layer deposition at 300 °C substrate temperature on the Pt electrode. Ferroelectric crystallization of the films was achieved without post-annealing, which is highly beneficial to the application of the films in non-volatile memories and ultralow-power nanoelectronics. The existence of the ferroelectric orthorhombic phase with noncentrosymmetric space group Pbc2₁ in the as-deposited ZrO₂ thin films was confirmed by high-resolution transmission electron microscopy.     

Solubility and diffusion coefficient of supercritical-CO2 in polycarbonate and CO2 induced crystallization of polycarbonate

The solubility and diffusion coefficient of supercritical CO₂ in polycarbonate (PC) were measured using a magnetic suspension balance at sorption temperatures that ranged from 75 to 175 °C and at sorption pressures as high as 20 MPa. Above certain threshold pressures, the solubility of CO₂ decreased with time after showing a maximum value at a constant sorption temperature and pressure. This phenomenon indicated the crystallization of PC due to the plasticization effect of dissolved CO₂. A thorough investigation into the dependence of sorption temperature and pressure on the crystallinity of PC showed that the crystallization of PC occurred when the difference between the sorption temperature and the depressed glass transition temperature exceeded 40 °C (T − T_g ≥ 40 °C). Furthermore, the crystallization rate of PC was determined according to Avrami's equation. The crystallization rate increased with the sorption pressure and was at its maximum at a certain temperature under a constant pressure.     

Physical characterization and arcjet oxidation of hafnium-based ultra high temperature ceramics fabricated by hot pressing and field-assisted sintering

For this study, HfB₂-based ultra high temperature ceramic (UHTC) samples were prepared by hot pressing and field-assisted sintering (FAS) with 10–20 vol.% SiC (baseline), 5 vol.% TaSi₂, and 5 vol.% iridium. Dense billets were tested for hardness and mechanical strength. When compared, the FAS method consistently yielded materials with a grain size 1.5–2 times finer than samples processed via hot pressing. In general, room temperature flexural strengths of these materials were found to be lower (～400 MPa) than similar fully dense HfB₂–SiC materials, with strengths between 500 and 700 MPa. Oxidation resistance testing of flat-face models was conducted in a simulated re-entry environment, at Q_{Cold Wall} ～250 W/cm² for 5 min. Samples processed by FAS had reduced oxide thickness and SiC depletion zones compared to the baseline HfB₂–20SiC material. In all cases oxide thickness was reduced by ～3× and SiC depletion zone thickness was reduced ～3× over the baseline.     

Microstructural characterization and crystallization behaviour of (1 − x)TeO2–xWO3 (x = 0.15, 0.25, 0.3 mol) glasses

DTA, XRD and SEM investigations were conducted on the (1 − x)TeO₂–xWO₃ glasses (where x = 0.15, 0.25 and 0.3). Whereas the 0.75TeO₂–0.25WO₃ and 0.7TeO₂–0.3WO₃ glasses show no exothermic peaks, an indication of no crystallization in their glassy matrices, two crystallization peaks were observed on the DTA plot of the 0.85TeO₂–0.15WO₃ glass. On the basis of the XRD measurements of the 0.85TeO₂–0.15WO₃ glass samples heated to 510 °C and 550 °C (above the peak crystallization temperatures), α-TeO₂ (paratellurite), γ-TeO₂ and WO₃ phases were detected in the sample heated to 510 °C and the α-TeO₂ and WO₃ phases were present in the sample heated to 550 °C. SEM micrographs taken from the 0.85TeO₂–0.15WO₃ glass heated to 510 °C showed that centrosymmetrical crystals were formed as a result of surface crystallization and were between 3 μm and 15 μm in width and 12 μm and 30 μm in length. On the other hand, SEM investigations of the 0.85TeO₂–0.15WO₃ glass heated to 550 °C revealed the evidence of bulk massive crystallization resulting in lamellar crystals between 1 μm and 3 μm in width and 5 μm and 30 μm in length. DTA analyses were carried out at different heating rates and the Avrami constants for the 0.85TeO₂–0.15WO₃ glass heated to 510 °C and 550 °C were calculated as 1.2 and 3.9, respectively. Using the modified Kissinger equation, activation energies for crystallization were determined as 265.5 kJ/mol and 258.6 kJ/mol for the 0.85TeO₂–0.15WO₃ glass heated to 510 °C and 550 °C, respectively.     

High temperature investigation of SiO2-Al2O3-ZnO-Na2O glass for ceramic-glaze: in‐situ/ex-situ synchrotron diffraction and conventional approaches

The temperature dependent behaviour of a complex aluminosilicate glass (SiO₂-Al₂O₃-ZnO-Na₂O system), which is a reference for ceramic glaze technology, has been determined, by combining techniques that cover a scale ranging from atomistic to macroscopic. The system shows a linear thermal expansion up to about 600 °C. The glass transition temperature is at 620 °C, as observed from Differential Scanning Calorimetry. Ex situ synchrotron diffraction experiments found a further transformation consisting of albite crystallization above 810 °C. This reaction is very slow and induces permanent structural modifications in the material at both intermediate and short ranges, as shown by in situ synchrotron diffraction experiments. These observations explain why ceramic glaze technology still faces challenges for large scale manufacturing and show the critical thermal range where interventions should be focussed. Eventually, melting takes place at 1190 °C, from hot stage microscopy.     

Enhanced transport critical current density of (Bi,Pb)-2223/Ag superconductor tapes added with nano-sized Bi2O3

Ag sheathed superconductor tapes with starting composition (Bi, Pb)-2223(Bi₂O₃)_0.01 were prepared. Bi₂O₃ with average size 150 nm was used in this work. The Bi₂O₃ amount was chosen based on our initial study on nano-sized Bi₂O₃ added pellets which showed an optimal superconducting property for 0.01 wt% addition. Non-added tapes were also prepared for comparison. The tapes were investigated by X-ray diffraction method, scanning electron microscopy and transport critical current density, J_c measurements (30 K to 77 K). The influence of different sintering times (50, 100, and 150 h) on J_c under applied magnetic field (0–0.75 T at 77 K) parallel and perpendicular to the surface of the tapes was also investigated. J_c of added tapes was found to increase significantly as compared with the non-added tapes. The Bi₂O₃ added tapes sintered for 150 h exhibited the highest J_c at 30 K of 57,900 A/cm² as compared with 19,400 A/cm² for the non-added tapes sintered for 100 h. The improvements in flux pinning and connectivity between grains due to nano Bi₂O₃ addition led to the enhancement of J_c.     

Synergetic effects of SiC and Csf in ZrB2-based ceramic composites. Part I: Densification behavior

The effects of processing variables on densification behavior of hot pressed ZrB₂-based composites, reinforced with SiC particles and short carbon fibers (C_sf), were studied. A design of experiment approach, Taguchi methodology, was used to investigate the characteristics of ZrB₂–SiC–C_sf composites concentrated upon the hot pressing parameters (sintering temperature, dwell time and applied pressure) as well as the composition (vol% SiC/vol% C_sf). The analysis of variance recognized the sintering temperature and SiC/C_sf ratio as the most effective variables on the relative density of hot pressed composites. The microstructural investigations showed that C_sf can act as a sintering aid and eliminate the oxide impurities (e.g. B₂O₃, ZrO₂ and SiO₂) from the surfaces of raw materials. A fully dense composite was achieved by adding 10 vol% C_sf and 20 vol% SiC to the ZrB₂ matrix via hot pressing at 1850 °C for 30 min under a pressure of 16 MPa. Moreover, the in-situ formation of interfacial ZrC, which also improves the sinterability of ZrB₂-based composites, was studied by energy-dispersive X-ray spectroscopy analysis and verified thermodynamically.     

Two step hot pressing sintering of dense fine grained WC–Al2O3 composites

WC–40 vol% Al₂O₃ composites were prepared by high energy ball milling and the following two step hot pressing sintering (TSS). The tungsten carbide (WC) and commercial alumina (Al₂O₃) powders composed of amorphous Al₂O₃, boehmite (AlOOH) and χ-Al₂O₃ were used as the starting materials. The feasibility of two step sintering (TSS) method to WC–40 vol% Al₂O₃ composites was demonstrated, optimum TSS regime was discussed and phase transformation during TSS process was investigated. The results showed that both the pre-sintering at a proper first step temperature (T₁) to obtain a critical initial density and isothermal hot pressing at an appropriate second step temperature (T₂) to inhibit grain boundary migration (GBM) were of significant importance. When the as milled WC–40 vol% Al₂O₃ powders were hot pressed under TSS₄ regime, a relative density of 99% and a grain size of 2.38 μm were obtained, an excellent Vickers hardness of 19.71 GPa was achieved, combining a fracture toughness of 12 MPa m^1/2 with a flexural strength of 1285 MPa. Compared with the near full dense samples consolidated under CS₁ regime (1540 °C for 90 min), the grain size decreased, the Vickers hardness, fracture toughness and flexural strength were all improved due to the refined microstructure and the transgranular fracture mode. The amorphous Al₂O₃, AlOOH and χ-Al₂O₃ were transformed to α-Al₂O₃ completely during the sintering process.     

Visible-light-induced photocatalytic activity of TiO2−xNy prepared by solvothermal process in urea–alcohol system

Nitrogen-doped anatase, rutile and brookite titania photocatalyst TiO_2−xN_y which can be excited by visible light were prepared by mixing aqueous TiCl₃ solutions with urea ((NH₂)₂CO) and various type of alcohols followed by solvothermal treatment at 190 °C. The phase composition, crystallinity, microstructure and specific surface area of titania powders greatly changed depending on the pH and type of solvents. Violet, yellowish and grayish TiO_2−xN_y with excellent visible light absorption and photocatalytic activity were prepared. The TiO_2−xN_y powders prepared in urea–methanol solution showed excellent photocatalytic ability for the oxidative destruction of nitrogen monoxide under irradiation of visible light λ > 510 nm.     

Processing and dielectric characterization of Ba0.3Sr0.7TiO3 thin films on alumina substrates

Ba_0.3Sr_0.7TiO₃ (BST) thin films were prepared from the sols based on alkaline earth acetates and titanium propoxide in 2-methoxyethanol–acetic acid solvents and deposited on polished alumina substrates by spin coating. The perovskite phase crystallizes upon heating at/above 700 °C. By increasing the annealing temperature from 700 to 900 °C the grain size increases from 40 to 80 nm, due to the increased driving force for crystallization. The annealing time has got only a minor influence on grain size as a consequence of constrained conditions of the film. The dielectric permittivity and tunability (ɛ_0 V/ɛ_200 V) of BST films, measured at 1 MHz, strongly depend on the grain size, exhibiting the values of 345 and 1.47, and 722 and 1.93 for the films with 40 and 80 nm-sized grains, respectively.     

The effect of BaO and Al2O3 addition on the crystallization behaviour of cordierite glass ceramics in the presence of V2O5 nucleant

The effect of V₂O₅ nucleant on crystallization of stoichiometric cordierite glass ceramics in the presence of various amounts of BaO and Al₂O₃ additives were investigated by DTA, XRD and SEM. It was shown that 3 wt.% V₂O₅ and 1.5 wt.% BaO were the optimum amounts of the additives effective in inducing both surface and bulk crystallization in the above glass ceramics.This resulted in ～90 wt.% cordierite after a 3 h heat treatment at 1020 °C. The specimens possessing 4–5 wt.% Al₂O₃ in excess of the stoichiometric cordierite composition, developed mullite along with cordierite in the temperature range of 1045–1055 °C, whereas in the specimen containing 6 wt.% excess Al₂O₃, mullite was detected as the sole crystallization product.     

Effect of crystallization temperature on dielectric and energy-storage properties in SrO-Na2O-Nb2O5-SiO2 glass-ceramics

The SrO-Na₂O-Nb₂O₅-SiO₂ (SNNS) glass-ceramics were prepared through the melt-quenching combined with the controlled crystallization technique. XRD results showed Sr₆Nb₁₀O₃₀, SrNb₂O₆, NaSr₂Nb₅O₁₅ with tungsten bronze structure and NaNbO₃ with the perovskite structure. With the decrease of crystallization temperature, dielectric constant firstly increased and then decreased, while breakdown strength (BDS) was increased. High BDS of the glass-ceramics is attributed to the dense and uniform microstructure at low crystallization temperature. The optimal dielectric constant of 140±7 at 900 °C and BDS of 2182±129 kV/cm at 750 °C were obtained in SNNS glass-ceramics. The theoretical energy-storage density was significantly improved up to the highest value of 15.2±1.0 J/cm³ at 800 °C, which is about 5 times than that at 950 °C. The discharged efficiency increased from 65.8% at 950 °C to 93.6% at 750 °C under the electric field of 500 kV/cm by decreasing crystallization temperature.     

The effects of pulse repetition rate on the structural,surface morphological and UV photodetection properties of pulsed laser deposited Mg-doped ZnO nanorods

The Mg_0.05Zn_0.95O (MZO) nanorod array (NRA) films have been successfully grown onto SiO₂/ n-Si substrates by pulsed laser deposition (PLD) without any template or seed layer and the influence of pulse repetition rate (3 to 15 Hz) of a 248 nm KrF excimer laser on their crystallinity, surface morphology and UV photodetection properties were systematically investigated. All the samples show the hexagonal wurtzite phase with a preferential c-axis orientation and the optimum crystallization of the MZO NRAs occurs at 5 Hz. FE-SEM analysis revealed that the growth of MZO NRAs is strongly influenced by the pulse repetition rate. It was observed that the average film thickness increases almost linearly with the pulse repetition rate and the MZO nanorod arrays grown at 5 Hz exhibits best surface area. Moreover, the room temperature UV photodetection properties of the samples were investigated in metal–semiconductor–metal (MSM) planar configurations and are found to be strongly driven by the pulse repetition rate dependent crystalline and surface morphological features. The device current–voltage (I–V) characteristics were measured under dark and UV light conditions. Then, the photocurrent and responsivity were measured with the variation of optical power density and applied voltage, respectively. Transient photoresponse studies show an exceedingly stable and fast switching UV photoresponse for the photodetector having MZO nanorods grown at 5 Hz, which demonstrates highest responsivity of 17 mA/W upon 2 mW/cm² UV illumination (365 nm), at 5 V bias.