Combustion Synthesis of Aluminoborate Glass Matrices

Combustion synthesis or self-propagating high-temperature synthesis (SHS) has been used for the first time to produce glass–ceramic materials. The materials produced by the technique have a glassy matrix (aluminoborate glass) and crystalline TiB₂ particles, of about 0.5 m. The combustion characteristics and microstructures of the synthesized materials are presented.     

Preparation and microwave properties of Y-type magnetoplumbite microcrystalline glass ceramic with the composition 0.5(Ba2ZnZCo2-ZFe12O22)·0.5SiO2

The citrate sol-gel synthesis technique was used to prepared Ba₂Zn_ZCo_2-ZFe₁₂O₂₂/SiO₂ microcrystalline glass ceramic with Z=2.0, 1.6, 1.2, and 0.8. Several methods such as X-ray diffractometry, scanning electron microscopy and HP vector network analyzer were used to obtain detailed information on the crystallography and magnetic properties of Ba₂Zn_ZCo_2-ZFe₁₂O₂₂/SiO₂ microcrystalline glass ceramic. The correlation of the formation of Ba₂Zn_ZCo_2-ZFe₁₂O₂₂/SiO₂ microcrystalline glass ceramic with thermal treatment was studied. The complex dielectric constant and complex permeability of microcrystalline glass ceramic-paraffin wax composites were measured by the transmission/reflection coaxial line method in the range from 100MHz to 6 GHz. The effects of composition, annealing temperature and measuring frequency on complex permittivity and permeability of Ba₂Zn_ZCo_2-ZFe₁₂O₂₂/SiO₂ microcrystalline glass ceramic were also investigated. Received 23 July 2001 / Accepted: 24 July 2001     

Microstructural and dielectric behaviour of glass ceramics in the system PbO-BaO-TiO2-B2O3-SiO2

Glasses in the system PbO-BaO-TiO₂-B₂O₃-SiO₂ with and without P₂O₅ as nucleant have been prepared. The glass samples were ceramized based ondta studies. The ferroelectric phase crystallizing out has been found to be BaTiO₃ fromxrd. The optical and scanning electron micrographs show the presence of BaTiO₃ as major phase. In these glass ceramic samples, dielectric constant and dissipation factor are approximately constant with temperature and frequency upto the glass transition temperatureT _g and thereafter increase sharply with temperature and finally level off. The addition of P₂O₅ as nucleant and molar ratio of (PbO + BaO) to TiO₂ has marked influence on the dielectric behaviour and composition of ferroelectric phase crystallizing out.     

Physical and spectroscopic properties of multi-component Na2O–PbO–Bi2O3–SiO2 glass ceramics with Cr2O3 as nucleating agent

Transparent glass ceramics, synthesized from melt quenching followed by heat treatment, of the composition 10Na₂O–30PbO–10Bi₂O₃–(50 − x)SiO₂:xCr₂O₃ (mol%), where 0 ⩽ x ⩽ 0.5, were characterized with XRD, DTA, SEM and EDS. Physical and spectroscopic studies, viz., optical absorption, electron paramagnetic resonance (EPR), FTIR and Raman were investigated. The characterization of the host glass ceramic has revealed that the formation of a major phase of sodium silicate along with two minor phases such as lead silicate and bismuth oxide. By integrating Cr₂O₃ to the host glass additional crystal phases viz., NaCrO₂, Na₂Cr₂O₇ and Pb(CrO₄) which are the complexes of Cr³⁺ and Cr⁶⁺ ions were also developed. As the concentration of nucleating agent is increased, a part of the Cr⁶⁺ ions is found to reduce in to Cr³⁺ ions. Spectroscopic studies have revealed that with an increase in the concentration of Cr₂O₃ from 0.1 to 0.5 mol%, there is a gradual increase in the intensity of vibrational modes of various asymmetric structural units of silicate, bismuthate and chromate in the glass ceramic network at the expense of symmetrical structural units. The analysis of the results of these studies has indicated that in the samples containing higher concentration of Cr₂O₃, chromium ions exists predominantly in Cr³⁺ state and occupy the octahedral positions in glass ceramic matrix and such glass ceramic samples are suitable for lasing action.     

The formation of TiC/Al2O3 microstructures by a self-propagating high-temperature synthesis reaction

The effect of processing variables on reaction rate and ceramic microstructure are examined for the self-propagating high-temperature synthesis reaction 3TiO₂+4Al+3C 3TiC+2Al₂O₃. Reaction controlling methods used are reactant particle size, the use of diluents (to lower the combustion temperature) and the use of reactant preheating (to increase the combustion temperature). The ceramic microstructure has an unusual laminar structure which is generally only observed during unstable combustion wave propagation.     

Self-propagating high temperature synthesis and magnetic properties of Ni0.35Zn0.65Fe2O4 powders

Ni-Zn ferrite powders were synthesized by self-propagating high temperature synthesis (SHS) method. X-ray diffraction, TEM and vibrating sample magnetometry (VSM) were used to characterize the phase composition, microstructure and magnetic properties of the combustion products. The effect of the combustion temperature (T _c), the major parameter of the SHS process, on particle size, phase composition and magnetic properties of the products was also studied. The results showed that particle size grew with the increasing combustion temperature. The maximum saturation magnetization,M _s, increased with combustion temperature indicating the growth of grain size and high degree of ferritization, while residual magnetization,M _r, and coercive force,H _c, decreased. Compared with other methods, Ni_0.35Zn_0.65Fe₂O₄ ferrite powders with improved magnetic properties can be obtained by SHS at 1000°C.     

Thermal conductivity of (TeO2)0.7(WO3)0.2(La2O3)0.1 glass

The thermal conductivity of (TeO₂)_0.7(WO₃)_0.2(La₂O₃)_0.1 tellurite glass has been measured by a steady-state technique between 77 and 320 K. In this temperature range, the thermal conductivity varies from 0.38 to 1.29 W/(m K). The results were analyzed using fitting equations tested on quartz glass, which made it possible to extend the temperature range of thermal conductivity determined in this study.     

Self-propagating high temperature synthesis of Ni0.35Zn0.65Fe2O4 ferrite powders

The stoichiometric Ni_0.35Zn_0.65Fe₂O₄ ferrite powders were synthesized by SHS method. In the process of SHS, the effects of the molar ratio Fe/Fe₂O₃ in the starting mixture, oxygen pressure, grain size and relative density of the raw materials on combustion temperature, combustion wave velocity, phase composition and microstructure of the combustion products were investigated. X-ray diffraction, scanning electron microscope, TEM, vibrating sample magnetometry were used to characterize the microstructure and magnetic properties of the products. The results showed that as the molar ratio Fe/Fe₂O₃ increases, the combustion temperature and combustion wave velocity increased. The same results can be observed when the oxygen pressure increased from 0.1 to 0.9 MPa. The increase of grain size and relative density of raw materials resulted in the decrease of combustion temperature and combustion wave velocity. Compared with other methods, SHS process leads to ferrite powders with improved magnetic properties.     

Fabrication of Al2O3–ZrB2 in situ composite by SHS dynamic compaction: A novel approach

Al₂O₃–ZrB₂ in situ composites of 97% of theoretical density were successfully fabricated by a novel self-propagating high temperature synthesis (SHS) dynamic compaction, using less expensive raw materials zirconium oxide, boron oxide, and aluminium. The process is fast, energy efficient, where no furnace sintering is required. The process inhibits and controls the grain growth and microstructure. The densification behaviour and correlation with microstructure of the SHS dynamic compacts were compared with the furnace sintered composite samples where the composite powder was prepared by SHS process. The furnace sintered samples showed coarser grain growth and maximum density of 94.5% of theoretical density was achieved. The SHS dynamic compacted in situ composite had much finer grains in the range of 0.5–3 μm with density 95.5% of the theoretical value. The average grain size was found to decrease from 10 μm to 1.4 μm for alumina and from 5.4 μm to 1.0 μm for zirconium diboride from furnace sintering to SHS dynamic compaction, respectively. Addition of Al₂O₃ as a diluent during SHS reaction enhanced the density to 97%. During SHS dynamic compaction, the amount of liquid and the time interval at which the sample stays at high temperature are the controlling factor of the final microstructure and the densification of the composite.     

Formation mechanism of Ti2AlC under the self-propagating high-temperature synthesis (SHS) mode

Ti₂AlC was fabricated by the self-propagating high-temperature synthesis (SHS) technique from a compacted powder mixture consisting of Ti:Al:C = 2:1:1 (molar ratio) of mortar mixing, planetary mixing and various packing densities. A thermocouple was placed directly into the green compact body in order to monitor the combustion temperature during the SHS process. When the green compact with planetary mixing treatment and a packing density of 17% as well as 60% was used, propagation of the reaction exothermicity could occur, and the starting compact completely changed to Ti₂AlC. Formation mechanism of Ti₂AlC using a SHS technique was discussed. In addition, the melting point of the resultant sample was determined to be 1570 °C.     

Structure and mechanical characteristics of ceramic Nb2O5 and Nb2(1 − y)Ta2y O5

The structure and mechanical characteristics of ceramic Nb₂O₅ and ceramic materials produced from coprecipitated Nb_{2(1 ? y)}Ta_2y O₅ pentoxides by a conventional ceramic processing technique and exposure to high-intensity light have been studied by scanning probe microscopy and Raman spectroscopy.     

Mictomagnetism in a new BaO.Fe2O3.B2O3 glass.

An homogeneous BaO ·Fe₂O₃ ·B₂O₃ glass containing 30% Fe₂O₃ is prepared by a splat-cooling technique. X-ray and electron diffraction reveal the product to be amorphous at room temperature. The crystallization, as shown by DTA studies, begins at 750 K, and up to 950 K, a temperature at which BaFe₁₂O₁₉ is shown to be present, the crystallizing products are mainly evolutive.The magnetic measurements show a probably mictomagnetic behaviour at low temperature with a maximum of magnetic susceptibility at 12 (±1) K. At higher temperature the susceptibility obeys a Curie-Weiss law with large negative Weiss constant and low Curie constant per Fe³⁺ ions. After crystallization the product is ferrimagnetic and could be used as a permanent magnet.Mössbauer study reveals that the glass mainly consists of Fe³⁺ ions in distorted sites and a hyperfine structure at low temperature; the magnetic ordering temperature is estimated to be about 44 (±1) K.     

Sintering behaviors,microstructures and dielectric properties of CaO-B2O3-SiO2 glass ceramic for LTCC application with various network modifiers content

CaO-B₂O₃-SiO₂ (CBS) glass ceramic with various glass network modifiers were synthesized to develop a kind of material sintered at low temperature for multi-layer ceramic substrates. The influence of double alkali metal oxides (Na₂O and K₂O) on sintering characteristics, microstructures and dielectric properties of CBS glass ceramic were investigated. XRD analysis showed that the major crystalline phase was β-CaSiO₃ and CaB₂O₄, and the strength of the peak increased with the addition of network modifiers. DSC curve revealed that the process of crystallization and softening were obviously affected. SEM photographs exhibited that the microstructures were composed of amorphous phases, pores and crystalline phases. FTIR showed that the additives changed the continuity of CBS glass structure. An appropriate amount of Na₂O and K₂O additives could significantly improve the sintering characteristics and dielectric properties. CBS samples with 1.5 wt% (Na₂O?+?K₂O) sintered at 850 °C for 15 min presented the best performances with the bulk density of 2.53 g/cm³, ε_r?=?5.94 and tanδ?=?1.22?×?10^?3 at 10 GHz.

     

Relaxor ferroelectric-like behavior in 10PbTiO3–10Fe2O3–30V2O5–50B2O3 glass for energy storage applications

Recently, demand increased for dielectric materials used in energy storage devices at high voltage applications. Appearance of polar clusters in glass matrix could promote its use in energy storage applications. Conventional quenched glass sample of composition 10PbTiO₃–10Fe₂O₃–30V₂O₅–50B₂O₃ were successfully developed. The glassy nature was confirmed by XRD and DSC measurements. Boson peak observed at low frequency from the Raman spectra confirms polar cluster formation. Dielectric properties of prepared glass were investigated in a wide range of frequency and temperature. Broad and diffuse peak of dielectric permittivity shifted to the higher temperatures, denoting the typical relaxor ferroelectrics like behavior. Sample shows energy storage density of about 164.7 mJ/cm³ at room temperature. Quenched glass sample shows typical anti-ferromagnetic behavior.

     

Nanocrystallization as a method of improvement of electrical properties of Fe2O3–PbO2–TeO2 glasses

Selected glasses of Fe₂O₃–PbO₂–TeO₂ system have been transformed into nanomaterials by annealing at a temperature close to the crystallization temperature (T_c). The effects of the annealing of the present samples on the structural and electrical properties were studied by transmission electron micrograph (TEM), X-ray diffraction (XRD), differential scanning calorimeter, density (d) and dc conductivity (σ). TEM and XRD of glass–ceramic naocrystals indicated nanocrystals embedded in the glassy matrix with average particle size of 20–35 nm. The glass–ceramic naocrystals obtained by annealing at T_c exhibit improvement of electrical conductivity up to four orders of magnitude than the starting glasses. This considerable improvement of electrical conductivity after nanocrystallization is attributed to formation of extensive and dense network of electronic conduction paths which are situated between Fe₂O₃ nanocrystals and on their surface. The conduction is attributed to non-adiabatic hopping of small polaron.     

Processing and properties of Y-TZP/Al2O3 composites

The processing and property measurement of Y-TZP/Al₂O₃ ceramic-ceramic composites was investigated. The wet chemical synthesis route was adopted for the preparation of 3Y-TZP matrix dispersed with Al₂O₃ in three different volume fractions. Characterization of the resultant powders was carried out and their densification behaviour was studied by sintering in air in the temperature range 1200–1600 °C. The role of alumina as grain-growth inhibitor for Y-TZP, and the mechanical response of these ultrafine-grain ceramic composites in terms of K_lc characteristics, have been discussed.     

Dielectric and impedance spectroscopic studies of (Sr1−x Pb x )TiO2 glass ceramics with addition of Nb2O5

Glasses were made by melt-quench method in the system [(Sr_1?x Pb _x )O·TiO₂]-[2SiO₂·B₂O₃]-5[K₂O-BaO] (0·0 ≤ x ≤ 0·4) with addition of 1 mol% Nb₂O₅. Perovskite strontium lead titanate in solid solution phase has been crystallized in borosilicate glassy matrix with suitable choice of composition and heat treatment schedule. Addition of 1 mol% of Nb₂O₅ enhances the crystallization of lead strontium titanate phase in the glassy matrix. Scanning electron microscopy (SEM) is performed to study the surface morphology of the crystallites and crystalline interface to the glass. Dielectric properties of these glass ceramics were studied by measuring capacitance and dissipation factor as a function of temperature at a few selected frequencies. Nb₂O₅ doped strontium lead titanate glass ceramic shows a high value of dielectric constant. It is of the order of 10,000 while the dielectric constant of undoped glass ceramic sample is of the order of 500. Complex impedance and modulus spectroscopic techniques were used to find out the contributions of polarization of crystallites and glass crystal interfaces to the resulting dielectric behaviour.     

Effect of B2O3 addition on microhardness and structural features of 40Na2O-10BaO-xB2O3-(50-x)P2O5 glass system

Phosphate glasses having composition, 40Na₂O-10BaO-xB₂O₃-(50-x)P₂O₅, wherex = 0–20 mol% were prepared using conventional melt quench technique. Density of these glasses was measured using Archimedes principle. Microhardness (MH) was measured by Vicker’s indentation technique. Structural studies were carried out using IR spectroscopy and³¹P and¹¹B MAS NMR. Density was found to vary between 2.62 and 2.77 g/cc. MH was found to increase with the increase in boron content.³¹P MAS NMR spectra showed the presence of middle Q² groups and end Q¹ and Q⁰ groups with P-O-B linkages. FTIR studies showed the presence of BO₃ and BO₄ structural units along with the depolymerization of phosphate chains in conformity with³¹P MAS NMR.¹¹B NMR spectra showed increase in BO₄ structural units with increasing boron content. The increase in MH with B₂O₃ content is due to the increase of P-O-B linkages and BO₄ structural units as observed from MAS NMR studies resulting in a more rigid borophosphate glass networks.     

Processing and Characterization of Plasma Spray Coatings of Glass Microspheres Premixed with Al2O3 Particles

Various ceramics and metals are being deposited as functional, protective, and near-homogenous coatings on engineering components by exploiting the characteristic properties of plasma medium. Such coatings are known to exhibit improved wear, thermal, and corrosion resistance. Although a lot of studies have been reported on coatings made up of a large number of metals and ceramic particles, hardly any effort is made to coat glass microspheres on metals despite their high hardness. In view of this, the present work was undertaken to study the preparation and characterization of a new class of coatings made up of borosilicate glass microspheres (BGM) premixed with micro-sized aluminum oxide (Al₂O₃) in different proportions. Deposition of these BGM and BGM/Al₂O₃ coatings is carried out at five different levels of torch input power. Coatings are characterized in terms of their thickness, hardness, adhesion strength, and porosity. The coatability of BGM and the BGM/Al₂O₃ mixture on metallic substrates is assessed by evaluating the coating deposition efficiency. This work reveals that the torch input power and the Al₂O₃ content in the feedstock affect the major coating characteristics, and premixing of Al₂O₃ with BGM results in better coating properties.     

Photoluminescence of Ti3+ in P2O5-Na2O-Al2O3 glass

The optical properties (absorption and luminescence) of Ti³⁺ in a P₂O₅ Na₂O-Al₂O₃ glass have been studied in the temperature range 12 to 300 K. A very broad infrared emission band at 860 nm, has been observed for the first time in an inorganic glass, when excitation is performed in the Ti³⁺ absorption band (T_2g → E_g transition in cubic field approximation). The spectroscopic characteristics of this system are compared with those of the Ti³⁺ as a dopant in AL₂O₃ single crystals.