Crystallization and sinterability of cordierite-based glass powders containing CeO2

Dense glass-ceramics for low firing temperature substrates were prepared by the addition of CeO₂ flux to a glass of the MgO-Al₂O₃-SiO₂ system. The glass powders were fabricated by melting at 1500°C and ball milling. Glass powder compacts prepared by dry pressing were heated at 800–1000°C for 0.5–4 h and sintered at 900–1000°C for 3 h. The crystallization behaviour and sinterability of the glass powder compacts were analysed by thermal and thermomechanical techniques. X-ray diffractometry and scanning electron microscopy. The addition of CeO₂ prevents the formation of -cordierite phase in the glass-ceramics and improves the formation of -cordierite phase. The activation energy of the glass containing CeO₂ for crystallization was lower than that of the CeO₂-free glass. Therefore, crystallization properties were enhanced. Because the crystallization onset temperature increased and the softening temperature decreased on the addition of CeO₂, the sinterability increased and dense glass-ceramics were fabricated below 1000°C. The properties of the glass-ceramics containing CeO₂ appeared to be correct for low firing temperature substrates.     

Preparation and densification of superconducting YBa2Cu3Ox ceramics

The solid-state reaction method to form the superconducting oxide YBa₂Cu₃O _x was studied. It was found that the starting cupric and yttrium components accelerated the decomposition of the BaCO₃ component. At a constant heating rate of 10 ° Cmin^–1 in thermogravimetric analysis, the temperature of complete decomposition,T _f, was lowered from greater than 1000 ° C in pure BaCO₃ to between 915 and 985 ° C. The effectiveness in decreasingT _f can be ranked in the order of oxalate, carbonate and oxide. The highest sintered density achieved in this study was 6.03 g cm^–3 (/_th = 95%) at 990 ° C and 5.85 g cm^–3 (/_th = 92%) at 960 ° C. The source of cupric ion had the largest effect on densification. The use of cupric carbonate resulted in a consistently high Archimedes density of about 6.00gcm^–3 and large dimensional shrinkage of about 20% at 990 ° C for 12h. In contrast, the use of cupric oxide gave the lowest density and smallest shrinkage. Within the same powder lot, higher sintered density and smaller dimensional shrinkage were observed in samples with higher initial green density and compaction pressure. However, the data suggested that the enhanced densification and higher density achieved by the use of cupric carbonate and oxalate cannot be accounted for by the different physical characteristics of the powders and the mechanics of powder compaction, measured collectively by the green density.     

Fabrication, characterization and sintering of glass-ceramics for low-temperature co-fired ceramic substrates

Cordierite-based glass-ceramics with non-stoichiometric composition doped with rare earth oxide (CeO₂) and heavy metal oxide (Bi₂O₃) respectively were fabricated from glass powders. After sintering and crystallization heat treatment, various physical properties, including compact density and apparent porosity, were examined to evaluate the sintering behavior of cordierite-based glass-ceramics. Results showed both that the additives heavy metal oxide and rare earth oxide promoted the sintering and lowered the phase temperature from - to -cordierite as well as affecting the dielectric properties of sintered glass-ceramics. The complete-densification temperature for samples was as low as 900 °C. This material has a low dielectric constant (5.3), a low dielectric loss (0.2%) and a low thermal expansion coefficient (2.8–3.52×10^–6 K^–1), and can be co-fired with high conductivity metals such as Au, Ag, Cu, Ag/Pd paste at low temperature (below 950 °C), which makes it a promising material for low-temperature co-fired ceramic substrates.     

Preparation and microwave dielectric properties of (Pb0.45Ca0.55)(Fe1/2Nb1/2)O3 ceramics by citrate-gel processing route

A synthetic procedure has been developed for the (Pb_0.45Ca_0.55)(Fe_1/2Nb_1/2)O₃ (PCFN) system in terms of liquid-mixed citrate precursor. The stability of the citrate gels against phase separation is investigated as a function of the pH value and the citrate/metal ratio in the initial solution. The elaboration process is described in detail. A single-phase PCFN powder with a particle size of 30–50 nm can be synthesized at the low temperature of 850 °C using the citrate-gel processing route, which is 200 °C lower than that for the conventional process. Sintering properties and microwave dielectric properties of specimens derived from the citrate-gel process were also studied in this paper. A dense compound with a bulk density up to 6.20 g cm^-3 could be obtained when the specimens are sintered at 1000 °C/4 h. The dense single-phase PCFN ceramics were found to have microwave dielectric properties of =86.2, Q · f=3450 GHz and _f=4 ppm^°C^-1.     

The preparation and properties of some lithium zinc silicate glass-ceramics

Lithium zinc silicate glasses are of interest for the preparation of moderately high thermal expansion glass-ceramics which are suitable for sealing to a number of nickel-based superalloys. The effect of composition, in particular the variation of nucleating species, on the crystallization behaviour of a number of these glasses has been examined using differential thermal analysis, X-ray diffraction, and electron microscopy. Various crystal phases have been identified, including cristobalite, quartz, tridymite and ₀ Li₂ZnSiO₄. In addition, most of the glass-ceramics also contain an unidentified phase which may be related to the-series of lithium zinc silicates. Heat-treatment schedules have been derived on the basis of these results in order to produce a number of glass-ceramic materials. The resultant thermal expansion characteristics of the glass-ceramics have been monitored using dilatometry, and expansions in the range 12.3 to 17.1×10^–6° C^–1 (20 to 460° C), have been obtained, depending on the precise glass composition and heat-treatment schedule employed. In addition, the mechanical properties of a number of selected samples have been monitored, employing a biaxial flexure technique.     

Characteristics of powder and sintered bodies of hydrothermally synthesized Mn-Zn ferrites

To improve the sinterability of powders fabricated by the conventional mixed-oxides method, ultrafine Mn-Zn ferrite powders were hydrothermally synthesized from metal nitrates solution using ammonia as a precipitant. The R value (alkalinity) was introduced to adjust the amount of added OH^– in the reaction suspension. The characteristics of the powders synthesized at different hydrothermal conditions and the properties of the sintered bodies were investigated. The results show that the R value and hydrothermal time have a great effect on the compositions and phases of hydrothermally synthesized Mn-Zn ferrite powders. Powders synthesized from a starting suspension with a higher content of Zn ions (or lower content of Mn²⁺) may approach to a stable spinel structure with a lower Mn/Zn ratio as the hydrothermal time is longer. Factors affecting the position of the diffraction angle (2) of the spinel Mn-Zn ferrite (311) of powders may include both the compositions of spinel ferrite structure and crystallite sizes (or particle sizes) of powders. Some possible reasons were suggested to explain the dependence of composition and phase of hydrothermally synthesized Mn-Zn ferrite powders on the R value and hydrothermal time. The temperature that the green compact begins to shrink at increases with increasing R value, and ranges from 510°C (R = 2) to 650°C (R = 6). After being sintered at 950°C for 2 h in N₂ atmosphere, the relative sintered density of each specimen reaches a value of 94.5–99.8%.     

Preparation and crystallization of ultrafine Li2O-Al2O3-SiO2 powders

Ultrafine powders of Li₂O-Al₂O₃-SiO₂ (LAS) glass-ceramic were prepared by the sol-gel process using tetraethoxysilane, titanium butoxide, lithium, magnesium, aluminium (and zinc) inorganic salts as starting materials. The effect of pH on the sol-gel transition and particle sizes of the Li₂O-Al₂O₃-SiO₂ system was studied. The nucleation and crystallization process of LAS powders were also investigated by differential thermal analysis and X-ray diffraction. The results show that a surface nucleation process occurs for ultrafine LAS powders. The LAS glass-ceramics fabricated from ultrafine LAS powders have a low thermal expansion coefficient, <10×10^–7 °C.     

Sintering and crystallization phenomena in Silceram glass

An investigation has been carried out on the feasibility of employing a CaO-MgO-Al₂O₃-SiO₂ glass-ceramic (known as Silceram) as a matrix for a fibre composite produced by a powder route. Some important properties of the parent glass, e.g. surface energy, viscosity, have been measured as well as the kinetics and structural aspects of the sintering and crystallization processes. It was found that the crystallization of the main phase, diopside, occurred from the surface of individual particles and internally with activation energies of 392 to 452 kJ mol^–1 and 258 to 323 kJ mol^–1 respectively. However, the surface crystallization was only dominant when the glass powder was fine (<>m). The work has demonstrated that this glass-ceramic shows promise as the matrix component of a composite because

Sintering and crystallization of mullite powder prepared by sol-gel processing

Mullite powder with the stoichiometric composition (3Al₂O₃.2SiO₂) was synthesized by a sol-gel process, followed by hypercritical drying with CO₂. Within the limits of detection by X-ray diffraction, the powder was amorphous. Crystallization of the powder commenced at 1200 °C and was completed after 1 h at 1350 °C. In situ X-ray analysis showed no intermediate crystalline phases prior to the onset of mullite crystallization and the pattern of the fully crystallized powder was almost identical to that of stoichiometric mullite. The synthesized powder was compacted and sintered to nearly theoretical density below 1250 °C. The microstructure of the sintered sample consisted of nearly equiaxial grains with an average size of 0.2 m. The effect of heating rate (1–15 °C min^–1) on the sintering of the compacted powder was investigated. The sintering rate increased with increasing heating rate, and the maximum in the sintering curve shifted to higher temperatures. The sintering kinetics below 1150 °C can be described by available models for viscous sintering.     

Mechanical and elastic properties of transparent nanocrystalline TeO2-based glass-ceramics

Mechanical and elastic properties of transparent TeO₂-based glass-ceramics (15K₂O · 15Nb₂O₅ · 70TeO₂) consisting of nanocrystalline particles (each particle size: 40–50 nm) and showing optical second harmonic generation were evaluated by means of usual Vickers indentation and nanoindentation tests. The precursor glass has Vickers hardness H _v of 2.9 GPa, Young's modulus E of 54.7 GPa, the fracture toughness K _c of 0.25 MPam^1/2 and Poisson's ratio of 0.24. The transparent nanocrystalline glass-ceramic heat-treated at 420°C for 1 h has H _v = 3.8 GPa, E = 75.9 GPa and K _c = 0.34 MPam^1/2, and the opaque glass-ceramic heat-treated at 475°C for 1 h has H _v = 4.5 GPa, E = 82.9 GPa and K _c = 0.68 MPam^1/2, demonstrating that poor mechanical and elastic properties of the precursor TeO₂-based glass are improved through sufficient crystallization. The fracture surface energy, brittleness and elastic recoveries (about 44%) after unloading (the maximum load: 30 mN) of transparent nanocrystalline glass-ceramics are almost the same as those of the precursor glass, implying that the interaction among nanocrystalline particles is not so strong.     

Microstructure and properties of an infra-red transmitting chalcogenide glass-ceramic

A chalcogenide glass-ceramic (0.3 PbSe-0.7 Ge_1.5 As_0.5 Se₃) which transmits in the infra-red region between 8 and 12 m was produced from a phase separated parent glass. The glass transition temperature (T _g) was increased from 280 to 340°C by crystallizing the phase with the lowerT _g. Further heat-treatment produced a glass-ceramic that was up to 60% crystalline and contained PbSe, PbSe₂ and GeSe₂ crystals with a gran size of 0.5 m. The infra-red transmission of the glass-ceramic decreased with increased crystallinity. The glass-ceramic modulus of rupture (38 MN m^–2) was increased to as much as twice that of the glass and the Vickers hardness increased by 30% to 280 kg mm^–2.     

Sol-gel-derived hydroxyapatite powders and coatings

Hydroxyapatite (HAP) and tri-calcium phosphate (TCP) powders and coatings with a Ca/P molar ratio from 1.56 to 1.77 were prepared by the sol-gel technique using calcium 2-ethylhexanoate (Ca(O₂C₈H₁₅)₂) and 2-ethyl-hexyl-phosphate as calcium and phosphorus precursors, respectively. The structural evolution and phase formation mechanisms of HAP and tri-calcium phosphate in calcined powders and coatings on Si wafer and Ti-alloy substrates (Ti-30Nb-3Al and Ti-5Al-2.5Fe) were characterized by X-ray diffraction, Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). The elimination of organics was studied by differential thermal analysis (DTA) and thermogravimetry (TGA). Two different formation mechanisms of crystallization are proposed. In sols with Ca/P 1.67, -tricalcium phosphate is formed as the major phase and hydroxyapatite as a minor phase by calcination at 700°C. At 900°C these phases react to form AB-type carbonated hydroxyapatite (Ca_10–2x/3[(PO₄)_6–x (CO₃) _x ][(OH)_2–x/3–2y (CO₃) _y ]). A release of CO₂ substituting PO₄ ^3– occurs between 900°C and 1100°C yielding carbonate apatite, Ca₁₀(PO₄)₆[(OH)_2–2y (CO₃) _y ], whereas CO₂ substituting OH^– groups in the apatite structure is released above 1200°C. In sols with Ca/P 1.70, rather than carbonate apatite, B-carbonated hydroxyapatite Ca_10–2x/3[(PO₄)_6–x (CO₃) _x ](OH)₂ is formed, which subsequently decomposes into HAP and CaO above 1200°C. The optimum sintering conditions for coatings on Ti-alloys are found to be 600°C for 10 minutes, since, at higher temperature, oxidation of titanium and the formation of rutile (TiO₂) occur. Dip coating and sintering in two cycles yielded a homogeneous and dense coated film with a thickness of 250 nm.     

Silicon carbide fibre reinforced glass-ceramic matrix composites exhibiting high strength and toughness

Silicon carbide fibre reinforced glass-ceramic matrix composites have been investigated as a structural material for use in oxidizing environments to temperatures of 1000° C or greater. In particular, the composite system consisting of SiC yarn reinforced lithium aluminosilicate (LAS) glass-ceramic, containing ZrO₂ as the nucleation catalyst, has been found to be reproducibly fabricated into composites that exhibit exceptional mechanical and thermal properties to temperatures of approximately 1000° C. Bend strengths of over 700 MPa and fracture toughness values of greater than 17 MN m^–3/2 from room temperature to 1000° C have been achieved for unidirectionally reinforced composites of 50 vol% SiC fibre loading. High temperature creep rates of 10^–5 h^–1 at a temperature of 1000° C and stress of 350 MPa have been measured. The exceptional toughness of this ceramic composite material is evident in its impact strength, which, as measured by the notched Charpy method, has been found to be over 50 times greater than hot-pressed Si₃N₄.     

High-T c superconducting Bi(Al)-Ca-Sr-Cu-O glass-ceramic fibres drawn from glass preforms

Bi(Al)-Ca-Sr Cu-O glass-ceramic fibres over 100 cm in length were successfully drawn from a glass preform above the crystallization temperature,T _x. The diameters of the uniformly drawn fibres with circular cross-section could be controlled in the range from 25–200 m and the drawing speed was as high as 200 cm min^–1. In this work Al₂O₃ was used to modify the properties of the glass. It increased the glass transition and crystallization temperatures but did not significantly increase the glass working range. Shrinkage and increase of density during heat treatment of the glass fibres were observed. The annealed (825°C/12 h in air) Bi₄Al_0.1Ca₃Sr₃Cu₄O_y glass-ceramic fibre showed aT _c(onset) of 82 K and aT _c(zero) of 71 K.     

Microstructure,chemistry, elastic properties and internal friction of Silceram glass-ceramics

The temperature dependence of both Young's modulus (E) and internal friction (Q ^–1) from room temperature to 700° C has been determined by Förster's forced-resonance method for three Silceram glass-ceramics, produced by the direct controlled cooling of glass melts in the quaternary system CaO-MgO-Al₂O₃-SiO₂. These results are correlated with microstructural and phase chemistry data as well as calculated viscosity against temperature data. In particular, the viscosity of the residual glass is shown to predominate over its volume fraction in deter mining the temperature dependence ofE andQ ^–1 for a given Silceram. A simple model which enables the residual glass content of Silceram glass-ceramics to be estimated from a know ledge of the proportions of silicon, iron and magnesium in the corresponding glass melts is also proposed. Furthermore, the room-temperature bulk modulus (K) and Poisson's ratio of two Silceram glass-ceramics are calculated using experimentalE and shear modulus (G) values obtained using both Förster's method and another forced-vibration technique.     

Sintering characteristics and crystallization for sol–gel-derived powders for low-dielectric and low-temperature sintering ceramics

Samples in the MgO–Al₂O₃–SiO₂ system were prepared by the sol–gel technique. The coalescence, sintering characteristics, and crystallization were investigated by X-ray diffraction, thermal analysis, and scanning electron microscopy. The dielectric properties and density were measured with an impedance analyzer. Results demonstrated that the obtained cordierite powders synthesized with the sol–gel method distribute uniformly and its effective size is 474 nm. The glass powder could be sintered at 950 °C, and the polymorphic modification cordierite detected in the sintered sample was only stable hexagonal -phase. The sintering densification process was performed mainly in the temperature range from 800 °C to 930 °C, and follows the viscous floating principle. The dielectric constant of the sintered body is 4.2 and its dielectric loss is lower than 0.001 at high frequency (1.5 GHz).     

Crystallization of leucite as the main phase in glass doped with fluorine anions

The crystallization of a multicomponent glass containing 1.63 wt% of F^– anions was studied. The results show in powder glass with particle sizes less than 0.15 mm, that surface crystallization is dominant, whereby two phases: leucite and dioside are formed. In glass powder or particle size about 0.15 mm, three phases, phlogopite, diopside and leucite, are formed, accompanied by an abrupt decrease in the resistance of the glass to crystallization. If the particle sizes are in the range 0.15 to 0.45 mm, both surface and volume crystallization are significant, while with particle sizes >0.45 mm, volume crystallization is dominant. Two nucleation temperatures, T _n1 = 655°C and T _n2 = 675°C, were determined in the temperature range of 600–710°C. These temperatures satisfy the condition that T _n T _g . Crystallization of bulk glass occurs in the temperature range of T _c = 870–1100°C, the crystal phases appearing in the sequence: phlogopite, followed by diopside, followed by leucite. Kinetical and microstructural studies show that the crystallization process is controlled by volume diffusion.     

Nonlinear electrical properties of ZnO varistors fast-fired by using millimeter-wave sintering process

Fast firing of Bi₂O₃-based ZnO varistor materials, which includes zero minutes soaking at 1100°C with 120°C/min heating and 145°C/min cooling rate, was made possible using millimeter-wave sintering (mS) technique. The overall sintering time of the process is less than 18 minutes, and the varistor characteristics obtained are = 38, J _L = 5.55 × 10^–6 A/cm² and V _bk = 600 V/mm, whereas the intrinsic parameters of the materials are _b = 2.84 eV, N _d = 1.85 × 10²⁴ m^–3 and N _s = 7.02 × 10¹¹ cm^–2. By contrast, conventional sintering (cS) process needs higher sintering temperature (1200°C), longer soaking time (60 min) and slower ramping rate (30°C/min) to obtain ZnO materials with the same marvelous nonlinear properties as those prepared by mS-process. Moreover, millimeter-wave sintering (24 GHz, mS) process enhances the densification kinetics and grain growth behavior more efficiently than the microwave sintering (2.45 GHz, S) process, resulting in better varistor characteristics for ZnO materials. However, sintering by millimeter-wave for too long period induces overfiring of the samples, which results in a density reversion phenomenon. Such a phenomenon leads to the decrease in surface state (N _s) and the potential barrier height (_b), which are presumed to be the mechanism leading to the degradation of ZnO materials' nonlinear properties.     

Fabrication and mechanical properties of silicon carbide–silicon nitride nanocomposites

A powder mixture of ultrafine –SiC–35 wt% –Si₃N₄ containing 6 wt% Al₂O₃ and 4 wt% Y₂O₃ as sintering additives were liquid–phase sintered at 1800°C for 30 min by hot–pressing. The hot–pressed composites were subsequently annealed at 1920°C under nitrogen–gas–pressure to enhance grain growth. The average grain–size of the sintered bodies were ranged from 96 to 251 nm for SiC and from 202 to 407 nm for Si₃N₄, which were much finer than those of ordinary sintered SiC–Si₃N₄ composites. Both strength and fracture toughness of fine–grained SiC–Si₃N₄ composites increased with increasing grain size. Such results suggested that a small amount of grain growth in the fine–grained region (250 nm for SiC and 400 nm for Si₃N₄) was beneficial for mechanical properties of the composites. The room–temperature flexural strength and fracture toughness of the 8–h annealed composites were 698 MPa and 4.7 MPa · m^1/2, respectively.     

Infra-red transmitting glass-ceramics of the As-Ge-Se system nucleated by zirconium selenide

The aim of this study was to produce an infra-red transmitting chalcogenide glass-ceramic nucleated by an appropriate nucleant. In order to crystallize chalcogenide glasses in a controlled manner a preliminary series of experiments, in which the metals of group IVB of the periodic table of elements were used as nucleants, showed that zirconium is an effective additive. Studies of glass compositionsxZrSe₂-(100–x) [As_0.1Ge_0.3Se_0.6] indicated that for values ofx 0.25 and 0.50 mol%, glass-ceramics with high thermomechanical properties were produced. By heat treating at 400° C for 15 h, the glass As_0.1Ge_0.3Se_0.6 with 0.25 mol% ZrSe₂ was transformed into a glass-ceramic with a fracture toughness of 0.848 MN m^–3/2, a sag point of 530° C and satisfactory infra-red transmission. X-ray diffraction analysis revealed that the precipitated crystal phase was germanium selenide (GeSe₂). The parasite absorption band with a maximum around 800cm^–1 generally present in the infra-red spectra was eliminated by adding 0.1 wt% metallic zirconium. The infra-red transmision and thermomechanical properties for glass-ceramics of compositions with 0.50 mol% ZrSe₂ were poorer than for those with 0.25 mol% ZrSe₂, for the same heat treatment condtions. The kinetics and activation energies of crystallization were studied by means of electron microscopy. The mechanism of controlled crystallization proceeds by the precipitation of crystalline GeSe₂ on the ZrSe₂ nuclei formed by heat treatment of the initial chalcogenide glass supersaturated in ZrSe₂.