Crystallization of R2O–MgO–Al2O3–B2O3–SiO2–F (R=K+, Na+) glasses with different fluorine source

The effect of fluorine source on the crystallization behaviors of the R₂O–MgO–Al₂O₃–B₂O₃–SiO₂–F (R=K⁺, Na⁺) glasses is studied by differential thermal analysis (DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM) and Raman spectrum. Two glasses with the same composition were prepared by using NaF and MgF₂ as the source of fluorine, respectively. The glass with the fluorine source of NaF is quite stable during heat-treatment. Only large dendrite crystals form on the surface of this glass, while the interior of the glass still remains amorphous. In contrast, the glass with the fluorine source of MgF₂ exhibits uniform bulk crystallization and forms mica-containing glass-ceramics after heat-treatment. The structure difference between these two glasses was investigated by Raman spectrum. The glass with the fluorine source of NaF shows a simple Raman spectrum. Boroxol rings are found to be the dominant unit in this glass. In contrast, multiple peaks appear on the Raman spectrum of the glass with the fluorine source of MgF₂, and these peaks are resolved to the vibrations of three- or four-coordinated boron units, such as chain-type metaborate groups, pentaborate groups, diborate groups, highly changed orthoborate groups, pyroborate groups, as well as boroxol rings. The stability of the glass with the NaF fluorine source is attributed to the high stability of the boroxol rings in this glass.     

Macroliquation in the system Al2O3SiO2

Presence of droplets of one glass in another, both often partially crystallised, was detected in the high-silica region of the system alumina-silica. Cristobalite and mullite were on the other hand found to coexist with the liquids, suggesting that perhaps metastable equilibrium or even non-equilibrium conditions prevailed. It was concluded that if a simple binary cristobalite-mullite eutectic does exist at all in this system, its temperature might be in the region of 1675°C, as suggested by Welch (1960).     

Phase transformation in the Al2O3–ZrO2 system

Co-precipitation methods have been used to produce 20 mol% Al2O3–80 mol% ZrO2 mixed oxides, from aqueous solutions of zirconium oxychloride and aluminium chloride, followed by precipitation with ammonia. The resulting gel was calcined at increasing temperatures, and X-ray diffraction confirmed that the structure remained amorphous up to 750°C and then crystallized as a single-phase cubic zirconia solid solution, but with a reduced unit-cell dimension. At higher temperatures, the unit-cell dimension increased and, above 950°C, this phase started to transform to a tetragonal zirconia (t-ZrO2) phase, again of reduced cell dimensions compared with t-ZrO2, with simultaneous appearance of small amounts of -Al2O3. Above 1100°C, the tetragonal phase transformed to monoclinic zirconia on cooling, and the amount of -Al2O3 increased. Above 1200°C, the -Al2O3 transformed to the stable -Al2O3. These results confirm that aluminium acts as a stabilizing cation for zirconia up to temperatures of about 1100°C. © 1998 Chapman & Hall     

Phase relationships in the quasi-ternary LaO1.5–SiO2–MgO system at 1773 K

Phase relationships in the LaO_1.5–SiO₂–MgO quasi-ternary system at 1773 K were investigated by powder x-ray diffraction (XRD) analysis applying single- and multiple-phase Rietveld methods. Most of the formed phases satisfied the Gibbs’ phase rule, except for the samples containing LaO_1.5 and a liquid phase at 1773 K. The detection of segregated MgO phases was difficult in the XRD profiles of the compositional samples around the oxyapatite single phase because the MgO peaks were weak and heavily overlapped by peaks from the oxyapatite and La(OH)₃ phases. The solid solubility limit of MgO in oxyapatite was determined not only from the chemical composition of the oxyapatite phase, which was confirmed by XRD, but also from several phase boundary compositions among the two-phase and three-phase regions based on the Gibbs’ phase rule. Formation of a liquid phase at 1773 K was observed in a wide range of compositions and considered when constructing the phase diagram.     

Phase relationships in the quasi-ternary LaO1.5–SiO2–MgO system at 1773 K

Abstract

Phase relationships in the LaO_1.5–SiO₂–MgO quasi-ternary system at 1773 K were investigated by powder x-ray diffraction (XRD) analysis applying single- and multiple-phase Rietveld methods. Most of the formed phases satisfied the Gibbs’ phase rule, except for the samples containing LaO_1.5 and a liquid phase at 1773 K. The detection of segregated MgO phases was difficult in the XRD profiles of the compositional samples around the oxyapatite single phase because the MgO peaks were weak and heavily overlapped by peaks from the oxyapatite and La(OH)₃ phases. The solid solubility limit of MgO in oxyapatite was determined not only from the chemical composition of the oxyapatite phase, which was confirmed by XRD, but also from several phase boundary compositions among the two-phase and three-phase regions based on the Gibbs’ phase rule. Formation of a liquid phase at 1773 K was observed in a wide range of compositions and considered when constructing the phase diagram.     

Dielectric resonators with complex crystal structures in the La2O3–Al2O3–TiO2 system for microwave applications

LaTi₂Al₉O₁₉ and La₃Ti₅Al₁₅O₃₇ ceramics in the La₂O₃–Al₂O₃–TiO₂ system have been prepared by conventional solid state ceramic route. The structure and microstructure of the compositions have been studied using powder X-ray diffraction and scanning electron microscopic techniques. Laser Raman studies have been employed to understand the complex crystal structure of these compositions in molecular level. The microwave dielectric properties of the sintered ceramic compacts were measured by Hakki and Colemann post resonator and TE_01δ cavity techniques using a vector network analyzer. LaTi₂Al₉O₁₉ and La₃Ti₅Al₁₅O₃₇ ceramics possess excellent microwave dielectric properties such as relatively high unloaded quality factors 7,762 and 7,415, low dielectric constant 15.7 and 22.1 and low temperature coefficient of resonant frequency values ?22 and +18.9 ppm/°C, respectively.     

Effects of MgO on properties of Li2O–Al2O3–SiO2 glass–ceramics for LTCC applications

Li₂O–Al₂O₃–SiO₂ (LAS) glass–ceramics for low temperature co-fired ceramics (LTCC) application were prepared by melting method, and the effects of MgO on the sinterability, microstructure, dielectric property, thermal expansion coefficient (CTE) and mechanical character of this glass–ceramics have been studied. The X-ray diffraction images represent that the main phase is β-spodumene solid solutions. And some ZrO₂ and CaMgSi₂O₆ phases in LAS glass–ceramics are detected. The LAS glass–ceramics without additive (MgO) sintered at 800° had the dielectric properties: dielectric constant (ε_r) of 5.3, dielectric loss (tanδ) of 2.97 × 10^?3 at 1 MHz, CTE value of 1.06 × 10^?6 K^?1, bulk density of 2.17 g/cm³, and flexural strength of 73 MPa. 5.5 wt% MgO-added LAS glass–ceramic achieves densification at 800° exhibited excellent properties: low dielectric constant and loss (ε_r = 7.1, tanδ = 2.02 × 10^?3 at 1 MHz), low CTE (2.89 × 10^?6 K^?1), bulk density = 2.65 g/cm³ as well as high flexural strength (145 MPa). The results indicate that the addition of MgO is helpful to improve the dielectric and mechanical properties. The formation of CaMgSi₂O₆ crystal phase with higher CTE leads to the increase of CTE value of LAS glass–ceramics due to the increasing MgO content, and the increase of CTE is favourable for matching with silicon (3.1 × 10^?6 K^?1). The prepared LAS glass–ceramics have the potential for LTCC application.     

Effects of Y2O3 and La2O3 addition on the crystallization of Li2O·Al2O3·4SiO2 glass-ceramic

Effects of adding Y₂O₃ and La₂O₃ on the crystallization of -quartz solid solution (ss) and the subsequent -quartz ss to -spodumene transformation of Li₂O·Al₂O₃·4SiO₂ glass-ceramic were investigated. Adding 4 mol% YO_3/2 or 8 mol % LaO_3/2 effectively improved the control of the crystallization process of the glass. Y₂O₃ did not effectively induce bulk crystallization of -quartz ss, but can reduce the rate of surface crystallization. La₂O₃ completely suppressed the surface crystallization and promoted a uniform, bulk crystallization of -quartz ss. For both the Y₂O₃- and La₂O₃-doped glasses, the kinetics for glass crystallization to -quartz ss was delayed as the doping level increased. Except for the 8 mol % LaO_3/2-doped glass in which no -spodumene was formed, the kinetics for the -quartz ss to -spodumene transformation for the doped glasses was enhanced compared with that for the undoped glass. For the 4 and 8 mol % YO_3/2-doped compositions, the relative amount of -spodumene to -quartz revealed an anomalous decrease trend with heating temperature in a particular temperature range. This can be explained by the surface crystallization characteristic, which induced an overlap of crystallization and -quartz ss to -spodumene transformation. Glass doped with 8 mol % LaO_3/2 exhibited an Avrami exponent of about 2.4 and an activation energy for crystal growth of -quartz ss of about 418 kJ mol^–1.     

Effect of MgO contents on the mechanical properties and biological performances of bioceramics in the MgO–CaO–SiO2 system

The aim of this research was to investigate the effect of the chemical composition on the mechanical properties, bioactivity, and cytocompatibility in vitro of bioceramics in the MgO–CaO–SiO₂ system. Three single-phase ceramics (merwinite, akermanite and monticellite ceramics) with different MgO contents were fabricated. The mechanical properties were tested by an electronic universal machine, while the bioactivity in vitro of the ceramics was detected by investigating the bone-like apatite-formation ability in simulated body fluid (SBF), and the cytocompatibility was evaluated through osteoblast proliferation and adhesion assay. The results showed that their mechanical properties were improved from merwinite to akermanite and monticellite ceramics with the increase of MgO contents, whereas the apatite-formation ability in SBF and cell proliferation decreased. Furthermore, osteoblasts could adhere, spread and proliferate on these ceramic wafers. Finally, the elongated appearance and minor filopodia of cells on merwinite ceramic were more obvious than the other two ceramics.     

Zeolite synthesis in the system pyrrolidine-Na2OAl2O3SiO2H2O

Crystallization of zeolites from the system pyrrolidine-Na₂OAl₂O₃SiO₂H₂O was investigated. Five zeolites, ZSM—5, ZSM—35, ZSM—39, ZSM—48 and KZ—1, were synthesized as single phases, whose crystallinity was very high. In this work zeolite ZSM—48 was prepared from the pyrrolidine system for the first time. It was suggested that pyrrolidine might stabilize certain aluminosilicate polyanions which have building units of oxygen five-membered rings. It was also presumed that all zeolites, except ZSM—39, have a pore opening of oxygen ten-membered rings.     

Low temperature sintering and characteristics of K2O–B2O3–SiO2–Al2O3 glass/ceramic composites for LTCC applications

The K₂O–B₂O₃–SiO₂, K₂O–B₂O₃–SiO₂–2 %Al₂O₃, K₂O–B₂O₃–SiO₂–4 %Al₂O₃ glasses with different Al₂O₃ content were prepared. Different proportions (50, 55, 60, 65, 70 %) of the three glasses were respectively mixed with alumina ceramic-filler, then the mechanical and dielectric properties were investigated. The results showed K₂O–B₂O₃–SiO₂–2 %Al₂O₃ glass/alumina filler (glass:alumina = 60:40) had the excellent comprehensive properties, so further study was continued with part of alumina ceramic-filler replaced by the silica ceramic-filler on this composite. Then the X-ray diffraction analysis revealed that the alumina and silica fillers existed as the crystal phase, and the densification was seriously damaged when the silica content reached to three quarters of the fillers. With the increase of the silica-filler, the composites’ density and dielectric constant exhibited uniform decrease, but thermal expansion coefficient (TEC) uniformly increased. When the glass:alumina:silica was equal to 60:30:10, a best composite property was presented as a bulk density of 2.582 (g cm^?1), a dielectric constant of 6.1 and a dielectric loss of 2 × 10^?3 at 1 MHz, a flexural strength of 168 MPa, and a TEC of 8.62 × 10^?6 °C^?1.     

High strength glass–ceramics in the system MgO/Y2O3/Al2O3/SiO2/ZrO2 without quartz as crystalline phase

This paper is the first report on high strength glass–ceramics which mainly contain zirconia as crystalline phase. This is in contrast to previous reports on high strength glass–ceramics which contain low quartz as the crystalline phase. Glasses in the system MgO/Y₂O₃/Al₂O₃/SiO₂ with zirconia as nucleating agent were melted. The samples were crystallised in a first step at 950 °C for different periods of time and in the second step at 1060 °C for 1 h. The prepared glasses and glass–ceramics were characterised using differential thermal analysis, dilatometry, X-ray diffraction, and scanning electron microscopy. As shown by X-ray diffraction after the first step in every analysed composition, zirconia was precipitated. With increasing crystallisation time and temperature and depending on the composition, the other phases such as β- or α-quartz solid solutions, spinel and indialite were precipitated. With higher yttria concentrations and increasing crystallisation temperatures during the second crystallisation step, the main crystalline phase was zirconia whereas the quartz solid solution was no longer observed. Bending strengths up to 450 MPa and Young′s moduli up to 115 GPa were obtained after the two subsequent crystallisation steps. Higher yttria concentrations resulted in an increase in density and microhardness. The maximum Vickers hardness was 10.5 GPa.     

On the behaviour of vacancies in quenched AlAl2O3 alloys

Thin foils of SAP, an alloy of Al with dispersed Al₂O₃-particles, were quenched from temperatures above 500°C into brine at −20°C and subsequently examined by transmission electron microscopy. It was found that vacancy loops can be obtained by quenching SAP if the alloy is quenched from high temperatures with a high quenching rate and if the interparticle spacing is not too small. The “interstitial strain” field normally formed around the particles by differential thermal contraction effects during cooling was not observed after quenching from temperatures close to the melting point. It is suggested that the strain field around the particles acts as a sink for vacancies and the strain is relieved by the high excess vacancy concentration. During quenching from lower temperatures, on the other hand, the concentra tion and the mobility of the excess vacancies appear not to be sufficiently high to prevent the rapid set-up of the strain field which becomes large enough around the bigger particles to generate dislocations.     

The K2O·Al2O3-Al2O3 system

The phase relationships in the system K₂O · AL₂O₃-Al₂O₃ between 1200 and 1700° C have been experimentally established. The homogeneity range of potassium -alumina is limited by the 83 and 91 mol % Al₂O₃ compositions. The eutectic point between the K₂O-Al₂O₃ and -alumina was found to be at 1450° at about 62 mol % Al₂O₃ composition. An X-ray diffraction pattern analysis of potassium -alumina is shown.     

Influence of Al2O3/SiO2 ratio on the microstructure and properties of low temperature co-fired CaO–Al2O3–SiO2 based ceramics

In this work, in order to obtain the materials for low temperature co-fired ceramics applications, CaO–Al₂O₃–SiO₂ (CAS) based ceramics were synthesized at a low sintering temperature of 900 °C. The influences of Al₂O₃/SiO₂ ratio on the microstructure, mechanical, electrical and thermal properties were studied. According to the X-ray diffractomer and scanning electron microscopy results, the addition of the Al₂O₃ is advantageous for the formation of the desired materials. Anorthite(CaAl₂Si₂O₈) is the major crystal phase of the ceramics, and the SiO₂ phase is identified as the secondary crystal phase. No new crystal phase appears in the ceramics with the increasing Al₂O₃ content. More or less Al₂O₃ addition would all worsen the sintering, mechanical and dielectric properties of CAS based ceramics. The ceramic specimen (Al₂O₃/SiO₂ = 20/18.5) sintered at 900 °C shows good properties: high bending strength = 145 MPa, low dielectric constant = 5.8, low dielectric loss = 1.3 × 10^?3 and low coefficient of thermal expansion value = 5.3 × 10^?6 K^?1. The results indicate that the prepared CAS based ceramic is one of the candidates for low temperature co-fired ceramic applications.     

Solid state reactions in the ZrO2 · SiO2 - αAl2O3 system

Solid state reactions between ZrO₂· SiO₂ and Al₂O₃ in mixed powders were studied by quantitative X-ray diffraction, density measurements and qualitative EDAX. Data were obtained at temperatures ranging from 1400 to 1600° C for 5 h; the initial molar ratios of the reactants (Al₂O₃/ZrO₂ · SiO₂) varying from 0 to 5. The results indicate that: (1) ZrO₂· SiO₂ and Al₂O₃ react and form ZrO₂, crystalline 3Al₂O₃ · 2SiO₂ and a noncrystalline mullite phase; (2) the non-crystalline mullite phase is an important transitional phase towards equilibrium under subsolidus conditions. In the experimental conditions used the amount of the non-crystalline phase varies by as much as about 15%. This phase is of great importance in the mechanisms of reaction sintering between ZrO₂ · SiO₂ and Al₂O₃.     

Fabrication and properties of SiC fibre-reinforced Li2O·Al2O3·6SiO2 glass-ceramic composites

Ta₂O₅, Nb₂O₅ and TiO₂ were used separately as additives to a Li₂O·Al₂O₃·6SiO₂ glass-ceramic composition, to act as nucleating dopants and to aid the formation of an interfacial carbide layer (TaC and NbC) between the fibre and matrix in SiC fibre uniaxially reinforced glass-ceramic composites, The composites exhibited high modulus of rupture (>800 MPa) and fracture toughness (K _IC > 15 MPam^1/2). The interfacial amorphous carbon rich layer and carbide layer were responsible for lowered interfacial shear strength but permitted high composite fracture toughness. The composite with the TiO₂ additive in the matrix showed a lower flexural strength (<500MPa) and a smaller K _IC (-11 MPam^1/2) which resulted from the high interfacial shear strength between the SiC fibre and the matrix due to the formation of the interfacial TiC layer.     

Effects of Li2O3–B2O3–SiO2 addition on dielectric properties and microstructure of MgO–TiO2–ZnO–CaO ceramics

The effect of Li₂O₃–B₂O₃–SiO₂ (LBS) liquid-phase additives on the sintering, microstructures, and dielectric properties of MgO–TiO₂–ZnO–CaO (MTZC) ceramics was investigated. It was found that the sintering temperature could be lowered easily, and the dielectric properties of MTZC ceramics could be greatly improved by adding a small amount of LBS solution additives. With the addition of 10 wt% LBS, the ceramics sintered at 900 °C showed favorable dielectric properties with ε_r = 21.7, Qf = 5.0 × 10⁴ GHz, and TCF = ?21.6 ppm/ °C. The distructive physical analysis showed an excellent co-firing interfacial behavior between the MTZC ceramic and the Ag electrode. It indicated that MTZC ceramics with LBS solution additives have a number of potential applications on passive integrated devices based on the low-temperature co-fired ceramics technology.