Toughened glass-ceramics in the ZrO2-Al2O3-SiO2 system prepared by the sol-gel process

Monolithic glass-ceramics containing Al₂O₃ or TiO₂ were prepared in the ZrO₂-SiO₂ system by the sol-gel process from metal alkoxides. Tetragonal ZrO₂ was precipitated by heat treatment at 900–1200 °C and its crystal growth was increased by adding TiO₂ or Al₂O₃. Further heating at higher temperature resulted in the precipitation of zircon and monoclinic ZrO₂ which was transformed from tetragonal ZrO₂. The addition of Al₂O₃ had less effect on both the tetragonal-to-monoclinic ZrO₂ transformation and the precipitation of zircon. The fracture toughness increased as the size of tetragonal ZrO₂ particles increased and then decreased with the appearance of monoclinic ZrO₂ or zircon. The fracture toughness of the glass-ceramics was measured in the glass-forming regions of the ZrO₂-Al₂O₃-SiO₂ system. The fracture toughness was sensitively dependent on both Al₂O₃ and ZrO₂ content, of which the highest value achieved was 9 MPa m^1/2 for the 50ZrO₂·10Al₂O₃·40SiO₂ composition.     

Al2O3-Ta2O5-rich glass-ceramic with interconnected pores and its sintering

Substitution of SiO₂ in the ternary sodium borosilicate system with Al₂O₃ plus Ta₂O₅ was found to produce glass which decomposed into microphases and/or crystallized after heat treatment. At least one of the phases present was water soluble. The structure of the material was glassy with the presence of a small crystalline content. Crystalline forms found during powder X-ray diffraction analysis of heat treated, leached and then sintered materials were orthorhombic NaTaO₃ plus AIBO₃, orthorhombic NaTaO₃ and orthorhombic Na₂O · 4Ta₂O₅ plus rhombic 9AI₂O₃ · 2B₂O₃, respectively. The specific surface areas of the leached materials ranged between 96 and 304 m²g^–1, while the mean pore radii of interconnected pores were calculated to be between 2.0 and 8.4 nm. A sintering rate of between 1520 and 1580° C for 5 min were estimated from void volume and bulk density measurements.     

The crystallization of Gd2 3 -Al2O3 glasses

Glasses of Gd₂O₃ · x Al₂O₃ compositions where x represents 5/3, 4 and 6, were prepared using a rapid quenching apparatus and a laser beam. The crystallization process of the glasses was studied by means of differential thermal analysis (DTA), X-ray diffraction analysis and electron microscopy. The crystallizations of Gd₂O₃ · 5/3Al₂O₃, Gd₂O₃ · 4Al₂O₃ and Gd₂O₃ · 6Al₂O₃ are complex and exhibit one, two and three exothermic peaks in DTA measurement with increasing Al₂O₃ concentration, respectively. The crystallization process of Gd₂O₃ · 5/3Al₂O₃ glass involved the direct formation of the gadolinium aluminium garnet, 3Gd₂O₃ · 5l_{2 3} (GdAG), which is not obtained by the ordinary solid phase reaction. After crystallization of Gd₂O₃ · 4Al₂O₃ and Gd₂O₃ · l_{2 3} glass, both phases become a mixture of Gd₂O₃ · Al₂O₃ (perovskite type) and -Al₂O₃.     

Interfacial phenomenology of SiC fibre reinforced Li2O·Al2O3·6SiO2 glass-ceramic composites

The microstructures of SiC fibre-reinforced Li₂O·Al₂O₃·6SiO₂ glass-ceramic composites with Ta₂O₅, Nb₂O₅, TiO₂ and ZrO₂ dopants were investigated. An amorphous carbon-rich layer, from 100–170 nm thick, was observed in the interfacial region between fibre and matrix. A second interfacial layer of TaC, NbC, or TiC precipitates, appeared adjacent to the C-rich layer. Low bond strength between these two interfacial layers resulted in low interfacial shear strength, and this in turn led to an increase in toughness of the composites containing 4 mol% Ta₂O₅ or Nb₂O₅ dopant. 2 mol% Ta₂O₅ dopant in this composite acted as a nucleating agent for the matrix but was not adequate to form an appreciable volume of TaC particles in the interfacial region, hence a flexural strength decrease was observed. The composite containing TiO₂ dopant exhibited low flexural strength and fracture toughness resulting from the formation of a TiC layer which had a larger coherent bond strength with the interfacial C-rich layer, and attacked the structural integrity of the fibres.     

Structure of plasma-sprayed oxides in the MgO-Al2O3-SiO2 system

Cordierite (2MgO · 2Al₂O₃-SiO₂), forsterite (2MgO · SiO₂) and spinel (MgO · Al₂O₃) were rapidly solidified into a thick film through plasma spraying. The crystal structure of the assprayed deposits was examined using X-ray diffractometry and transmission electron microscopy, and shows that as-sprayed cordierite is amorphous, as-sprayed spinel is a crystalline phase, and as-sprayed forsterite is a mixture of amorphous and crystalline phases. Moreover, transmission electron microscopy reveals uniform amorphous cordierite, a nonuniform grainsize distribution of crystalline spinel and a twinned texture of as-sprayed forsterite.     

Enhanced dielectric properties of modified Ta2O5 thin films

 Tantalum oxide (Ta₂O₅) is a promising high dielectric constant material for the DRAM applications because of its ease of integration compared to other complex oxide dielectrics. The dielectric constant and thermal stability characteristics of bulk Ta₂O₅ samples were reported to enhance significantly through small substitutions of Al₂O₃. However, this improvement in the dielectric constant of (1-x)Ta₂O₅-xAl₂O₃ is not clearly understood. The present research attempts to explain the higher dielectric constant of (1-x)Ta₂O₅-xAl₂O₃ by fabricating thin films with enhanced dielectric properties. A higher dielectric constant of 42.8 was obtained for 0.9Ta₂O₅–0.1Al₂O₃ thin films compared to that reported for pure Ta₂O₅ (25–30). This increase was shown to be closely related to a-axis orientation. Pure Ta₂O₅ thin films with similar a-axis orientation also exhibited a high dielectric constant of 51.7, thus confirming the orientation effect. The leakage current properties and the reliability characteristics were also found to be improved with Al₂O₃ addition. Received: 24 November 1998 / Reviewed and accepted: 7 December 1998     

The phase equilibrium diagram of the ternary subsystem CaO-CaO·Al2O3-11 CaO·7 Al2O3·CaF2

On the basis of the experimental results obtained from the study of high-temperature equilibrium relations of seven pertinent joins the phase diagram of the subsystem CaO-CaO·Al₂O₃-11 CaO·7 Al₂O₃·CaF₂ has been constructed. In this diagram the delineation of the boundary curves of the primary fields of CaO, 3 CaO·Al₂O₃, CaO·Al₂O₃ and the 11 CaO·7 Al₂O₃·CaF₂ solid solution has been improved over our previously published phase diagram of the system CaO-Al₂O₃-CaF₂ [1]. The isotherms have also been drawn more precisely to give a better idea about the topography of the portion investigated.     

Structural changes of glass-ceramics of the Cu2O-Al2O3-SiO2 system on heating in air

The behaviour of copper ions in low thermal expansion glass-ceramics prepared from Cu₂O · Al₂O₃ ·_nSiO₂ glasses, or in-spodumene type Cu₂O · Ae₂O₃ ·nSiO₂ crystals included in the glass-ceramics on heating in air was investigated. On hewing at 300 to 500° C, the copper ions behaved as in the corresponding glasses. Cuprous ions in the glass-ceramics or-spondumene type crystals were oxidized into the cupric state, and at the same time an equal amount of cuprous ions to those oxidized were expelled out of the specimen for the requirement of electrical charge neutrality and then reacted with oxygen to form CuO on the surface. The oxidation of cuprous ions and the decrease of the copper content of the-spondumene type crystals brought about a considerable decrease in the lattice spooings.     

Characterization and sintering of a porous glass-ceramic in the system Na2O-B2O3-Ta2O5

Substitution of SiO₂ in the ternary sodium borosilicate system with Ta₂O₅ was found to produce glasses, which after heat treatment separated into immiscible microphases, one of which was water soluble. The structure of the leached material after heat treatment was a well developed low temperature form of Ta₂O₅. After firing at temperatures between 1100 and 1550°C X-ray diffraction analysis showed the presence of low and high temperature forms of Ta₂O₅ and of orthorhombic Na₂Ta₅O₂₁. The high solubility of up to 40 wt% Ta₂O₅ in the sodium-borate matrix resulting in clear glasses is of practical interest. The specific surface areas of the leached materials ranged between 5.54 and 35.57 m²g⁻¹ while in an additionally Al₂O₃ doped material the value of 307 m²g⁻¹ was measured. Mean pore radii of interconnected pores were calculated to be between 18.63 to 41.12 nm in the Ta₂O₅-rich materials while the additional Al₂O₃ doping decreased the value to 2.71 nm. A sintering temperature between 1500 and 1550°C is estimated from void volume measurements after a series of firing steps at temperatures between 1100 and 1550°C were undertaken.     

Electrical characteristics of SrxBi2.4Ta2O9 thin film and Pt/Sr0.85Bi2.4Ta2O9/Al2O3/Si structure

Sr _x Bi_2.4Ta₂O₉ (0.7 x 1.3) thin films were processed by metalorganic decomposition and their ferroelectric characteristics were investigated. The Sr-deficient Sr _x Bi_2.4Ta₂O₉ films exhibited well-developed ferroelectric hysteresis curves compared to those of the Sr-excess films, and Sr_0.85Bi_2.4Ta₂O₉ film had the optimum electrical characteristics among Sr _x Bi_2.4Ta₂O₉ films. Electrical characteristics of the Pt/SBT/Al₂O₃/Si structure using Sr_0.85Bi_2.4Ta₂O₉(SBT) film were investigated for metalferroelectric-insulator-semiconductor field-effect-transistor (MFIS-FET) applications. Memory window of C-V hysteresis characteristics of the Pt/SBT/Al₂O₃/Si structure became large with decreasing the Al₂O₃ thickness, and the Pt/SBT(400 nm)/Al₂O₃ (10 nm)/Si structure gave memory window of 2.2 V at sweeping voltages of ±5 V. The Pt/SBT/Al₂O₃/Si structure can be proposed for MFIS-FET applications.     

Dielectric characteristics of composite ceramics in the Ba(Mg1/3Ta2/3)O3-BaO · Nd2O3 · 5TiO2 system

Dielectric characteristics of composite ceramics in the system Ba(Mg_1/3Ta_2/3)O₃-BaO · Nd₂O₃ · 5TiO₂ were investigated to search for a new candidate system for microwave dielectric ceramics with modifiable dielectric constant, low dielectric loss and small temperature dependence. The dielectric constant could be adjusted in the range 25–81 by controlling the concentration of BaO · Nd₂O₃ · 5TiO₂, while the dielectric loss remained of the order of 10^–4 for some compositions. Moreover, the dielectric properties in the present system could be significantly improved by post-densification thermal treatment.     

Sintering and compensation effect of donor and acceptor codoped 3 mol% Y2O3-ZrO2

Addition of 0.15–0.5 mol% acceptor oxide, Al₂O₃, to 3 mol% Y₂O₃-ZrO₂ results in enhanced densification at 1350°C. The enhancement is accounted for by a liquid phase sintering mechanism. While the addition of donor oxide, Ta₂O₅, of 0.15–2.5 mol% at 1300–1600°C results in the decrease of final density and in the destabilization of the tetragonal (t) phase of the 3 mol% Y₂O₃-t-ZrO₂ (TZP). X-ray diffractometry (XRD) reveals that the Ta₂O₅-added 3 mol% Y₂O₃-ZrO₂ contains monoclinic (m) ZrO₂ phase and a second Ta₂Zr₆O₁₇ phase. The decrease is attributed to the increase of m-ZrO₂ content in these samples. Complete phase transformation from t-ZrO₂ to m-ZrO₂ observed in samples added with 2.5 mol% Ta₂O₅ is interpreted by the compensation effect based on donor and acceptor codoping defect chemistry.     

Sintering and crystallization of (SrO·SiO2)-(SrO·Al2O3·2SiO2) glass-ceramics

In the ternary SrO-Al₂O₃-SiO₂ system the pseudobinary join composition of 50 wt % SrO·SiO₂–50 wt % SrO·Al₂O₃·2SiO₂ (SS-SA2S) showed a glass melting temperature of 1500 °C and a crystallization peak temperature of 1100 °C. The (SS-SA2S) glass-ceramic pellets prepared by cold pressing and pressureless sintering, showed very low porosity. The (SS-SA2S) glass-ceramics containing B₂O₃ and those containing B₂O₃ and TiO₂ revealed crystallization peak temperatures of 1000 °C and unexpectedly high porosity. By applying Kissinger analyses to the DTA data the activation energy values for crystallization of the three glass-ceramics were determined to range from 196 to 255 kJ/mol. The Ozawa analyses on the DTA data gave the Avrami parameter values at 3.69 to 3.95. The X-ray diffraction (XRD) patterns from the three glass-ceramics revealed formation of the equilibrium crystalline phases of SrO·SiO₂ and SrO·Al₂O₃·2SiO₂ (monocelsian).     

Sintering and compensation effect of donor- and acceptor-codoped 3mol% Y2O3-ZrO2

Addition of 0.15–0.5 mol% acceptor oxide, Al₂O₃, to 3 mol% Y₂O₃-ZrO₂ results in enhanced densification at 1350 °C. The enhancement is accounted for by a liquid phase sintering mechanism. The addition of donor oxide, Ta₂O₅, of 0.15–2.5 mol % at 1300–1600 °C results in the destabilization of tetragonal (t-) phase and the decrease of final density in 3 mol% Y₂O₃-TZP (tetragonal ZrO₂ polycrystals). X-ray diffractometry (XRD) reveals that the Ta₂O₅-added 3 mol% Y₂O₃-ZrO₂ contains monoclinic (m-) ZrO₂ and a second phase of Ta₂Zr₆O₁₇. The decreasing in final density is attributed to the increase of m-ZrO₂ content. Complete destabilization of t-ZrO₂ to m-ZrO₂ in samples added with 2.5 mol% Ta₂O₅ is interpreted by the compensation effect based on donor- and acceptor-codoping defect chemistry.     

Fabrication and Dielectric Properties of Multilayer Ta2O5/Al2O3 Nanostructures

Multilayer Ta₂O₅/Al₂O₃ nanostructures are produced on Si(100) and Al by molecular layering, and their dielectric properties are investigated. The mechanisms of layer growth are discussed in terms of surface reactions.     

Compositional dependence of bioactivity of glasses in the system CaO-SiO2-Al2O3: itsin vitro evaluation

In order to investigate fundamentally the effect of Al₂O₃ on the bioactivity of glasses and glass-ceramics, the compositional dependence of bioactivity of glasses in the system CaO-SiO₂-Al₂O₃ was studiedin vitro. It is already known that the essential condition for glasses and glass-ceramics to bond to living bone is the formation of an apatite layer on their surfaces in the body, and that the surface apatite layer can be reproduced even in an acellular simulated body fluid which has almost equal ion concentrations to those of the human blood plasma. In the present study, bioactivity of the glasses was evaluated by examining apatite formation on their surfaces in the simulated body fluid with thin-film X-ray diffraction, Fourier transform infrared reflection spectroscopy and scanning electron microscopic observation. Only CaO-SiO₂-Al₂O₃ glasses containing Al₂O₃ less than 1.5 mol % formed the surface apatite as well as Al₂O₃-free CaO-SiO₂ glasses, but CaO-SiO₂-Al₂O₃ containing Al₂O₃ more than 1.7 mol % did not form it as well as an SiO₂-free CaO-Al₂O₃ glass. This indicates that only a small amount of addition of Al₂O₃ to glass compositions suppresses the bioactivity of glasses and glass-ceramics by suppressing apatite formation on their surfaces in the body.     

Preparation, sintering and fracture behavior of Al2O3/LaAl11O18 ceramic composites

Al₂O₃/25 vol% LaAl₁₁O₁₈ composites were prepared by pressureless sintering at 1550°C with composite powders obtained by copercipiated method using La(NO₃) · 6H₂O and Al(NO₃)₃ · 9H₂O as starting materials. The enhanced reactive activity of Al₂O₃ and chemically homogeneous mixing of the constituents made LaAl₁₁O₁₈ phase to be formed at low temperature in composite powders. AlF₃ additive was used to reduce the transformation temperature of transition alumina. The LaAl₁₁O₁₈ grains in the composite powder obtained at 1500°C showed rodlike morphology distributed homogeneously in Al₂O₃ powder. The samples sintered at 1550°C for 4 h with CAS (CaO-Al₂O₃-SiO₂) sintering aid can obtain a high relative density. The effects of the sintering time on the grain growth of Al₂O₃ and the fracture toughness of the composites were studied and the results showed that LaAl₁₁O₁₈ grains reduced the growth of Al₂O₃ grains and the rodlike grains increased the fracture toughness. The improvement in fracture toughness of the composites was mainly attributed to the mechanism of crack deflection.     

Energetic electronic processes and negative resistance in amorphous TaTa2O5Au and AlAl2O3Au diodes

Voltage-controlled negative resistance (VCNR) can be established in metal-insulator-metal structures by applying a potential to the diode, which is usually in vacuum. Light emission due to electroluminescence (E.L.), and electron emission into vacuum, accompany the formation of conductivity; these energetic electronic phenomena are closely related to the “forming” of VCNR and to the resultant current-voltage characteristics. For Al₂O₃ diodes with impure oxides, VCNR forms at constant voltage, independent of oxide thickness; for clean oxides, forming depends on field and on the metal counterelectrode. The intensity and energy distribution of light emitted from TaTa₂O₅Au diodes have been measured, and are compared to E.L. from AlAl₂O₃Au diodes. The voltage threshold for the appearance of E.L. in TaTa₂O₅Au diodes is 1.2 V, for AlAl₂O₃Au it is 1.4 V. The respective voltages for maximum current, V_m, are 1.9 V and 2.8 V. In Al₂O₃, the E.L. spectrum covers the visible range with peaks at 1.8 eV, 2.3 eV, and 4.0 eV. For Ta₂O₅, the E.L. intensity is constant over most of the visible, but has a maximum between 1.6 eV and 1.8 eV which is also the energy of E.L. of TaTa₂O₅Au diodes before the establishment of VCNR. For both Al₂O₃ and Ta₂O₅ diodes, electron emission into vacuum is anomalous since emission is detectable at diode voltages of ∼2 V. Electron emission in the two insulators, though qualitatively similar, shows differences that depend on differences in trapping levels and band gaps of the insulators.     

Electrical characteristics of Ti-O/Ta2O5 thin film sputtered on Ta/Ti/Al2O3 substrate

The electrical characteristics of Ti-O/Ta₂O₅ films sputtered on Ta/Ti/Al₂O₃ substrate were investigated. Ta (tantalum) was used for the bottom and upper electrodes for the purpose of simplifying the fabrication process and Al₂O₃ substrates were used, which are needed in integral passive devices. Ta/Ti-O/Ta₂O₅/Ta/Ti/Al₂O₃ capacitors were annealed at 700 °C for 60 s in vacuum. The X-ray diffraction pattern (XRD) results showed that as-deposited Ta had a highly preferred orientation, but Ta₂O₅ film had amorphous structure, which was transformed to crystallization structure by rapid thermal heat treatment. We examined the log J-E and C-V characteristics of the dielectric thin films deposited on the Ta bottom electrode. From these results, we concluded that the leakage current could be reduced by introducing a Ti-O buffer layer. The conduction mechanisms of Ta/Ti-O/Ta₂O₅/Ta/Ti/Al₂O₃ capacitors could be interpreted appropriately by hopping conduction in lower field (E<1×10⁵ V/cm) and space-charge-limited current in higher fields (1×10⁵ V/cm<E).     

The effect of interfacial mixing on Ag/Al2O3 by N+ implantation

The Ag film/Al₂O₃ implanted with an N ion energy of 110 keV. The ion-induced interfacial mixing were examined using AES, STD and XPS. The frictional coefficient of implanted Ag/Al₂O₃ in air was determined by Drive friction precise measuring apparatus (DFPM). The influence of N⁺ implantation on the interfacial chemistry and adhesion of Ag films on Al₂O₃ substrates was examined, compounds formed by introducing a thin layer between the Ag and the Al₂O₃ at 1 × 10¹⁷ N·cm^–2. This investigation resulted in extensive interfacial grading, and new chemical bonding across the Ag/Al₂O₃ interfaces. The case for the Ag/Al₂O₃ reaction lead to the metal ceramic of 13Al₂O₃·AIN and -AgAlO₂ formation though non equilibrium processes of implantation, and friction decreased in initial cycle where ion implantation of lower vacuum lead to surface carbon film. The combination of these effects provided an adhesion increase that was approximately 3 times that obtained in unimplanted Ag/Al₂O₃ specimens.