The effect of MgAl2O4 on the formation kinetics of Al2TiO5 from Al2O3 and TiO2 fine powders

The formation of Al_2(1–x)Mg _x Ti_(1+x)O₅ solid solutions from Al₂O₃-TiO₂-MgAl₂O₄ powder mixtures of 1 m particle size and moderate purity has been studied at 1300°C for different final composition values: x=0 (pure Al₂TiO₅), 10^–3, 10^–2 and 10^–1. Analysis of the kinetic data and microstructural observation indicates that MgAl₂O₄ affects the mechanism of Al₂TiO₅ formation by providing active nuclei for the growth of the new phase. These nuclei are probably constituted by Mg_0.5AlTi_1.5O₅, i.e. the equimolar Al₂TiO₅-MgTi₂O₅ solid solution, and are formed by reaction between MgAl₂O₄ and TiO₂ at temperatures above 1150 °C. As the value of x increases, the number of titanate particles per unit volume accordingly increases and the conversion of the original oxides is faster. At values of x10^–2, the prevailing mechanism is the nucleation and growth of Al₂TiO₅ nodules for fractional conversion up to 0.8. Further conversion of the residual Al₂O₃ and TiO₂ particles dispersed into the titanate nodules is slower and controlled by solid-state diffusion through Al₂TiO₅. At x=0.1, a large number of nucleation sites is present, and solid-state diffusion through Al₂TiO₅ becomes important even in the initial stage of reaction, as the diffusion distances are strongly reduced. The study of Al₂TiO₅ formation under non-isothermal conditions in the temperature range 1250–1550°C shows that reaction proceeds between 1300 and 1350 °C for x=0.01 and between 1250 and 1300 °C for x=0.1. Densification of the titanate becomes important at temperatures above 1300°C for x=0.1, but only above 1450 °C for x=0.01.     

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.     

MoSi2 diffusion barrier for the passivation of copper at elevated temperatures

Sputter deposited MoSi₂ coatings (200 and 400 nm thick) on copper have been studied in an attempt to prevent or at least reduce the oxidation of copper. Samples were exposed to an air ambient at temperatures ranging from 600–850 °C for up to 15 min. Sputter-deposited MoSi₂ was amorphous upon deposition and crystallized on annealing. Silicon from the MoSi₂ was found to diffuse into the copper causing the MoSi₂ to transform to lower suicides. The primary oxidation product for MoSi₂-coated samples was CuO (with small amounts of Cu₂O), which is in contrast to uncoated copper where Cu₂O is the main oxidation product. The amount of copper consumed by oxidation, for a 200 nm MoSi₂ barrier relative to uncoated copper, was reduced by 140 times at 600 °C and 30 times at 800 °C. A 400 nm MoSi₂ coating yielded an improvement of 420 times at 600°C, and 200 times at 850 °C. For the 400 nm barrier exposed to air for 15 min, this corresponds to a 35 nm CuO layer at 600°C and a 300 nm thick oxide layer at 850 °C.     

SiO2-Al2O3 metastable phase equilibrium diagram without mullite

A metastable binary phase diagram between SiO₂ (cristobalite) and-Al₂O₃ (corundum) in the absence of any mullite phase is presented. A eutectic is indicated at a temperature of 1260° C and a composition of 18 wt% ( 12 mol%) Al₂O₃. The liquidi of the proposed metastable system were positioned on the basis of the thermodynamic data calculated from the stable equilibrium diagram of Aksay and Pask [2]. Experimental evidence is also presented. A SiO₂-Al₂O₃ melt containing 80 wt% Al₂O₃ cooled at a slow rate in sealed molybdenum crucibles shows crystalline Al₂O₃ plus a glass phase whose composition followed the calculated extension of the stable Al₂O₃ liquidus to lower temperatures. Compacts of cristobalite—corundum mixtures were fired at subsolidus temperatures to estimate the eutectic temperature experimentally. The proposed metastable phase diagram effectively explains the formation of non-crystalline phases in subsolidus reactions, and microstructure obtained on solidification of high alumina melts.     

Preparation and study of YSTZ-Al2O3 nanocomposites

Yttria stabilized zirconia-alumina (YSTZ-Al₂O₃) nanocomposite system with various Al₂O₃ concentrations has been synthesized by sol-gel route. The experimental techniques XRD, DTA, TGA, FT-Raman, FT-IR, SEM, Vickers hardness measurements, density measurements and Impedance spectroscopy were used to characterize the synthesized specimens. DTA result shows two exothermic reactions: one around 760°C and another around 960°C. XRD results confirm that the specimen starts to crystallize on heating above 750°C. Well resolved XRD reflections corresponding to tetragonal (t) ZrO₂ were obtained after the specimens were heated at 1000°C. FT-Raman results confirmed that the crystallites developed above 750°C was t-ZrO₂. It was observed from the XRD and DTA results that the bulk and grain boundary region crystallize independently in two different temperatures with a difference in temperature of about 200°C. The crystallization temperatures increase with Al₂O₃ contents. At 1300°C, the pure YSTZ and 5 and 10 wt % Al₂O₃ added YSTZ specimens underwent structural transformation from tetragonal to monoclinic ZrO₂. But, the tetragonal symmetry remains stable at 1300°C with an addition of 15 wt % Al₂O₃. The system which retain its tetragonal symmetry at its processing temperature (1300°C) gives high hardness and maximum density values. Almost 100% theoretical density value was obtained at 1300°C with an addition of 15 wt % of Al₂O₃.     

Formation and hot isostatic pressing of ZrO2 solid solution in the system ZrO2-Al2O3

In the system of ZrO₂-Al₂O₃, cubic ZrO₂ solid solutions containing up to 40 mol% Al₂O₃ crystallize at low temperatures from amorphous materials prepared by the simultaneous hydrolysis of zirconium and aluminium alkoxides. At higher temperatures, they transform into tetragonal solid solutions. Metastable ZrO₂ solid solution powders containing 25 mol% Al₂O₃ have been sintered at 1000–1150 °C under 196 M Pausing the hot isostatic pressing technique. The solid solution ceramics consisting of homogeneous microstructure with an average grain size of 50 nm exhibited a very high fracture toughness of 23 MN m ^–1.5. They have been characterized by X-ray diffraction and electron probe surface analyses.     

Kinetics of ion-beam mixing at Ti-Si interfaces

Ion-beam mixing at Ti-Si interfaces induced by krypton ions has been studied as a function of fluence and temperature by means of sheet resistivity measurements and a 2 MeV⁴He⁺ Rutherford backscattering spectrometry. For mixing induced by 200 keV Kr⁺, substrate temperatures during irradiation between 30 and 400° C and with fluences ranging from 1.0×10¹⁵ to 7.5×10¹⁵ Kr⁺/cm², the amount of mixing has a linear fluence dependence at low fluences and square-root fluence dependence at high fluences. Above 100° C, the mixing is strongly temperature dependent. The activation energy for the temperature-dependent part is 0.115 eV. Also, 300 keV Kr⁺ ion mixing at room temperature with fluences ranging from 2.5×10¹⁶ to 1.0×10¹⁷ Kr⁺/cm² displays a linear fluence dependence in this fluence range and the formation of nonuniform Ti _x Si_1–x layers.     

Decomposition of Al2TiO5-MgTi2O5 solid solutions: a thermodynamic approach

The decomposition of Al_1–x Mg _x Ti_1+x O₅ solid solutions with x=0.0, 0.1, 0.2, 0.4, 0.5 and 0.6 was studied in the temperature range 900–1175 °C using a 250 h annealing test. As x increases from 0–0.2 there is a strong stabilizing effect and the decomposition temperature decreases from 1280 °C (Al₂TiO₅) down to 1125 °C. For 0.2x0.5 the decomposition temperature does not decrease further. For x=0.6 no decomposition was observed. For x0.5 decomposition is complete or almost complete at 1000 °C; at 900 °C transformation is kinetically hindered and solid solutions with x=0.2 and 0.4 are unaffected by the thermal treatment. A relationship between the decomposition temperature and the parameter x has been derived using the regular solution model to describe the Al_2(1–x)Mg _x Ti_(1+x)O₅ solid solution.     

Formation and characterization of oxynitride glasses in the Si-Ca-Al-O-N and Si-Ca-Al,B-O-N systems

Oxynitride compositions in the Si-Ca-Al-O-N and Si-Ca-Al, B-O-N systems were melted and furnace-cooled in BN crucibles at temperatures from 1650 to 1850° C under dry nitrogen atmosphereS. Glass formation, phase stability and crystallization were studied by characterizing the cooled melts by X-ray diffraction, DTA, and electron microscopy. Oxynitride batches with nitrogen content up to 11 at % formed glasses in the Si-Ca-Al-O-N system. Glasses in the Si-Ca-Al, B-O-N system could be formed only when the B₂O₃ content of the batch was less than 3 wt %. Oxynitride glasses in these boron-containing systems were characteristically inhomogeneous, difficult to process, and prone to crystallization. In both the systems, glasses exhibited glass transitions beginning at 1000° C and crystallization at 1300 to 1500° C. Nitrogen-containing crystalline phases were identified in devitrified glasses via microstructural and micro-mechanical analyses.     

Chemical and structural properties of the system Fe2O3-Cr2O3

Chemical and structural properties of the mixed metal oxides (1–x)Fe₂O₃+xCr₂O₃ were studied by different techniques. X-ray powder diffraction showed the existence of solid solutions, (Fe_1–x Cr _x )₂O₃, over the whole concentration region, 0x1. The gradual replacement of Fe³⁺ with Cr³⁺ ions in samples prepared at 900°C caused changes in unit-cell parameters; most of these changes took place in the region fromx0.3–0.9. The samples having the fraction of Cr₂O₃ in the region from 0.7–0.8, contained two closely related phases, with slightly different compositions. After an additional heat treatment at 1100°C, these samples contained only one phase.⁵⁷Fe Mössbauer spectroscopy showed a gradual decrease of hyperfine magnetic field with increasing Cr₂O₃ content. The sample having the fraction of Cr₂O₃ of 0.7, and prepared at 900°C, exhibited two separated sextets at room temperature, in comparison with other compositions showing one sextet. It was shown that Fourier transform infrared (FT-IR) spectroscopy is a powerful method for the investigation of structural changes in these solid solutions. The increase in the Cr₂O₃ content resulted in shifts of the corresponding infrared bands. In addition, a gradual transition of the spectrum typical for -Fe₂O₃ to the spectrum typical for Cr₂O₃ was shown. The transition effects observed in the FT-IR spectra were correlated with the X-ray powder diffraction and⁵⁷Fe Mössbauer spectroscopic results.     

Hydrothermal precipitation and characterization of nanocrystalline BaTiO3 particles

Crystalline BaTiO₃ powders were precipitated by reacting fine TiO₂ particles with a strongly alkaline solution of Ba(OH)₂ under hydrothermal conditions at 80°C to 240°C. The characteristics of the powders were investigated by X-ray diffraction, transmission electron microscopy, thermal analysis and atomic emission spectroscopy. For a fixed reaction time of 24 hours, the average particle size of BaTiO₃ increased from 50 nm at 90°C to 100 nm at 240°C. At synthesis temperatures below 150°C, the BaTiO₃ particles had a narrow size distribution and were predominantly cubic in structure. Higher synthesis temperatures produced a mixture of the cubic and tetragonal phases in which the concentration of the tetragonal phase increased with increasing temperature. A bimodal distribution of sizes developed for long reaction times (96 h) at the highest synthesis temperature (240°C). Thermal analysis revealed little weight loss on heating the powders to temperatures up to 700°C. The influence of particle size and processing-related hydroxyl defects on the crystal structure of the BaTiO₃ powder is discussed.     

Thermal stability in the Al-Al3Ti system

Directionally solidified samples of an Al-2 wt% Ti alloy were annealed at temperatures between 435° C and 660° C to investigate the thermal stability of phases formed during an incomplete peritectic transformation. The proportion of Al₃Ti present in the assolidified alloy is less than equilibrium up to about 480° C, and more than equilibrium at higher temperatures. Hence, Al₃Ti particles will be stable up to about 500° C and will tend to dissolve at higher temperatures. Diffusion due to non-equilibrium composition of the phase continues at all temperatures but is sluggish up to about 600° C. The diffusion coefficient of Ti in Al at 635° C is estimated to be 2×10^–11 cm² sec^–1.     

Effect of Composition Variations on Electrical and Optical Properties of La1−x (Ca, Sr) x MnO3 Thin Films

Thin films of La _0.67 Ca _0.33 MnO ₃₊ (d 200 nm) grown at 750° C on MgO(100) substrates by pulsed laser deposition were annealed in various oxygen atmospheres. The LaF ₃ , CaF ₂ and Mn overlayers (d 5-10 nm) have been evaporated onto the films in order to saturate the compound by La, Ca or Mn during subsequent annealing at 900° C in air. Further, the pulsed injection CVD method has been utilized to prepare series of La _1–x Sr _x MnO ₃ films with x = 0÷0.5 on LaAlO ₃ (100). The effect of composition variations on electrical resistivity and magnetoresistance of the films as well as on their optical transmission and reflection spectra in the energy range between 0.2 and 4.0 eV has been investigated.     

Reaction control of TiB2 formation from titanium metal and amorphous boron

TiB₂ powder was synthesized by a controlled formation reaction from titanium metal and amorphous boron. Precursory TiB₂ formed by the pretreatment of the mixed powder (mole ratio: B/Ti=2.0) at 600° C for 60 min in an argon stream. Hollow TiB₂ powder with an average grain size of 15m was obtained by subsequent heat treatment above 900° C for more than 60 min in an argon stream. The formation reaction of TiB₂ powder was further controlled by pretreatment of the mixed powder at 600° C for 60 min in a hydrogen and argon stream and subsequent heat treatment at 1000° C for 360 min in an argon stream, when hollow-free TiB₂ powder was formed by a milder formation reaction between amorphous boron and the reformed titanium metal with hydrogen diffused lattice.     

Morphology of ordering Fe-Si alloys

The microstructure of some Fe-Si alloys water-quenched from 700–1200°C and aged at 550–675°C for various times has been studied by means of transmission electron microscopy (TEM). It has been shown that at temperatures of 700°C and above the microstructure of the alloy is a solid solution with two-dimensional particles of a B2 ordered phase. These particles have a FeSi composition and lie along (100) planes of the matrix. At temperatures of 675°C and below a modulated microstructure is formed; in its enriched modulations the composition of the Fe₃Si phase is attained and D0₃ ordering occurs. Phase composition of some high-silicon alloys has been determined by X-ray diffraction phase analysis. It corresponds to + B2 or + D0₃. These results are plotted on the accepted Fe-Si phase diagram.     

Influence of deposition and annealing parameters on some properties of YBCO films prepared by spray pyrolysis

Superconducting YBCO films were prepared by chemical deposition of aerosol generated from nitrate solutions by pneumatic and ultrasonic atomizers. Single crystalline YSZ and MgO substrates were used. Two different substrate temperatures were applied, with the chemical composition of starting solutions being adjusted accordingly. Deposited precursor films were thermally processed under two basically different conditions, i.e., either in atmospheric O₂ atT900°C, or in atT600°C. The influence of these different deposition and annealing parameters onT _c andJ _c values, microstructure, and surface morphology is reported. By optimizing our technological procedure, a real possibility exists to prepare (by means of aerosol deposition) YBCO films withJ _c values of at least 10⁴–10⁵ A/cm², at processing temperatures (both deposition and annealing as well) not exceeding 600–700°C.     

Effect of Al2O3 addition on phase reaction of the AIN-Y2O3 system

It is well known that the existence of oxygen in the sintering of aluminium nitride (AIN) causes the degradation of thermal conductivity. To clarify the role of the oxygen, the effect of Al₂O₃ addition on the phase reaction of the AIN-Y₂O₃ system at high temperatures of 1900 and 1950°C was investigated. In the AIN-Al₂O₃ system, -AION and 27R polytypoid were formed at 1900 and 1950°C, respectively, and the ratio of each crystal increased with increasing Al₂O₃ content. The resultant phase was found to be in agreement with the phase diagram given by McCauley and Corbin. In the AIN-Y₂O₃ system, YAM phase or YAM plus unreacted Y₂O₃ were identified. Phase reaction in the AIN-Al₂O₃-2Y₂O₃ system was quite different from that of the AIN-Al₂O₃ system: namely, -AION and 27R coexisted even at 1950°C, which is inconsistent with McCauley and Corbin's diagram. It was confirmed by the dilatometric method and phase reaction data using some couples, that Al₂O₃ strongly affected the phase reaction in the AIN-Y₂O₃ system, indicating that the shrinkage of the Al₂O₃-doped specimen was initiated at somewhat lower temperatures. The grain morphology depended on the phase changes shown above, in which AIN and -AION were granular, and 27R polytypoid was plate-like.     

SiC matrix composites reinforced with internally synthesized TiB2

A new process of preparing particulate-reinforced ceramic composites by internal synthesis has been developed. SiC powder mixed with TiN and amorphous boron was hot-pressed above 2000° C in an argon atmosphere. The boron molar content in the mixture was designed to be more than twice that of TiN. In the process of hot-pressing, the following reaction took place between 1100 and 1700° C TiN+2B TiB₂+1/2N₂ The synthesis of TiB₂ was followed by the densification of SiC matrix with the aid of the excess boron. The new process provides SiC matrix composites in which fine TiB₂ particulates are dispersed. Compared with hot-pressed monolithic SiC, the composite containing 20 vol % TiB₂ exhibits a 80% increase in fracture toughness and about the same flexural strength of 490 MPa at 20° C in air and 750 MPa at 1400° C in a vacuum.     

Microstructure and Mechanical Properties of ZrO2(2Y)-Toughened Al2O3 Ceramics Fabricated by Spark Plasma Sintering

Spark plasma sintering (SPS) has been performed for 5 min at 1500°C and 30 MPa using submicrometer-sized Al₂O₃/ZrO₂(2Y) composite powders in the Al₂O₃-rich region. Dense ZrO₂-toughened Al₂O₃ (ZTA) ceramics show excellent mechanical strength; the strength of 1620 MPa is achieved in the ZTA with 50 mol% ZrO₂. The grain size of Al₂O₃ in ZTA decreases from 1.5 to 0.6 m with increased ZrO₂ content. Almost all the ZrO₂ grains (0.3 m) are located in the boundaries of the Al₂O₃ grains. Mechanical properties are discussed, with an emphasis on the relation between t-/m-ZrO₂ ratios and microstructures of ZTA.     

Soft chemical synthesis and characterization of lithium nickel oxide electrode materials

Soft chemistry was used to prepare ordered nanophase Li_0.6NiO₂ electrode materials by completing the oxidation of Ni²⁺ to Ni³⁺ and/or Ni⁴⁺ species with H₂O₂ oxidant during solution reactions at 50–60 °C and evaporation at 105–150 °C rather than during sintering. Both elemental analysis and electron spectroscopy for chemical analysis (ESCA) results indicate that oxidation was completed. The deconvoluted ESCA spectra of nickel ions exhibited a semi-quantitative ratio of Ni⁴⁺Ni³⁺=6040 which presented no significant change with increase of sintering time. After sintering for up to 11 h at 700°C, ordered Li_0.61Ni_0.96O_2.0 ceramics were formed (R3m, a ₀=0.2837 nm, c ₀=1.417 nm). Distribution of the crystallite size was in the range of 80–200 nm. As sintering times were increased, the crystallite shapes exhibited a more distinct morphology, and the ordering degree of the cations was enhanced, while the conductivity was sharply enhanced up to 2.0×10^{–1 –1} cm^–1 at 30 °C. Compared to conventional ceramic and solution methods, the ordered nanophase Li_0.61 Ni_0.96O_2.0 ceramics was obtained at 700 °C with shorter sintering times ( 11 h).