Dielectric properties of laser-ablated 0.5-μm-thick c-axis epitaxial Na0.5K0.5NbO3 films on high-resistivity (7.7 Ω cm) silicon SiO2/Si substrate are studied experimentally at frequencies up to 40 GHz. For measurements, planar 0.5-μm-thick gold electrodes (interdigital and straight slot) are photolithography defined on the top surface of Na0.5K0.5NbO3 films. The slot width between the electrodes is 2 or 4 μm. 13% capacitance change at 40 V dc bias and Q factor more than 15 are observed at 40 GHz, which makes the structure useful for applications in electrically tunable millimeter-wave devices

In the present study on AlN-based ceramic materials, an attempt was made to show the morphological influence of the precursors on the phase relationships during sintering as well as the microstructural development of the material. An investigation has been done of the phase transformations during pressureless sintering of wear-resistant ceramics based on the AlN-Al2O3-Y2O3 system in the AlN-rich field as well as a study of the effect of different precursors, such as Al2O3, AlCl3 and Al(NO3)3, on the sequence of phase transitions in the low temperature field (800-1400°C). The peculiarities of phase transitions in dynamic and isothermal schedules have also been discussed.

A modified computer-controlled high-temperature x-ray diffractometer with good stability and an upper temperature limit of more than 2300 K is described. A critical test of the system, determining the thermal expansion of Pt, Ni and AlN, showed close agreement with dilatometric and literature data. Lattice thermal expansion data of CrB2 and TiB2 up to 2100 K were also determined

The crystallisation kinetics of amorphous sol-gel PZT thin films were investigated using high-temperature X-ray diffraction. Crystallisation for different isotherms was monitored as a function of time. Phase transformation data were obtained from integrated X-ray peak intensities which were calibrated based on image analysis of the surface microstructure of the samples at the end of the isothermal treatments. An activation energy of 310 kJ/mol was obtained without assuming a specific kinetic model. From the transformation data, a TTT diagram was constructed for the ranges studied.

The kinetics of the second calcination step in the formation of PZT solid solution (with perovskite ABO3 lattice) has been investigated by using two different particle sizes of the B-site precursor (1·91 and 5·08 μm), the finer size being obtained by prolonged milling. In-situ analysis performed by high-temperature X-ray diffractometry in a non-isothermal mode (20-800°C) revealed a reduction of the calcination temperature by 100°C with a decrease in particle size of the precursor. In order to clarify the mechanism of the solid-state reaction to PZT, isothermal heat treatment of the mixtures was performed in the temperature range 540-700°C. The activation energies for the fine and the coarse powders were estimated as 150 and 210 kJ mol 1 respectively, and the reaction was found to follow the Jander model for diffusion-controlled solid-state reaction kinetics

Stable and metastable phase relationships in the ZrO2-TiO2 system were investigated using homogeneous samples with ZrO2 particles of different surface area. Phase analysis associated with the diffusionless and reversible monoclinic-tetragonal transition has been performed with high temperature X-ray diffractometry. The rate of the phase transition, depending on powder morphology, is presented. The character of the hysteresis loop associated with the martensitic phase change for pure ZrO2 and for the ZrO2-TiO2 system is discussed

High temperature x-ray diffraction was used to investigate the evolution of phases at 1050 C in three AlN-based ceramic powder mixtures containing Al2O3 and Y2O3 additives. The overall composition was 87.5 mol% AlN, 10% Al2O3 and Y2O3 2.5%; the alumina precursors in the three mixtures were Al2O3, AlCl3 and Al(NO3)3 respectively. The effects of the tape of precursor and the heat treatment atmosphere (vacuum or nitrogen) on the phases formed and the sequence of their formation over a period of two hours, are presented and discussed.

A dense and transparent hydroxyapatite produced by hot isostatic pressing was investigated with respect to its stability and thermal expansion behaviour in the range 20–1150 °C. The evaluation was made by high-temperature X-ray diffraction in a reconstructed test chamber with a BN sample holder and Pt as an internal (in situ) reference. No degradation of the hydroxyapatite was detected in the range investigated. The lattice parameters, a and c, were determined as a function of temperature. The linear thermal expansion coefficient, determined by dilatometry, was 17.1 × 10^–6°C^–1, which agreed well with the value 17.3 × 10^–6°C^–1 obtained by X-ray diffraction.

The sequence of the solid state reactions in the PbOZrO 2-TiO2-Nb2O5-NiO system has been investigated. The oxide mixing route utilised in sample preparation was selected in order to determine the basic reaction path in the formation of the PZTPNN perovskite phase. It has been established that the main intermediate phases formed prior to PZTPNN are PbTiO3 and pyrochlore Pb-Nb-based phases. The sequence in the pyrochlore formation was from tetragonal Pb3Nb2O8 (500 °C) to rhombohedral Pb2Nb2O7 (600-750 °C) and finally to cubic Pb3Nb4O13 (650-850 °C). The formation of the perovskite phase proceeded from mutual solubility of PbTiO3 and pyrochlore Pb3Nb4O13 phases, accompanied by dissolving of residuals (PbZrO3 and NiO) in the perovskite solid solution formed

Reaction bonding and post sintering of silicon nitride/oxynitride was investigated as a route to fabricate a material with good oxidation resistance and good high temperature strength. Silicon powders, mixed with 0, 2, 4, 8 and 16 wt% silica, were CIPed at 150 MPa and nitrided using the nitrogen demand principle. Four different nitriding gas compositions were used, consisting of different amounts of hydrogen, argon and helium mixed with nitrogen. Post sintering at atmospheric pressure and by HIP at 160 MPa were investigated to densify the materials. Samples were characterised by XRD, SEM and Hg-porosimetry in both the nitrided and sintered state

Single phase Na0.5K0.5NbO3 (NKN) thin films have been pulsed laser deposited on Al2O3(01-12), LaAlO3(001), and MgO(001) single crystal substrates as well as onto SiO2/Si(001) wafers to demonstrate films feasibility for voltage tunable microwave device applications. NKN film texture has been found to be quite different on three different single crystals: highly c-axis oriented on Al2O3, "cube-on-cube" epitaxial quality on LaAlO3, bi-axial textures on MgO, while NKN films grown on Si substrate with various thickness of SiO2 buffer layer possess highly c-axis oriented quadrupled structure. NKN film interdigital capacitors fabricated onto single crystal oxide substrates showed tunability of 30-40 % and dissipation factor of 0.01-0.02 at 1 MHz and applied electric field of 100kV/cm. Microwave frequency measurements for NKN/Si varactors yield 13 % tunability and dielectric loss tan delta as low as 0.012 at 40 GHz under 200 kV/cm applied bias.

We report on ferroelectric/giant magnetoresistive Nao.sKo.sNbCVLao.oSrojMniO} (NKN/LSMO) heterostructures gro\vn onto LaAlOj (001) single crystal using KrF pulsed laser ablation of stoichiometric ceramic target. Main processing parameters have been optimized to obtain smooth LSMO template layer, avoid NKN-LSMO interdiffusion, preserve NKN stoichiometry against the lost of volatile potassium and sodium and achieve reasonable reliability of NKN film performance. X-ray diffraction 0- 20 scans and rocking curves evidence for single-phase content and high c-axis orientation both in template LSMO and top NKN layers. Ferroelectric measurements yield remnant polarization P, of 1.5 (.iC/cm2 and spontaneous polarization Ps of 7 jiC/cm2 at electric field strength of 130 kV/cm. At room temperature, dielectric permittivity e' and dissipation factor tancJhave been found to vary from 595 to 555 and 0.046 to 0.029 respectively in the frequency range of 0.4 to 20 kHz. At 10 kHz dielectric permittivity linearly increases from 410 to 650 in the temperature range 77 K to 415 K while the dissipation factor below 320 K does not exceed 3%.

During laser machining of ceramics, cracks can occur in the material around the irradiated area. These cracks reduce the strength of machined parts and may possibly hinder their use as an engineering material. It is therefore very important to establish methods for the detection and avoidance of crack formation. As an aid for detecting crack formation, the acoustic emission method was used for two materials: soda lime glass and sialon. The results have led to the conclusion that it is possible to detect a signal coming from a crack in the investigated material and that the power of the detected signal is due to propagation of the crack. The relationship between laser parameters and crack formation was found with the aid of acoustic emission signal analysis. It was also found that it is possible to minimise or avoid crack formation by choosing suitable laser parameters

Thin (Na,K)NbOx perovskite films (NKN) have been deposited on SiO2/Si(0 0 1) substrates at low temperatures, from 350°C to 550°C, by RF magnetron sputtering. The effects of substrate temperature on microstructure, electrical-, and mechanical properties of the NKN films have been studied. X-ray diffraction analysis revealed that films deposited at temperatures in the range of 450-550°C were crystalline, growing as a single phase, with a preferred orientation of (0 0 1). Films deposited at 350°C, were shown to be amorphous. The growth temperature had a strong influence on the electrical properties of the NKN films and the relative dielectric constants of the obtained films were in between 38 and 78. Variations of the mechanical properties of the NKN films were observed for different substrate temperatures: The elastic moduli and the hardness values ranged from 205±26 to 93±29 GPa, and from 12±2 to around 2 GPa, for films deposited at 550°C and 450°C, respectively.

Thin films of (Na,K)NbOx (NKN) were grown by reactive RF magnetron sputtering on polycrystalline Pt80Ir20 substrates, at relatively low growth temperatures between 300°C and 450°C. The results show that the electrical performance and the microstructure of the films are a strong function of the substrate temperature. X-ray diffraction of films grown up to 400°C revealed the formation of only one crystalline NKN-phase with a preferred (002)-orientation. However, a mixed orientation together with a secondary, paraelectric potassium niobate phase, were observed for NKN films deposited at 450°C. The differences in the microstructure explains the variations in the dielectric constants and losses: The single phase NKN films displayed a dielectric constant and a dielectric loss of 506 and 0.011, respectively, while the films with mixed phases exhibited values of 475 and 0.022, respectively. The possibility of fabricating NKN films with relatively high dielectric properties at low growth temperatures, as demonstrated here, is of high technological importance

By investigating the nitridation processes using a large mass of silicon under different atmospheres, it was found that the nature of heat conduction and mass transport were influenced by the size of the silicon powder compact and/or the number of compacts. Two main reaction maxima were observed and both accelerating as well as retarding influences on the reaction rate could be identified. The extent and starting temperature of the first reaction maximum was found to be greatly influenced by gas composition. Also investigated was the influence of different nitridation atmospheres (N2+Ar, N2+H2, N2+H2+Ar and N2+He) on microstructure of large silicon powder compacts. A homogeneous and complete reaction could be achieved by tailoring the gas composition

Due to the strongly exothermic nature of silicon nitridation, a clear understanding of the nitridation process and an effective control regime, especially when the nitridation involves a rather large mass of silicon are of commercial interest. The nitridation process was investigated by reacting "large masses" of silicon powder compacts (about 1.5 kg) in a nitriding atmosphere in which some auxiliary gases such as Ar or He were added. Because of the exothermic heat, the reaction kinetics were found to be influenced by the amount of silicon in the nitridation cycle. The effect of the additional gases (Ar and He) is enhanced as the mass of silicon loaded in the furnace increases. The kinetics and the corresponding mechanisms of the whole nitridation cycle are discussed and a mechanism for the second reaction maximum is proposed. 22 refs.

In order to obtain detailed information about the kinetics and the reaction nature of a complex reaction process like reaction bonding of silicon nitride, mathematical modelling of the process is necessary. The previous quantitative models for this process have been based only on the mechanism that the nitrogen diffuses through the solid silicon nitride without taking into account the multiple reaction mechanisms. In the present study, a comprehensive kinetic model, which is based on analysis of the multiple mechanisms in a silicon powder compact reacting with nitrogen gas and forming silicon nitride, is constructed for a solid-gas reaction bonding process with specific application to the reaction-bonding of silicon nitride. The model will incorporate the rate equation for each mechanism into a constitutive equation from which more complete information of process kinetics can be predicted. The results predicted by the present model have been compared with previous experimental results and satisfactory agreement obtained

For a number of reaction-bonded ceramics, the reaction starts from the surface of the particles, forming a shell of products surrounding a reactant core. The reaction is continued by inward growth of the shell by means of diffusion of the reactant elements from the particle surface to the interface between the shell and core and vice versa. Since there exists a volumetric change due to the reaction, the products forming at the shell/core interface will be severely constrained. Strain and stress fields are then built up within the particles. A physical and mathematic model to calculate the strain and stress during and after the reaction bonding process is constructed. The preliminary results of estimation of stresses within the particles and their possible effect on reaction kinetics are presented and briefly discussed

A physical model is presented that predicts the stress distribution created in a particle during its reaction with a surrounding reactant to form a uniform layer of reaction product on its surface, when the reaction involves a volume change. The results of the model are applied specifically to the case of silicon reacting with nitrogen to form Si3N4. The model predicts the generation of a high, tensile hydrostatic stress in the Si core as well as high tensile radial stress and compressive tangential stress in the nitride layer. Although the model is restricted to elastic deformation only and therefore predicts unrealistically high stresses in some cases, the results are anyway of relevance in the consideration of possible non-elastic processes such as creep and fracture and also in assessing the possible effect of stress on the reaction equilibrium. It is predicted that the nitride reaction layer would fracture during the nitridation process. A second model is also presented predicting the residual stresses arising during cooling of a partially reacted particle as a result of the difference in thermal expansion of the reactant core and the reaction product layer. In the case of the reaction of silicon to silicon nitride these thermal expansion mismatch stresses are significant but small compared to the stresses due to the chemical reaction.

In this work, five yttria powders with slightly different states of agglomeration, inherited from various procedures of dewatering the same precursor, were densified by a combination of vacuum sintering followed by hot isostatic pressing (HIP). In order to relate the densification behavior of each powder to its state of agglomeration, all powders were characterized by tap density measurements, X-ray diffraction, nitrogen adsorption, and laser scattering, while the microstructures of the corresponding densified samples were studied by optical and scanning electron microscopy. The five yttria powders produced sintered samples that differed remarkably from each other in terms of transparency. These discrepancies were related to the degree of fineness in the powders at two different levels. At the level of primary particles, fine and weakly agglomerated powder was very sinterable, causing abnormal grain growth to occur only in the very late stage of sintering. However, the resulting entrapped pores and reduction due to vacuum sintering were responsible for poor optical properties. At the agglomerate level, a bimodal size distribution was identified for all powders. For powders showing severe agglomeration of the primary particles, increasing the relative content of the smaller size population of agglomerates was found to trigger abnormal grain-growth earlier during presintering. This was attributed to the density around large agglomerates exceeding a critical threshold in the green bodies. Finally, transparency was achieved in samples for which presintering was stopped before grain growth became abnormal. This confirmed that the key to successfully obtaining transparency was to keep porosity intergranular, which could be removed subsequently by HIP treatment.

Three different approaches have been investigated to synthesize alumina-chromium composites by displacement reactions between aluminium and chromia. The reactions have been performed either by pure mechanosynthesis or pure thermal ignition or also by a combined mechanical and thermal activation process. Differential thermal analysis, dilatometry and high temperature X-ray diffraction have been used to show the various reaction mechanisms and chemical transformations. High-energy milling of the powder modifies the reactivity of the system. A partial pre-milling treatment not only reduces the reaction temperature but can also induce a different reaction path. It has been shown that an appropriate mechanical-thermal treatment leads to the synthesis of an alumina-chromia composite by a pure solid-state process: intermediate reaction stages exhibit little if any liquid phases.