Low-Temperature Crystallization of Forsterite and Orthoenstatite

MgO–SiO₂ precursor gels were prepared by mixing tetramethoxysilane (TMOS) or tetraethoxysilane (TEOS), H₂O, and magnesium metal in methanol. Forsterite (Mg₂SiO₄) and orthoenstatite (MgSiO₃) were crystallized from their precursors at temperatures as low as 500° and 700°C, respectively. The chemical compositions of the crystallized phases were richer in MgO content than those of the starting materials. Inductively coupled plasma analysis showed that an amorphous SiO₂-rich phase was present, together with crystalline phases. We speculate that the amorphous SiO₂-rich phase has an important role in the low-temperature crystallizations of these magnesium silicates. Characterization of the preparation process via liquid-state ²⁹Si nuclear magnetic resonance (NMR) indicated that the breakage of the ≡Si–O–Si≡ bond was caused by the addition of magnesium metal. Solid-state ²⁹Si NMR showed that the Mg–O–Si bond might form in as-prepared specimens.     

Structure Characterization of HSQ Films for Low Dielectrics Uses D4 as Sacrificial Porous Materials

Hydrogensilsesquioxane (HSQ) (low- k ) films were prepared by spin-on deposition using D4 (octamethyl cyclotetrasiloxane) as a sacrificial porous material. The dielectric constant of silica films significantly changed from 3.0 to 2.2. Fourier transform infrared spectroscopy was used to identify the network structure and cage structure of the Si–O–Si bond and other bonds that may appear. We studied the structural and electrical properties of the spin-coated films prepared by mixing HSQ and D4 films after oxygen plasma exposure for 5 min, and studied the structural recovery of the damage by annealing at 350°C for 1.5 h in a nitrogen (N₂) ambient. This structure results in significant lowering of the dielectric constant ( k ) on annealing at 350°C for 1.5 h in an N₂ ambient and improvement in the leakage current density.     

Properties and Network Constitution of rf-Sputtered Amorphous Films in the System Silicon Dioxide–Aluminum Orthophosphate

Amorphous films in the system SiO₂–AlPO₄ were prepared by means of the rf-sputtering method, and their physical properties, such as density, refractive index, and temperature coefficient of Young's modulus, and infrared spectra were measured. Also, the K α X-ray emission spectra of silicon and aluminum were measured in order to investigate the coordination state of these cations in the amorphous films. The density and the refractive index were close to those of amorphous SiO₂ and AlPO₄ and the compositional dependence showed a small deviation from linearity. The temperature coefficients of Young's modulus were positive for all of the samples. The infrared absorption spectra of all of the samples were similar to those of SiO₂ glass and amorphous AlPO₄ film, and there was no evidence of the presence of P═O bonds. The coordination states of silicon and aluminum ions in the present amorphous films were the same as those in fused silica and AlPO₄ crystal, respectively. The results of the properties, infrared absorption spectra, and X-ray emission spectra suggest that SiO₄ tetrahedrons and AlO₄–PO₄ connecting tetrahedral dimers constitute the network of the present amorphous films. A small deviation of the physical properties from an additive rule was thought to result from the difference in the bond character between the newly formed Si–O–Al and Si–O–P bonds and the bonds in the end members, Si–O–Si and Al–O–P.     

Variation in In-Plane Stress during Densification and Crystallization of Titania Gel Coatings Deposited on Different Substrate Surfaces

Titania gel films were prepared from a coating solution with a starting mole ratio of Ti(OC₃H₇ⁱ)₄:H₂O:HNO₃:C₂H₅OH=1:1:0.2:30 on Si(100) wafers by spin coating, with and without sol–gel-derived, fired silica, and titania underlayers. In situ measurement was conducted on the in-plane stress in the titania gel films during heating up to 500°C at 5°C/min. Basically, for all the gel films, the in-plane, tensile stress increases once with increasing temperature, and then decreases around the temperature of crystallization. The increase in stress was thought to result from the capillary pressure due to solvent evaporation and from the densification of the coatings. The reduction in stress around the temperature of crystallization was ascribed to the accelerated atomic diffusion in the coatings and to the difference in thermal expansion coefficient between the coatings and the Si wafer. However, when the stress–temperature curves were examined more in detail, differences were found in the variation in stress, depending on the underlayers. Then, the question of how the underlayers could induce the crystallization and relax the strain in the coatings was discussed.     

Densification of the PLZT Films Derived from Polymer-Modified Solution by Tailoring Annealing Conditions

The Pb_0.91La_0.09(Zr_0.65,Ti_0.35)_0.9775O₃ films were prepared from the solution containing polyvinylpyrrolidone, with M _w of 360 000. A fast heating rate of 40°C/min was used to heat our samples to temperatures in the range of 550°–750°C for different durations. Both a rapid heating rate and isothermal holding at a proper temperature are found to be essential for obtaining dense and crack-free PLZT films. The TG-DTA and Fourier-transform infrared results indicated that the fast heating rate could keep a certain amount of polyvinylpyrrolidone residues undecomposed to a higher temperature (<700°C), which was found to be beneficial for the densification of the films during subsequent isothermal hold. The possible mechanisms responsible for the enhanced densification have been discussed. In addition, the effects of annealing temperature on the electrical and optical properties were also investigated.     

Refractory α-SiAlON Containing La2O3

Refractory Y-α-SiAlON with elongated grain morphology was obtained by utilizing La₂O₃ as a densification aid, which resulted in excellent room-temperature and high-temperature strength. Room-temperature strength of 1000 MPa was achieved when La₂O₃ was augmented by adding Y₂O₃ or removing AlN. With only La₂O₃, a temperature-independent strength of 800–950 MPa was maintained up to 1100°C, then gradually decreasing by 25% when reaching 1300°C. The R-curve measurements of fracture toughness showed relatively little dependence on microstructure, consistent with a strong interface that suppresses grain boundary decohesion. Compared with other densification aids such as SiO₂, Al₂O₃, Sc₂O₃, Y₂O₃, and Lu₂O₃, a finer microstructure was obtained by using La₂O₃. High nitrogen content in the residual La–Si–Al–O–N glass in equilibrium with the nitrogen-rich α-SiAlON is suggested to be the cause of these findings.     

Reaction and Formation of Crystalline Silicon Oxynitride in Si–O–N Systems under Solid High Pressure

Oxidized amorphous Si₃N₄ and SiO₂ powders were pressed alone or as a mixture under high pressure (1.0–5.0 GPa) at high temperatures (800–1700°C). Formation of crystalline silicon oxynitride (Si₂ON₂) was observed from amorphous silicon nitride (Si₃N₄) powders containing 5.8 wt% oxygen at 1.0 GPa and 1400°C. The Si₂ON₂ coexisted with β-Si₃N₄ with a weight fraction of 40 wt%, suggesting that all oxygen in the powders participated in the reaction to form Si₂ON₂. Pressing a mixture of amorphous Si₃N₄ of lower oxygen (1.5 wt%) and SiO₂ under 1.0–5.0 GPa between 1000° and 1350°C did not give Si₂ON₂ phase, but yielded a mixture of α,β-Si₃N₄, quartz, and coesite (a high-pressure form of SiO₂). The formation of Si₂ON₂ from oxidized amorphous Si₃N₄ seemed to be assisted by formation of a Si–O–N melt in the system that was enhanced under the high pressure.     

Structural Features of C–S–H(I) and Its Carbonation in Air—A Raman Spectroscopic Study. Part I: Fresh Phases

The Raman spectra of a series of mechanochemically prepared calcium silicate hydrate samples of type C–S–H(I) with C/S ratios ranging from 0.2 to 1.5 reveal changes in structure with changes in the C/S ratio. These support the model of Stade and Wieker based entirely on the tobermorite structure. The main characteristic feature of the spectra is the Si–O–Si bending vibration at about 670 cm⁻¹. Comparisons with bending frequencies of some known crystalline phases composed of single silicate chains led to an estimation of the mean Si–O–Si angles in the C–S–H(I) phases to be ∼140°. Finite silicate chains (Q²) dominate the structures of the samples at C/S ratios 0.2–1.0, the spectra showing characteristic bands from 1010 to 1020 cm⁻¹. When the samples are measured in air, the spectra exhibit carbonate bands arising from surface carbonation. The ν₁[CO₃] bands obscure the characteristic Raman scattering of silicate units near 1080 cm⁻¹, which is clearly evident in the fresh samples analyzed in closed capillaries. At C/S>1.00, dimers (Q¹) are the main building unit of the silicate anionic structure, with a characteristic band at 889 cm⁻¹. At C/S ratios 1.33 and 1.50, portlandite (Ca(OH)₂) is also observed.     

Densification of Nanocrystalline Yttria by Low Temperature Spark Plasma Sintering

We investigated the densification of undoped, nanocrystalline yttria (Y₂O₃) powder by spark plasma sintering (SPS) at sintering temperatures between 650°C and 1050°C at a heating rate of 10°C/min and an applied stress of 83 MPa. In spite of the low sinterability of the undoped Y₂O₃, a remarkable densification of the powder started at about 600°C, and a theoretical density of more than 97% was achieved at a sintering temperature of 850°C with a grain size of about 500 nm. The low temperature SPS is effective for fabricating dense Y₂O₃ polycrystals.     

Nitridation of the Sol–Gel-Derived Titanium Oxide Films by Heating in Ammonia Gas

Dip-coated sol–gel-derived TiO₂ films on an alumina substrate were converted to nonstoichiometric titanium nitride (TiN _x ( x ≦ 1)) films by heating at approxmately 1000°C in NH₃ gas. TiO₂ films made from TiO₂ sols prepared from Ti(O– i -C₃H₇)₄ and stabilized by diethanolamine were more easily nitrided than those from sols containing HCl as a deflocculant reagent. This appears to be a result of the more porous structure of the former films.     

Perovskite-Type Strontium Zirconate as a New Material for Thermal Barrier Coatings

Perovskite-type SrZrO₃ was investigated as an alternative to yttria-stabilized zirconia (YSZ) material for thermal barrier coating (TBC) applications. Three phase transformations (orthorhombic↔pseudo-tetragonal↔tetragonal↔cubic) were found only by heat capacity measurement, whereas the phase transformation from orthorhombic to pseudo-tetragonal was found in thermal expansion measurements. The thermal expansion coefficients (TECs) of SrZrO₃ coatings were at least 4.5% larger than YSZ coatings up to 1200°C. Mechanical properties (Young's modulus, hardness, and fracture toughness) of dense SrZrO₃ showed lower Young's modulus, hardness, and comparable fracture toughness with respect to YSZ. The "steady-state" sintering rate of a SrZrO₃ coating at 1200°C was 1.04 × 10⁻⁹ s⁻¹, which was less than half that of YSZ coating at 1200°C. Plasma-sprayed coatings were produced and characterized. Thermal cycling with a gas burner showed that at operating temperatures ∼1250°C the cycling lifetime of SrZrO₃/YSZ double-layer coating (DLC) was more than twice as long as SrZrO₃ coating and comparable to YSZ coating. However, at operating temperatures >1300°C, the cycling lifetime of SrZrO₃/YSZ DLC was comparable to the optimized YSZ coating, indicating SrZrO₃ might be a promising material for TBC applications at higher temperatures compared with YSZ.     

Characterization and Electrophoretic Deposition of Poly(Phenylsilsesquioxane)–Titania Hybrid Particles Prepared by the Sol–Gel Method

Poly(phenylsilsesquioxane)–titania (PhSiO_3/2–TiO₂) hybrid particles were prepared from phenyltriethoxysilane and titanium tetra- n -butoxide by the sol–gel method. Fourier transform infrared spectra showed that PhSiO_3/2 and the TiO₂ components were hybridized through Si–O–Ti bonds. The refractive index of the particles was monotonically increased from 1.57 to 1.62 with an increase in the TiO₂ content. The PhSiO_3/2–TiO₂ particles were electrophoretically deposited on indium tin oxide (ITO)-coated glass substrates to form opaque, thick films about 3 μm in thickness. When the mole ratio x in (1− x )PhSiO_3/2· x TiO₂ was equal to or less than 0.05, the deposited PhSiO_3/2–TiO₂ films became transparent with a heat treatment at 400°C because of the thermal sintering of the particles.     

Experimental Investigation and Thermodynamic Calculation of the Titanium–Silicon–Carbon System

The 1100°C isothermal section and the isopleths at 5, 10, and 15 at.% C in the Ti–Si–C system were determined by DTA and XRD methods. Five invariant reactions (L (liquid) = Si + SiC + TiSi₂ at 1330°C, L = TiSi + TiSi₂+ Ti₅Si₃C _x at 1485°C, L + Ti₅Si₃C _x = Ti₃SiC₂+ TiSi₂ at 1485°C, L + Ti₃SiC₂= TiSi₂+ SiC at 1473°C, and L + TiC = bcc-(Ti) + Ti₅Si₃C _x at 1341°C) were observed. The transition temperature for L + TiC = Ti₃SiC₂+ SiC was measured by the Pirani technique. Optimized thermodynamic parameters for the Ti–Si–C system were then obtained by means of the CALPHAD (calculation of phase diagrams) method applied to the present experimental results and reliable literature data. The calculations satisfactorily account for most of the experimental data.     

Preparation of High-Purity ZrSiO4 Powder Using Sol–Gel Processing and Mechanical Properties of the Sintered Body

Effects of the concentration of ZrOCl₂, calcination temperature, heating rate, and the size of secondary particles after hydrolysis on the preparation of high-purity ZrSiO₄ fine powders from ZrOCl₂.8H ₂ O (0.2 M to 1.7 M ) and equimolar colloidal SiO₂ using sol–gel processing have been studied. Mechanical properties of the sintered ZrSiO₄ from the high-purity ZrSiO₄ powders have been also investigated. Single-phase ZrSiO₄ fine powders were synthesized at 1300°C by forming ZrSiO₄ precursors having a Zr–O–Si bond, which was found in all the hydrolysis solutions, and by controlling a secondary particle size after hydrolysis. The conversion rate of ZrSiO₄ precursor gels to ZrSiO₄ powders from concentrations other than 0.4 M ZrOCl₂.8H₂O increased when the heating rate was high, whereupon the crystallization of unreacted ZrO₂ and SiO₂ was depressed and the propagation and increase of ZrSiO₄ nuclei in the gels were accelerated. The density of the ZrSiO₄ sintered bodies, manufactured by firing the ZrSiO₄ compacts at 1600° to 1700°C, was more than 95% of the theoretical density, and the grain size ranged around 2 to 4 μm. The mechanical strength was 320 MPa (room temperature to 1400°C), and the thermal shock resistance was superior to that of mullite and alumina, with fairly high stability at higher temperatures.     

Fabrication and High-Temperature Phase Stability of Mo(Al,Si)2—MoSi2 Intermetallics

A two-step processing technique was used to make dense, homogeneous intermetallics in the Mo(Al,Si)₂—MoSi₂ system. A variation of self-propagating high-temperature synthesis was used, in which starting pellets were nucleated at room temperature to make intermetallic powders from metallic precursors, followed by uniaxial hot pressing at 1600°—1800°C to achieve densification. The samples were held at the hot-pressing temperatures for several hours; therefore, this study also provided qualitative phase-stability information. The solubility limit of Al for Si in MoSi₂ was <5% at 1800°C. Samples that had 10% Al substituted for Si yielded approximately equal amounts of MoSi₂ and Mo(Al,Si)₂.     

Glass Transition Temperatures at Rapid Heating Rates

Glass transition temperatures (T_g) determined at high rates of heating or cooling are essential for applications involving stress relief during cooling from firing of glass-to-metal seals and dental restorations. The measurement of T_g at heating rates as high as 600°C min⁻¹ is reported. These high-rate data, when combined with previously reported low-rate (≤20°C min_-1) values, indicate that the Arrhenius-type relation between T_g and heating or cooling rate is maintained into the high-rate regime.     

Reactions Which Form Both Silicide and Nitride Layers

Reactions of Ti which form both a silicide and a nitride after rapid thermal annealing have been characterized using Auger depth profiling, X-ray diffraction, and transmission electron microscopy. Reactions of Ti with Si annealed in a nitrogen ambient at temperatures above 700°C form a thin TiN layer on TiSi₂ (C54). A similar structure is formed by annealing Ti deposited on thin Si₃N₄ films on Si in Ar at 1000°C. The reaction of nitrogen directly with TiSi₂ begins at temperatures above 950°C, but at temperatures greater than 1050°C, TiSi₂ completely reacts to form TiN. The reaction of Ti with thick Si₃N₄ films was studied in greater detail. At lower temperatures in Ar, mostly Ti₅Si₃ is formed at the interface, with some contaminant oxygen and nitrogen released from the reaction, uniformly dissolved throughout the unreacted Ti. Also, a very thin TiN layer exists at the Ti₅Si₃/Si₃N₄ interface. At higher temperatures, a three-layered structure, TiN/TiSi _x /TiN/(unreacted Si₃N₄) develops, allowing a conducting layer to be formed on an insulator. A mechanism explaining the formation of the three-layered structure is discussed. The important reactions are the fast dissolution of nitrogen into the unreacted Ti and the thermal instability of the Ti silicide/Si₃N₄ interface.     

Deposition of Pb(Zr,Ti)O3 Thin Films by Metal-Organic Chemical Vapor Deposition Using β-diketonate Precursors at Low Temperatures

Lead zirconate titanate (PZT) thin films were deposited by metal-organic chemical vapor deposition (MOCVD) using β-diketonate precursors and 0₂ at temperatures below 500°C on variously passivated Si substrates. PZT thin films could not be deposited on bare Si substrates, owing to a serious diffusion of Pb into the Si substrate during deposition. Pt/SiO₂/Si substrates could partially block the diffusion of Pb, but a direct deposition of PZT thin films on the Pt/SiO₂/Si substrates resulted in a very inhomogeneous deposition. A TiO₂ buffer layer deposited on Pt/SiO₂/Si substrates could partially suppress the diffusion of Pb and produce homogeneous thin films. However, the crystallinity of PZT thin films deposited on the TiO₂-buffered Pt/SiO₂/Si substrate was not good enough, and the films showed random growth direction. PZT thin films deposited on the PbTiO₃-buffered Pt/SiO₂/Si substrates had good crystallinity and a- and c-axis oriented growth direction. However, the PZT thin film deposited at 350°C showed fine amorphous phases at the grain boundaries, owing to the low chemical reactivities of the constituent elements at that temperature, but they could be crystallized by rapid thermal anneaiing (RTA) at 700°C. PZT thin film deposited on a 1000-å PbTiO₃,-thin-film-buffered Pt/SiO₂/Si substrate at 350°C and rapid thermally annealed at 700°C for 6 min showed a single-phase perovskite structure with a composition near the morphotropic boundary composition.     

Pressureless Sintering of Boron Carbide

B₄C powder compacts were sintered using a graphite dilatometer in flowing He under constant heating rates. Densification started at 1800°C. The rate of densification increased rapidly in the range 1870°–2010°C, which was attributed to direct B₄C–B₄C contact between particles permitted via volatilization of B₂O₃ particle coatings. Limited particle coarsening, attributed to the presence or evolution of the oxide coatings, occurred in the range 1870°–1950°C. In the temperature range 2010°–2140°C, densification continued at a slower rate while particles simultaneously coarsened by evaporation–condensation of B₄C. Above 2140°C, rapid densification ensued, which was interpreted to be the result of the formation of a eutectic grain boundary liquid, or activated sintering facilitated by nonstoichiometric volatilization of B₄C, leaving carbon behind. Rapid heating through temperature ranges in which coarsening occurred fostered increased densities. Carbon doping (3 wt%) in the form of phenolic resin resulted in more dense sintered compacts. Carbon reacted with B₂O₃ to form B₄C and CO gas, thereby extracting the B₂O₃ coatings, permitting sintering to start at ∼1350°C.     

Structural Changes of Sol–Gel-Derived TiO2–SiO2 Coatings in an Environment of High Temperature and High Humidity

Structural changes in sol–gel-derived TiO₂–SiO₂ coatings were found to proceed in an environment of high temperature and high humidity as follows: (1) dissociation of Si–O–Ti bonds in the coating by the attack of water vapor, (2) formation of Ti–O–Ti bonds, and (3) nucleation and growth of anatase TiO₂. The coating obtained with the addition of poly(ethylene glycol), PEG, reacts with water vapor more easily than the coating obtained without PEG, since the former is more porous than the latter due to the decomposition of PEG during heat treatment.