Silicon dioxide as a high temperature stabilizer for silver films

Silver films cannot at present be used as high temperature reflectors because of severe agglomeration in the presence of oxygen at temperatures in excess of 200 °C. Cr₂O₃, Al₂O₃, SiO₃ and CrO_x were tested for their effectiveness as thin barrier layers, and SiO₂ is the best of those tested. The use of SiO₂ allows the process of hole healing to compete with the normal hole growth process. Hole healing does not last indefinitely but is superseded by a slower rate hole growth process with an activation energy of 49 kcal mol^-1.Because of this last mechanism, stabilized silver is less than 1% transmitting after 50 h at 650 °C in air, whereas bare silver agglomerates to 66% transmittance after only 3 min under the same conditions. Therefore 500 Å SiO₂ films can be used as long term stabilizers for silver films in oxygen atmospheres with temperatures of up to 650 °C.     

Synthesis of LaCrO3 films using spray pyrolysis technique

LaCrO₃ thin films on electrolyte yttria-stabilized zirconia (YSZ) substrates were prepared by spray pyrolysis technique in the temperature range of 600–750 °C using lanthanum and chromium nitrates as precursors. Thin films obtained at 600–650 °C appear to be a mixture of cubic La(OH)CrO₄ and cubic LaCrO₄ phases, which transforms to pure cubic LaCrO₄ with the substrate temperature increasing to 700–750 °C. After being annealed at 900 °C for 2 h, all films convert to a single cubic LaCrO₃ phase. The change of Cr2p spectra in X-ray photoelectron spectroscopy (XPS) analysis shows the similar phase transformation process. Reaction processes with respect to the substrate temperature were proposed according to X-ray diffraction (XRD) and XPS analysis. The surface morphology of the films was found to depend strongly on the substrate temperature, which would be the deciding factor of the film growth mechanisms.     

Perovskite-type lanthanum chromium-based oxide films prepared by ultrasonic spray pyrolysis

Ultrasonic spray pyrolysis was used to form 2–8 m thick LaCrO₃ films on different substrates from La(CrO₄)(NO₃)·nH₂O precursor films. There was an optimum substrate temperature for the formation of uniform precursor films by the spray pyrolysis. When the substrate temperature was lower than 250 °C, dry precursor films were not formed, while above 250 °C the deposition rate decreased with substrate temperature. The precursor films were converted to perovskite-type oxide films by heat treatment at 800 °C in a nitrogen atmosphere. Both A-site substituted (La_0.8Ca_0.2) CrO₃ and B-site substituted La(Cr_0.5Mn_0.5)O₃ oxide films were formed in a similar manner. Electronic conductivity of the oxide films was improved by repetitions of the spray pyrolysis and heat treatment in nitrogen.     

Hydrothermal synthesis of chromium dioxide powders and their characterisation

Chromium dioxide (CrO₂) powders have been synthesised by decomposing CrO₃ and Cr₂O₅ powders under hydrothermal conditions in the temperature range of 300–500°C and pressure range of 250–1200 bars. Oxides of antimony and iron have been used as modifiers to induce acicular morphology. A novel method of using alkali metal salts such as chlorides and carbonates as mineralisers produces CrO₂ with superior magnetic characteristics. The particle size distributions have been correlated with the magnetic properties of the materials. The products obtained have properties rendering them useful for magnetic recording applications.     

Synthesis, structure and exchange bias in Cr2O3/CrO2/Cr2O5 particles

We report on the synthesis, structure and magnetic properties of a novel exchange bias system with Cr₂O₃/CrO₂/Cr₂O₅ interfaces. Chromium oxide particles with mixed chromium valences were prepared by sintering CrO₃ in air. X-ray diffraction patterns show that CrO₃ lost its oxygen gradually with increasing temperature and time through Cr₃O₈, Cr₂O₅, CrO₂, and finally Cr₂O₃ at temperatures above 760 K. X-ray photoelectron spectra indicate a low CrO₂ content and a binding energy of 579.3 eV for Cr 2p_3/2 photoelectrons in Cr₂O₅. Chromium dioxide was found to stably coexist with Cr₂O₃ and Cr₂O₅ in the particles. Magnetic measurements show hysteresis loop shifts in the sample, indicating an exchange bias induced by antiferromagnetic Cr₂O₃/Cr₂O₅ in ferromagnetic CrO₂. An exchange bias of 9 mT at 5 K and a coercivity of 26.3 mT were observed in the chromium oxide particles containing CrO₂.     

Selective-black absorbers using sputtered cermet films

Selective-black absorbers have high solar absorptivity and low infrared emissivity. We have prepared excellent selective-black absorbers by using r.f. sputtering to deposit MgO-Au and Cr₂O₃-Cr cermet films, which are highly absorbing in the solar spectrum and highly transparent in the infrared, on metal substrates. Both types of films are found to consist of individual crystallites less than 200 Å in size. Absorbers with solar adsorptivity of over 0.9 and infrared emissivity of less than 0.1 have been prepared by depositing films of 75 vol.% MgO-25 vol.% Au on molybdenum-coated stainless steel. These absorbers are stable in air up to 400°C. Absorbers with similar optical properties have been obtained by depositing films of 71 vol.% Cr₂O₃-29 vol.% Cr on nickel-coated stainless steel, provided that these films are covered with a Cr₂O₃ antireflection coating. These absorbers are stable in air up to 300°C. Computer calculations indicate that even better selective properties can be obtained by using Cr₂O₃-Cr films with a graded composition profile rather than a discrete Cr₂O₃ coating.     

Insights into structure and high-temperature oxidation behavior of plasma electrolytic oxidation ceramic coatings formed in NaAlO<Subscript>2</Subscript>–Na<Subscript>2</Subscript>CrO<Subscript>4</Subscript> electrolyte

Plasma electrolytic oxidation (PEO) ceramic coatings were formed on Ti₂AlNb alloy in various NaAlO₂ electrolytes containing 2 g L^?1, 4 g L^?1, and 6 g ^?1 Na₂CrO₄ additive, respectively. The influence of Na₂CrO₄ additive in NaAlO₂ electrolyte on structure and high-temperature oxidation behavior at 800 °C was investigated. The Na₂CrO₄ additive in the NaAlO₂ electrolyte not only promotes the formation of γ-Al₂O₃ phase and densification of ceramic coatings, but also participates directly in the growth of ceramic coating to form new Cr₃O and (Al_0.948Cr_0.052)₂O₃ phases. The PEO ceramic coatings formed in NaAlO₂ electrolytes with 2 g L^?1 and 4 g L^?1 Na₂CrO₄ additive show better oxidation resistance than those PEO coatings formed in a NaAlO₂ basic electrolyte based on isothermal oxidation tests at 800 °C up to 150 h. A thin and uniform isothermally oxidized layer is formed in the interlayer adjacent the substrate, which protects the substrate from the inward diffusion of oxygen and the outward diffusion of metallic elements. The PEO ceramic coatings formed in NaAlO₂ electrolyte with 4 g L^?1 Na₂CrO₄ additive exhibit the least mass gain among all the specimens, which is only a half of the ceramic coating formed in a NaAlO₂ basic electrolyte without any Na₂CrO₄ additive.     

High-temperature mechanical properties of La0.7Sr0.3Cr1−yCoyO3 in reducing environments

La_0.7Sr_0.3Cr_1−yCo_yO₃ (y=0–0.2) was synthesized so that it could be sintered at temperatures as low as 1500°C, producing a highly dense system. The materials were fabricated into bars and mechanically tested using a four-point bend test, from room temperature to 1000°C. High mechanical strength was obtained at 1000°C; however, degradation of the mechanical properties was observed in the system at cobalt B-site dopant concentrations as low as 5 mol%, in reducing atmospheres at 1000°C. La_0.7Sr_0.3Cr_0.95Co_0.05O₃, for example, showed degradation after 55 min in H2 and 75 min in H2/3%H2O atmospheres, with total disintegration of the sample soon after. The degradation, and hence loss in mechanical strength, was postulated as being due to reduction of the perovskite, because CoO was observed as a second phase. La_0.7Sr_0.3CrO₃ showed no such degradation in the mechanical strength even after 10 days in H₂ at 1000°C. Hence, the increased sinterability of cobalt addition on the B-site of La(Sr)CrO₃ is offset by the reduction of stability (and mechanical properties) under either H₂ or H₂/3%H₂O environments. This revised version was published online in November 2006 with corrections to the Cover Date.     

Epitaxial growth of Cr2O3 thin film on Al2O3 (0001) substrate by radio frequency magnetron sputtering combined with rapid-thermal annealing

We prepared epitaxially grown Cr₂O₃ thin films on Al₂O₃ (0001) substrate by using radio frequency magnetron sputtering at room temperature followed by a rapid-thermal annealing process. It is shown that the phase and the crystallinity of as-grown samples depend on the flow ratios of the working gas (Ar/O₂). X-ray diffraction (XRD) results show that oxygen-rich CrO₃ phase having preferred orientation is formed when sputter-grown at room temperature under the working gas (Ar/O₂) flow ratios of 9:1 and 7:3. XRD Φ-scanning results exhibit that annealing causes the oxygen-rich CrO₃ phase to transform into epitaxial Cr₂O₃ phase, which is confirmed by X-ray photoemission spectroscopy. The samples grown at the gas flow ratio of 9:1 exhibit very smooth surfaces with root mean square roughness of 0.1-0.3 nm.     

Influence of ion-beam sputtering deposition parameters on highly photosensitive and transparent CdZnO thin films

CdZnO thin films with a nominal thickness of ~200 nm were grown on c-plane sapphire substrates by dual ion-beam sputtering deposition technique. The effect of substrate temperature (300–600 °C) and gas ambience on structural, morphological, compositional and opto-electronic properties was studied. X-ray diffraction patterns confirmed that all the films were polycrystalline in nature and were preferentially oriented along the c-axis. It was revealed that the films grown at Ar/O₂ ratio of 4:1 were structurally more ordered and the film quality was found to be the best at 500 °C. The compositional studies specify that approximately 11.8 at.% of cadmium were present in the film deposited at 300 °C in Ar–O₂ mixture. Investigations on optical properties by photoluminescence and absorption studies indicate band gap shrinkage with the increase in argon partial pressure and substrate temperature. It was found that photosensitivity of the deposited films was highly dependent on growth conditions. The photosensitivity was found to be 5000-fold higher for CdZnO film grown at 600 °C in Ar–O₂ ambience compared to the best reported result, and this was promising to realize high-performance opto-electronic devices on such CdZnO films.     

Oriented growth of thin films of samarium oxide by MOCVD

Thin films of Sm₂O₃ have been grown on Si(100) and fused quartz by low-pressure chemical vapour deposition using an adducted β-diketonate precursor. The films on quartz are cubic, with no preferred orientation at lower growth temperatures (∼ 550°C), while they grow with a strong (111) orientation as the temperature is raised (to 625°C). On Si(100), highly oriented films of cubic Sm₂O₃ at 625°C, and a mixture of monoclinic and cubic polymorphs of Sm₂O₃ at higher temperatures, are formed. Films grown on either substrate are very smooth and fine-grained. Infrared spectroscopic study reveals that films grown above 600° C are free of carbon.     

Structural and resistivity changes in heat-treated chromium-gold films

Chromium-gold films deposited on glass were subjected to heat treatment at temperatures between 280 °C and 450 °C. The resistivity dependence of the conductive gold film with time at temperature exhibits a maximum. The processes controlling the resistivity of the gold film are the diffusion of chromium into the gold and the depletion of chromium from the gold. This latter process is accompanied by the production of Cr₂O₃ on the outer surface of the gold film. Microscope observations suggest that the diffusion of the chromium in the gold takes place through grain boundaries as well as in the lattice until the chromium layer vanishes. The Cr₂O₃ first forms as a polycrystalline film. With further diffusion large discrete single crystals of Cr₂O₃ are observed on the gold film surface.     

Crystallization process and electro-optical properties of In2O3 and ITO thin films

Amorphous indium oxide (In₂O₃) and 10-wt% SnO₂ doped In₂O₃ (ITO) thin films were prepared by pulsed-laser deposition. These films were crystallized upon heating in vacuum at an effective heating rate of 0.00847 °C/s, while the evolution of the structure was observed by in situ X-ray diffraction measurements. Fast crystallization of the films is observed in the temperature ranges 165–210 °C and 185–230 °C for the In₂O₃ and ITO films, respectively. The crystallization kinetics is described by a reaction equation, with activation energies of 2.31 ± 0.06 eV and 2.41 eV and order of reactions of 0.75 ± 0.07 and 0.75 for the In₂O₃ and ITO films, respectively. The structures of the films observed here during heating are compared with those obtained upon film growth at different temperatures. The resistivity of the films depends on the evolution of the structure, the oxygen content and the activation of tin dopants in the films. A low resistivity of 5.5 × 10⁻⁴ Ω cm was obtained for the In₂O₃ and ITO films at room temperature, after annealing to 250 °C the resistivity of the ITO film reduces to 1.2 × 10⁻⁴ Ω cm.     

Structural and optical characterization of c-axis textured BaTiO<Subscript>3</Subscript> thin films on MgO fabricated by RF magnetron sputtering

In this paper, BaTiO₃ thin films were prepared by RF magnetron sputtering on MgO substrates and their properties such as the crystal structure and optical waveguide properties were investigated. The optimum deposition parameters, such as substrate temperature, deposition pressure, gas flow ratio, the RF power and the after annealing temperature, were obtained in order to get the best BaTiO₃ film quality. The XRD results show that highly c-axis textured BaTiO₃ thin films were successfully grown on MgO substrate. Films obtained under the optimum deposition parameters, substrate temperature of 650°C, RF power of 50 W, deposition pressure 18 mTorr and gas flow ratio O₂/(Ar+ O₂) of 15% namely, reaches a full width at half maximum intensity (FWHM) of BaTiO₃ (002) XRD peak of 0.25°. The FWHM of BaTiO₃ (002) XRD peak was further reduced to 0.24° via post-treatment with furnace annealing (at 800°C for 2 h) which indicates the film crystal quality is further improved. The bright and sharp TE modes measured by m-line spectroscopy of the BaTiO₃ film were observed indicating its possible application in optical waveguide.     

Effect of firing temperature on microstructure and dielectric properties of chromium oxide based glass composite thick films on stainless steel substrate

We report the influence of firing temperature on Al₂O₃–chromium oxide based (Cr₂O₃–Bi₂O₃–B₂O₃–SiO₂–Al₂O₃) glass composite (named as GC-1 composite) thick films of thickness (27?±?3) µm deposited onto 0.6 mm thick austenitic grade stainless steel (DIN 1.4301/AISI 304) substrate by screen printing technique, which can be used as a substitute to alumina substrate. Prior to formulation of glass composite, the chromium oxide based glass (named as GC-1) phase was prepared separately by melt-quench technique. X-ray diffraction analysis confirmed amorphous nature of the GC-1 glass. The thermo gravimetric analysis and differential scanning calorimetry of the GC-1 glass shows thermal stability over the temperature range of 20–1000 °C. We observed that the firing temperature significantly influences microstructural and dielectric properties of the GC-1 composite film. The deposited GC-1 composite films onto stainless steel base were fired at temperatures between the range of 550–750 °C, showed the surface resistivity in the range of (1.0–6.9?±?0.2) × 10¹² ohms per square. The microstructure of these composite films recorded using scanning electron microscopy and electrical properties recorded using LCR meter were correlated with each other. The study revealed that the film fired at 600 °C were found to be superior among the samples under investigation in terms of microstructure, stable relative permittivity [36 (±?1)] and low loss tangent [0.02 (±?0.002)] in frequency range of 1–200 kHz, and surface resistivity (~?5.1?×?10¹² ohms per square).     

Damping properties of epoxy-based composite embedded with sol–gel-derived Pb(Zr<Subscript>0.53</Subscript>Ti<Subscript>0.47</Subscript>)O<Subscript>3</Subscript> thin film annealed at different temperatures

Pb(Zr_0.53Ti_0.47)O₃ (PZT) thin films were prepared on Pt/Ti/SiO₂/Si substrate by sol–gel method. The effect of annealing temperature on microstructure, ferroelectric and dielectric properties of PZT films was investigated. When the films were annealed at 550–850 °C, the single-phase PZT films were obtained. PZT films annealed at 650–750 °C had better dielectric and ferroelectric properties. The sandwich composites with epoxy resin/PZT film with substrate/epoxy resin were prepared. The annealing temperature of PZT films influenced their damping properties, and the epoxy-based composites embedded with PZT film annealed at 700 °C had the largest damping loss factor of 0.923.     

Properties of indium tin oxide films prepared by ion-assisted deposition

Conducting transparent films of indium tin oxide were deposited by 100 eV oxygen-ion-assisted deposition. A refractive index of 2.13 at 550 nm was obtained for films deposited onto ambient temperature substrates. The refractive index decreased with increasing substrate temperature to a value of 2.0 at 400°C. The sheet resistance of films 135 nm thick decreased from 800 Ω/□ for layers deposited onto room temperature substrates to around 25 Ω/□ at 400°C. Structural studies revealed that ion-assisted deposition onto ambient temperature substrates produced amorphous films, and that at temperatures above 100°C the films exhibit In₂O₃ crystallinity. In addition, it was found that the number of voids in the ion-bombarded films was reduced relative to that in films produced by conventional reactive evaporation.     

Synthesis,structural and optical studies of sol–gel Gd2O3:Eu3+, Tb3+ films

Eu³⁺ and Tb³⁺ co-doped Gd₂O₃ films were elaborated by sol–gel process and dip-coating technique. The films were synthesized by hydrolysis of gadolinium pentanedionate. A homogeneous and stable sol was obtained by the reaction with acetylacetone. Gd₂O₃:Eu³⁺, Tb³⁺ films were crystallized around 500 °C; at an increase of temperature up to 700 °C, oriented growth of (4 0 0) face was observed. The obtained transparent Gd₂O₃: 2.5 at.% Eu³+, 0.005 at.% Tb³⁺ waveguide films at 700 °C display significant optical properties. Different crystallographic properties can be obtained in Gd₂O₃:Eu³⁺, Tb³⁺ films with varying sintering temperatures.     

Influence of post-deposition annealing on the microstructure and properties of Ga2O3:Mn thin films deposited by RF planar magnetron sputtering

Thin films of Ga₂O₃:Mn have been deposited on silicon (100) substrates without intentional heating by radio frequency (RF) planar magnetron sputtering from a Mn-doped Ga₂O₃ target in an oxygen-argon mixture atmosphere. Microstructure and properties of the deposited Ga₂O₃:Mn films were systematically investigated as a function of the post-deposition annealing temperature in the range between 500 °C and 1200 °C. X-ray diffraction (XRD) measurements showed that the as-deposited Ga₂O₃:Mn films were of an amorphous structure in nature. The Ga₂O₃:Mn films became crystalline by the post-deposition annealing above 800 °C and the crystallinity of the films was continuously improved up to the annealing temperature of 1200 °C. It was shown that the annealed Ga₂O₃:Mn films possessed a monoclinic β-Ga₂O₃ phase having a textured structure with (400) and (?401) crystallographic planes oriented preferentially parallel to the substrate surface. The lattice parameters of the monoclinic β-Ga₂O₃ phase in the 1200 °C annealed Ga₂O₃:Mn films were measured to be a = 12.152 Å, b = 3.043 Å, and c = 5.785 Å.     

Transparent and heat-reflecting indium tin oxide films prepared by reactive electron beam evaporation

Transparent and heat-reflecting indium tin oxide films were prepared by electron beam evaporation of In₂O₃9mol.%SnO₂ in an oxygen atmosphere of about 5×10^?4 Torr. A visible absorption of less than 2%, a thermal IR reflectance exceeding 90% and a d.c. resistivity of approximately 3×10^?4 Ω cm were obtained from films 0.4 μm thick deposited at a substrate temperature of 300°C. Films with similar properties could be prepared with substrate temperatures as low as 150°C.