Polished Alumina Substrates Research Articles

Influence of crystallization temperature and atmosphere on the phase composition, microstructure and electrical properties of Ni–Mn–O thin films

Nickel manganite thin films were prepared by chemical solution deposition from nitrate and acetate precursors on polished alumina substrates. By variation of crystallization temperature between 650 ​°C and 900 ​°C and atmosphere during cooling of the films, the morphology, phase formation and electrical properties of the films were investigated. The electrically conductive spinel phase was formed as a main phase independent of processing conditions exhibiting NTCR characteristics. Cooling down in nitrogen atmosphere favored the decomposition of the spinel phase into nickel oxide and a spinel phase with higher manganese content, but it also reduced the resistance drift of the samples. From this finding it can be deduced that a reduced oxygen partial pressure influences defect equilibria and by that decreases cation vacancy concentration. This supports the assumption that cation redistribution by a cation vacancy diffusion mechanism is a relevant mechanism of resistance drift in such thin films.

Comparison of morphology, electrical properties and sensitivity between bulk and thin-film Mn1.5Co1Ni0.5O4 thermistors

Mn1.5Co1Ni0.5O4 (MCN) bulk sensors and ceramic targets were prepared via the solid-state reaction method. MCN functional thin-film and Au electrode thin-film layers were fabricated on polished alumina substrates by employing a series of semiconductor processes; these involved physical vapour deposition, photolithography, and wet etching. The X-ray diffraction (XRD) patterns of the MCN thin film exhibit MCN spinel-structure peaks on the alumina substrate. Negative temperature coefficient temperature and resistance relationships of the bulk and thin-film thermistors were elucidated by measuring the resistance of thermistors in thermostatic oil both the resistance of the bulk sensor at 25 °C was measured to be 2077 Ω, while the thin-film sensor recorded resistance values exceeding 300 kΩ. The activation energy and B value of the thin-film sensor were slightly lower than those for the bulk sensors. The thermal response time constant of the bulk sensor was 5.13 s; meanwhile, for the thin film sensors, the time constants were 0.43 s, 0.46 s and 0.53 s, corresponding to the thicknesses of 528, 610 and 677 nm, respectively.

Tribological behavior of hydrogenated diamond-like carbon on polished alumina substrate with chromium interlayer for biomedical application

Diamond-like carbon (DLC) is a promising material for wear resistant films because of its high hardness and moderate elastic modulus. To enhance its performance in orthopedic implant applications, a better understanding of the tribological effect of DLC coating is essential. DLC with hydrogen, that is: hydrogenated amorphous carbon (a-C:H) was deposited on the polished alumina (Al2O3) substrate by magnetron sputtering. X-ray diffraction, energy dispersive spectroscopy and transmission electron microscopy were used to determine the phase compositions in the films. The a-C:H coated surface exhibits hydrophilic characteristic for protein absorption, with low water contact angles of 69.5°. It was found that the a-C:H, which has high hardness and moderate elastic modulus resulted in a relatively high H/E ratio of around 0.133. This result enables the film to conform well with the substrate and provide good wear resistance. The tribology test showed a significant reduction in the friction coefficient of the coated layer as compared to the uncoated surface rubbing on the Al2O3 counterface. The a-C:H coated pin against a-C:H coated block showed a stable and low friction coefficient under all loading conditions.

Impact of deposition temperature on crystalline structure of plasma-sprayed Al2O3 splats revealed by FIB-HRTEM technique

The effect of substrate surface temperature on the crystalline structure of plasma-sprayed Al2O3 splats was studied to clarify the phase selection process during the splat quenching for the development of crystalline structure. Al2O3 splats were deposited by atmospheric plasma spraying on the polished alumina substrate preheated to different temperatures from 573K to 1173K. The TEM samples of cross-sectional alumina splats were prepared by focused ion beam assisted with scanning electron microscopy (FIB-SEM). The crystalline structure of splats was examined using high resolution transmission electron microscopy (HR-TEM). Results showed that the alumina splats deposited at the substrate surface temperature higher than 573K were well bonded to the alumina substrate. Moreover, the crystalline structure changed significantly with substrate temperature. The crystalline structure of alumina splats changed from amorphous phase through meta-stable γ-Al2O3 crystallites to α-Al2O3 crystallites with the increase of the substrate temperature from 573K to 1173K. The present results clearly indicate that the crystalline structure formation depends on the surface temperature of the substrate on which droplet impacts. Thus, it is possible to control the splat crystalline structure by controlling substrate surface temperature prior molten droplet impact.

Platinum–zirconium composite thin film electrodes for high-temperature micro-chemical sensor applications

Stable metal interconnect thin films are critical in the development of various micro-machined devices that may operate continuously at elevated temperatures. The main objective of this work was to investigate the microstructural and electrical stability of a functionally gradient platinum (Pt)–zirconium (Zr) composite thin film electrode designed for resistive-type chemical sensors. Thin film electrodes were fabricated using a DC magnetron sputtering process. Zirconium was used as both the conventional adhesion promoter and the Pt grain modifier within the bulk electrode microstructure. The thin film deposition was completed on highly polished alumina substrates at 200°C. The various composite Pt thin films were further annealed at 1200°C after deposition for 1–24h for rapid evaluation of the microstructure stability. This temperature was chosen since the electrodes are expected to operate beyond 1000°C for high-temperature MEMS applications. Scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) were conducted to characterize the alterations in chemistry, microstructure and distribution of the constituent elements through the film thickness. The electrical resistivity of the as-deposited and thermally processed Pt thin films was measured by utilizing a van der Pauw's four-point probe technique. The work identified a Zr/Zr+Pt/Pt composite thin film with the 525nm total film thickness that demonstrated resistivity <5.08×10−7Ωm after being processed to 1200°C for 15h. A lift-off technique was finally used to produce a micro-electrode patterns with the optimized film structure for high-temperature applications.

(Invited) Nano-Derived Microsensors for Monitoring Gas Species in Harsh-Environments

The objective of our work is to develop nano-derived, micro-scale sensors to monitor various gases within extreme or harsh-environments. These environments include both high temperature and pressure applications, where sensors are required to monitor emissions in coal gasifiers and turbine generators. The environment may also include applications in extreme cold and low pressure applications, where sensors are required to monitor chemical leaks outside space stations, vehicles and satellites. This review includes both of these two extreme temperature and pressure environments and the nano-enhanced electrochemical and electromechanical sensor arrays developed to for these applications. In the case of the high-temperature emission sensing, nano-derived materials to detect gases such as H2, SOx, and H2S within high-temperature environments were investigated up to 1000°C. The micro-sensors investigated in this work are based upon a chemi-resistive sensor platform. The functionality of these micro-sensors is dependent upon the incorporation of refractory MOS nanomaterials within thin-film electrode patterns. The selective nanomaterials used for this work were synthesized by hydrothermal process. A range of baseline binary (WO3 and MoO3) and complex ternary tungstate, molybdates, and zirconate compositions were successfully synthesized by co-precipitation and/or hydrothermal methods. The sensing materials were deposited on macro- and micro-sensor platforms, and the chemi-resistive response of the sensing materials were investigated for SO2 and H2S exposure at levels of 100 to 3000 ppm and 50 to 300 ppm, respectively. The electrode used was an interdigitized electrode (IDE) pattern. Both CO and H2cross-sensitivity tests were also included.The sensing nanomaterials were also deposited over high-temperature stabile Pt IDEs, which were developed at WVU with dimensions in the millimeter to micron size range set upon a polished alumina substrate. The resistance change as a function of exposure time, temperature, and gas composition was monitored. The measurements were conducted at temperatures from 600 to 1000 °C, where enhanced sensitivities as high as 50-90% were achieved. Post-mortem microstructural and compositional analysis including XRD, XPS, SEM and EDS was completed to better understand both the sensing and degradation mechanisms.In the case of the low temperature and pressure leak sensors (for space applications), a micro-Surface Acoustic Wave (mSAW) sensors was investigated, which depends upon molecular interaction of the leaked species (such as fuel or life-support gasses) with the nano-enhanced surface of the sensor. In this case, the sensor would be dependent upon both mechanical and chemical interaction of the leaked specie with a micron-thick membrane, which would result in a high-frequency vibration. The mSAW structure is based upon a pair of inter-digitized electrodes (IDEs) set on a piezoelectric material, where a mechanical wave can be launched perpendicular to the surface onto the surface of the material. The interaction of the leaked fuel (or oxidant) with the membrane-supported mSAW sensor results in a frequency phase shift of the resonant frequency. The work describes the effect of the various electrode architectures (which includes bus bar height, finger dimensions, finger spacing and delay length) on the sensing response. In addition, the effect of key aspects of the piezoelectric film thickness, microstructure, and roughness, and further nano-enhancement to the sensor surface, was also investigated. The sensors were tested on a simulant gas between -150 to 200°C from standard pressure to 10-2Torr. Acknowledgements: The high-temperature emissions research was funded by US Department of Energy University Coal Research (UCR) program under contract no. DE-FE0003872. The space leak sensor work was sponsored by NASA Goddard Space Flight Center under grant no. NNX10AD17A and prime contract n. NNG10CR16C. The authors would also like to WVU Shared Facilities.

Vacuum Brazing of Alumina to Titanium for Implantable Feedthroughs Using Pure Gold as the Braze Metal

Pure gold brazing of 96% alumina ceramic with CP titanium using electrical resistance heating has been analyzed. Wetting studies of gold with niobium and titanium were performed. HTCC alumina ceramic was metalized with a 500nm Nb layer yielding hermetic joints with Ti ferrule having leak rate of the order of 1.6×10 -8 atm-cc/ sec. Wetting studies of Ti with Au and EDX analysis at the Au- Ti interface revealed that all the four Ti-Au intermetallic compounds forms at the interface and to avoid the growth of these intermetallic compounds brazing time should be kept minimum. Nb also shows good wetting behavior towards molten gold. Adhesion strength of Nb thin film over polished alumina substrate was measured using nanoscratch testing using a nanoindentor on 100nm Nb thin film on alumina. Scratch testing revealed that strong and adherent thin films of Nb over polished alumina can be deposited using sputtering. 

RELATION BETWEEN PROCESSING, MICROSTRUCTURE AND ELECTRIC FIELD-DEPENDENT DIELECTRIC PROPERTIES OF Ba0.3Sr0.7TiO3 THIN FILMS ON ALUMINA SUBSTRATES

ABSTRACT Ba0.3Sr0.7TiO3 thin films were prepared from alkaline earth acetates and titanium alkoxide in acetic acid/2-methoxyethanol solvents on polished alumina substrates by one-step, two-step and multi-step annealing. Films crystallize in the perovskite phase upon heating at 700°C notwithstanding the heating regime. In all cases the microstructure consists of equiaxed grains, however, the porosity decreases from one-step to multi-step annealed films. Increase in permittivity and tunability of fabricated films can thus be attributed to the increased density of the films obtained by a tailored heating regime and also to the improved chemical homogeneity introduced by the increased total thermal budget.

Processing and dielectric characterization of Ba 0.3Sr 0.7TiO 3 thin films on alumina substrates

Ba 0.3Sr 0.7TiO 3 (BST) thin films were prepared from the sols based on alkaline earth acetates and titanium propoxide in 2-methoxyethanol–acetic acid solvents and deposited on polished alumina substrates by spin coating. The perovskite phase crystallizes upon heating at/above 700 °C. By increasing the annealing temperature from 700 to 900 °C the grain size increases from 40 to 80 nm, due to the increased driving force for crystallization. The annealing time has got only a minor influence on grain size as a consequence of constrained conditions of the film. The dielectric permittivity and tunability ( ɛ 0 V / ɛ 200 V ) of BST films, measured at 1 MHz, strongly depend on the grain size, exhibiting the values of 345 and 1.47, and 722 and 1.93 for the films with 40 and 80 nm-sized grains, respectively.

Characterization of (Cr,Ta)N hard coatings reactively sputtered at low temperature

Two (Cr,Ta)N coatings with different composition were deposited in a Sputron plasma beam sputtering apparatus at a temperature below 200°C on polished tool steel disks, polished alumina substrates and silicon wafers. Microhardness, adhesion and internal stress were measured by Vickers microindentation, the scratch test and sample deflection, respectively. The mechanical and electrical properties were compared to the values of CrN and TaN binary nitride coatings prepared in the same device under similar conditions. Like CrN and TaN, (Cr,Ta)N crystallizes in a cubic (NaCl) structure. Oxidation resistance was tested in the temperature range 550–850°C. We observed that a chromium-rich protective oxide layer developed on the Cr 0.58Ta 0.42N coating, while on the Cr 0.17Ta 0.83N coating the oxidation was more rapid, with cracks appearing at temperatures as low as 600°C.

Electron Energy-Loss and Energy Dispersive X-Ray Spectros-Copy Studies of Diffusion Bonded Cu-Al2O3 Interfaces

Abstract The metal-ceramic interface between copper and alumina has been studied by TEM, Electron Energy-Loss Spectroscopy (EELS), and Energy Dispersive X-ray spectroscopy (EDX). of partic-ular interest is the potential role of silicon which is the major impurity in commercial alumina. This investigation focuses on the presence and distribution of silicon within the bulk alumina and at the diffusion bonded CU-AI2O3 interface using a Topcon 002B TEM equipped with an EDAX EDX detector and a Gatan Imaging Filter. The interface was created under vacuum by diffusion bonding of 100/xm copper foils (99.999% purity) pressed between polished alumina substrates (∼99.5% purity) for several hours at ∼90% of the melting temperature of the metal. An initial investigation of the bulk AI2O3 indicated that the majority of the Si occurred at the triple points. A typical example is shown in Fig. la. EDX spectra acquired with a 6nm probe from the center of the junction (Fig. lbi) and at the grain boundary indicate that the Si wets the grain boundary to a limited extent.

Oxidation of TiN, ZrN, TiZrN, CrN, TiCrN and TiN/CrN multilayer hard coatings reactively sputtered at low temperature

All the coatings and multilayer structures were deposited in a Sputron plasma-beam-sputtering apparatus at a temperature below 150 °C on polished alumina substrates ( R a= 25 nm) and polished tool steel discs. The coatings were annealed in an oxygen flow at temperatures in the range 500–850 °C. The oxidation kinetics of these coatings were studied by means of the weight gain as a function of the oxidation time at each temperature. On the same coatings, continuous in situ electrical resistivity measurements were taken and these indicate grain boundary oxidation. The composition and the depth profiles of the oxidation products formed were investigated using AES and XPS combined with sputter profiling. The oxide layers were found to grow according to a parabolic diffusion law with activation energies of 1.04, 1.89, 1.92, 2.37,2.91 and 3.98 eV for Ti 0.63Zr 0.37N, TiN, CrN, ZrN, TiN/CrN and Cr 0.72Ti 0.28N, respectively.

Surface morphology and gas-sensing characteristics of SnO 2−x thin films oxidized from Sn films

SnO 2−x gas sensors have been fabricated by thermal oxidation of Sn thin films. Sn thin films approximately 15 000A˚thick for commercial applications are deposited on a polished alumina substrate by using a high-vacuum resistance-heating evaporator. These films are oxidized at 500°C in various oxygen partial pressures in order to control the x value in SnO 2−x. The surface morphology and quantitative compositional analysis of SnO 2−x thin films as functions of oxygen partial pressures are systematically investigated by SEM, RBS and XPS. Gas sensitivities and response times of Pd-doped SnO 2−x thin films to H 2, CO, C 3H 8 and i-C 4H 10 gases are measured at 300°C. The relationship between gas sensitivities and the x values in SnO 2−x thin films is discussed.

Kinetics of the solid-state amorphizing reaction in thin films studied by electrical resistivity

We deposited Ni-Zr thin-film multilayers by electron beam evaporation onto polished alumina substrates. The multilayers were annealed at a constant heating rate, and in situ resistance measurements made to derive the growth kinetics of amorphous NiZr alloy at the Ni/Zr interfaces. Using a simple model that relates resistance change to amorphous-layer thickness, we derive the apparent diffusivity of nickel in amorphous Ni50Zr50, DNi =2.5×10−5 exp[−1.01(eV)/kT] cm2 s−1.