Srinivasan Raghavan

The effect of grain size, porosity and yttria content on the thermal conductivity of nanocrystalline zirconia

In order to accommodate the ever increasing inlet temperatures of gas turbines, air plasma sprayed (APS) or electron beam physically vapor deposited (EB-PVD) yttria stabilized zirconia thermal barrier coatings (TBC`s) are used to insulate the metallic surfaces. Because of its historic use as a TBC, the thermal diffusivity and conductivity of single crystal and polycrystalline stabilized zirconia have been the subject of numerous experimental investigations. However, to the knowledge of the authors, the thermal conductivity of nanocrystalline (gain size < 100 nm) zirconia has not yet been determined. To ascertain whether or when grain boundary effects begin to dominate thermal conductivity, k, values for a variety of nanocrystalline zirconias of different densities (60--100%), grain sizes (30--400 nm), and purities (0--15wt.% yttria) are compared in this work. Finally the measured values are compared with the thermal conductivities of commercially available air plasma sprayed (APS) and electron beam physical vapor deposited (EB-PVD) coatings.

THERMAL PROPERTIES OF ZIRCONIA CO-DOPED WITH TRIVALENT AND PENTAVALENT OXIDES

Zirconia doped with 6-8 wt% (3.2-4.2 mol%) yttria (6-8YSZ), the most common thermal barrier coating material, relies mostly on oxygen vacancies to provide the phonon scattering necessary for low thermal conductivity. The present study examines whether specific substitutional defects—in addition to, or instead of, oxygen vacancies—can provide similar or greater reductions in conductivity. To this end a series of zirconia samples co-doped with varying levels of yttrium (trivalent) and tantalum/niobium (pentavalent) oxides were synthesized, thereby allowing oxygen vacancy and substitutional atom concentration to be varied independently. The results show that Nb-Y and Ta-Y co-doped zirconia samples containing only substi- tutional defects produce stable single-phase tetragonal materials with thermal conductivities very close to that of the conventional 6-8YSZ. In these samples, Nb 51 and Td 51 are similarly effective in lowering thermal conductivity, in contradiction to phonon scattering theories that consider primarily mass effects and thereby predict significantly greater conductivity reduction due to Ta 51 doping than Nb 51 doping. Finally, Nb 51 /Ta 51 - Y 31 doped samples, which contain both oxygen vacancies and substitutional defects, are found not to be stable in single-phase form; however, the thermal conductivities of the two-phase tetragonal 1 cubic mixtures are again as low as that of the conventional 6-8YSZ.

Growth stresses and cracking in GaN films on (111) Si grown by metalorganic chemical vapor deposition. II. Graded AlGaN buffer layers

Intrinsic stress evolution during the growth of GaN by metal-organic chemical-vapor deposition on (111) Si, using an AlN buffer layer, was monitored in situ with a multiple-beam optical stress sensor. Data show that stress evolution takes place in two stages: an initial compressive regime up to about 100nm in thickness followed by a transition to a constant tensile stress, ∼0.3GPa, in films up to 1μm thick. Correlation of the stress evolution with surface morphological evolution by sequential atomic force microscopy images clearly shows that the incremental stress remains compressive in spite of grain coalescence, which is generally considered to be the dominant source of tensile stress in GaN films on sapphire. Rather, the most dominant feature accompanying the transition in stress from compressive to tensile, which takes place after grain coalescence, is an increase in the lateral size of individual islands. It is shown that this incremental tensile stress accompanied by an increase in lateral grain siz...

Intrinsic stresses in AlN layers grown by metal organic chemical vapor deposition on (0001) sapphire and (111) Si substrates

Stress evolution during metal organic chemical vapor deposition growth of AlN layers on (111) Si and (0001) sapphire substrates was investigated using in situ wafer curvature measurements in order to understand the origin of growth stresses. AlN layers 170±30nm thick were deposited over a temperature range of 600–1100°C at a growth rate of 0.2±0.05nm∕s. On (111) Si, AlN films grow under a constant tensile stress right from the beginning of growth in the temperature range investigated. In contrast, above 900°C on sapphire, an initial compressive growth stress is observed followed by a transition to a final tensile stress, while below 800°C the stress is tensile right from the beginning of growth as observed on Si. The origin of this stress behavior is explained in terms of a combination of epitaxial stress and grain coalescence stress. Calculation of the grain coalescence stresses show that the value is higher for growth on sapphire substrates than on Si substrates. Also, for growth on both substrates, a s...

The hot corrosion resistance of 20 mol% YTaO4 stabilized tetragonal zirconia and 14 mol% Ta2O5 stabilized orthorhombic zirconia for thermal barrier coating applications

Abstract Zirconia stabilized with 3.2–4.2 mol% (6–8 wt.%) yttria (3–4YSZ), the current material of choice for thermal barrier coating applications, is susceptible to hot corrosion by acidic oxides such as vanadia in the 700–900 °C range. The current study is a preliminary examination of the hot corrosion resistance to NaVO 3 –V 2 O 5 mixtures in the above temperature range of two alternative materials: a tetragonal zirconia co-doped with 10 mol% yttria+10 mol% tantala (20YTaO 4 Z) and an orthorhombic zirconia doped with 14 mol% tantala (14TZ). Results show that the 20YTaO 4 SZ is resistant to destabilization by NaVO 3, but is attacked at higher V 2 O 5 activities, resulting in the formation of YVO 4 and orthorhombic zirconia. Studies on the 14TZ itself then indicated that it is substantially more resistant than the YSZ to attack by environments more acidic (specifically V 2 O 5 rich) than pure NaVO 3 . However, it is less suitable than either 20YTaO 4 SZ or 3–4YSZ for environments that are more basic. A comparison of the resistance of the 14TZ, the 3–4YSZ and the 20YTaO 4 Z shows that the 20YTaO 4 SZ is more resistant to acidic oxides than the YSZ and more resistant to the basic oxides than the 14TZ.

Correlation of growth stress and structural evolution during metalorganic chemical vapor deposition of GaN on (111) Si

Compositionally graded AlGaN buffer layers enable the growth of thicker crack free layers of GaN on (111) Si than is possible with an AlN buffer layer. Using cross sectional transmission electron microscopy and in situ stress measurements, it is shown that a compressive growth stress is incorporated in the GaN layer when the graded AlGaN buffer layer is thick enough to accommodate all microstructural evolution, which is primarily a reduction in threading dislocation density with thickness during growth. Most of the dislocation density reduction is observed to occur when the film is growing under a compressive stress. This compressive stress arises from the changing lattice parameter due to grading and helps to offset the tensile stress generated by microstructural evolution. It also helps to decrease the tensile thermal expansion mismatch stress during cooling and thus reduces film cracking.

Effect of AlN interlayers on growth stress in GaN layers deposited on (111) Si

Thin (∼10nm) AlN interlayers have previously been used to mitigate stress and cracking in GaN epitaxial layers grown on Si substrates. However, multiple AlN interlayers are typically required for the growth of thick (>1μm) GaN as the initial compressive mismatch stress introduced by the AlN interlayer transitions to a tensile stress within 0.5μm. To better understand the reasons for the transition, in situ monitoring and transmission electron microscopy have been used to study stress and structural evolution in undoped GaN layers deposited on high temperature (1050–1100°C) AlN interlayers by metal-organic chemical-vapor deposition. The results show that transition of the initial compressive stress to a final tensile stress is associated with a reduction in the density of dislocations introduced either by the pseudosubstrate or the interlayer itself.

In situ observation of coalescence-related tensile stresses during metalorganic chemical vapor deposition of GaN on sapphire

Surface roughness and stress evolution were monitored in situ during the growth of GaN on sapphire substrates using low-temperature AlN buffer layers of varying thickness. A reduction in buffer layer thickness decreases the concentration of GaN nucleation sites which in turn increases the time to nuclei coalescence, thus varying the temporal evolution of surface roughness. By monitoring the accompanying changes in stress evolution, it is shown that island coalescence consisting of initial contact followed by subsequent surface roughness reduction is a source of tensile stress during growth of GaN films on sapphire. Such delayed coalescence also leads to an improvement in the structural properties of the material.

Effect of polarity on the growth of InN films by metalorganic chemical vapor deposition

The effect of surface polarity on InN growth on GaN by metalorganic chemical vapor deposition (MOCVD) was investigated. The polarity of the InN was found to follow that of the initial GaN template as determined by a comparison of experimental and simulated convergent beam electron diffraction patterns. Under identical MOCVD growth conditions, In-polar InN was observed to nucleate and grow on Ga-polar GaN as pyramidal-shaped islands with (101¯1) as the stable surface facet. In contrast, enhanced lateral growth and reduced surface roughness were observed for N-polar InN grown on N-polar GaN. InN films grown on (0001) sapphire substrates using a thin AlN buffer under identical conditions to those used for growth on the GaN templates also exhibited reduced surface roughnesses and were determined to be N polar. A qualitative model based on the difference in surface terminations and crystal structures is proposed to explain the observed differences in the structural properties and growth modes of the In-polar a...

Stress and morphology evolution during the heteroepitaxial growth of group III-nitrides

In this study, in-situ wafer curvature measurements were used to monitor the evolution of film stress during MOCVD growth of GaN and AlGaN. These studies were carried out using a multi- beam optical stress sensor (MOSS) incorporated onto a custom-designed vertical cold-wall MOCVD reactor. The MOSS system provides real-time information on growth rate and changes in substrate curvature which are related to film stress via a modified version of Stoneys equation. Post-growth structural characterization including atomic force microscopy, X-ray diffraction and transmission electron microscopy was used to correlate measured changes in film stress to film morphology evolution.