Development of a simplified procedure for rocket engine thrust chamber life prediction with creep

An analytical method for predicting engine thrust chamber life is developed. The method accounts for high pressure differentials and time-dependent creep effects both of which are significant in limiting the useful life of the shuttle main engine thrust chamber. The hot-gas-wall ligaments connecting adjacent cooling channels ribs and separating the coolant flow from the combustion gas are subjected to a high pressure induced primary stress superimposed on an alternating cyclic thermal strain field. The pressure load combined with strain-controlled cycling produces creep ratcheting and consequent bulging and thinning of these ligaments. This mechanism of creep-enhanced ratcheting is analyzed for determining the hot-gas-wall deformation and accumulated strain. Results are confirmed by inelastic finite element analysis. Fatigue and creep rupture damage as well as plastic tensile instability are evaluated as potential failure modes. It is demonstrated for the NARloy Z cases analyzed that when pressure differentials across the ligament are high, creep rupture damage is often the primary failure mode for the cycle times considered

Development of a simplified procedure for thrust chamber life prediction

An analytical design procedure for predicting thrust chamber life considering cyclically induced thinning and bulging of the hot gas wall is developed. The hot gas wall, composed of ligaments connecting adjacent cooling channel ribs and separating the coolant flow from the combustion gas, is subjected to pressure loading and severe thermal cycling. Thermal transients during start up and shut down cause plastic straining through the ligaments. The primary bending stress superimposed on the alternate in-plane cyclic straining causes incremental bulging of the ligaments during each firing cycle. This basic mechanism of plastic ratcheting is analyzed and a method developed for determining ligament deformation and strain. The method uses a yield surface for combined bending and membrane loading to determine the incremental permanent deflection and pregressive thinning near the center of the ligaments which cause the geometry of the ligaments to change as the incremental strains accumulate. Fatigue and tensile instability are affected by these local geometry changes. Both are analyzed and a failure criterion developed

The challenge of decomposition and melting of gallium nitride under high pressure and high temperature

Gallium nitride (GaN) is considered to be one of the most important semiconductors nowadays. In this report a solution of the long standing puzzle regarding GaN decomposition and melting under high pressure and high temperaturę is presented.This includes the discussion of results obtained so far. The possibility of a consistent parameterisation of pressure (P) evolution of the melting temperaturę (Tm) in basic semiconductors (GaN, germanium, silicon…), independently from signs of dTm/dP is alsopresented

Influence of Dopants on Defect Formation in GaN

Influence of p-dopants (Mg and Be) on the structure of GaN has been studied using Transmission Electron Microscopy (TEM). Bulk GaN:Mg and GaN:Be crystals grown by a high pressure and high temperature process and GaN:Mg grown by metal-organic chemical-vapor deposition (MOCVD) have been studied. Structural dependence on growth polarity was observed in the bulk crystals. Spontaneous ordering in bulk GaN:Mg on c-plane (formation of Mg-rich planar defects with characteristics of inversion domains) was observed for growth in the N to Ga polar direction (N polarity). On the opposite site of the crystal (growth in the Ga to N polar direction) Mg-rich pyramidal defects empty inside (pinholes) were observed. Both these defects were also observed in MOCVD grown crystals. Pyramidal defects were also observed in the bulk GaN:Be crystals

MnAs dots grown on GaN(0001)-(1x1) surface

MnAs has been grown by means of MBE on the GaN(0001)-(1x1) surface. Two options of initiating the crystal growth were applied: (a) a regular MBE procedure (manganese and arsenic were delivered simultaneously) and (b) subsequent deposition of manganese and arsenic layers. It was shown that spontaneous formation of MnAs dots with the surface density of 1$\cdot 10^{11}$ cm$^{-2}$ and $2.5\cdot 10^{11}$ cm$^{-2}$, respectively (as observed by AFM), occurred for the layer thickness higher than 5 ML. Electronic structure of the MnAs/GaN systems was studied by resonant photoemission spectroscopy. That led to determination of the Mn 3d - related contribution to the total density of states (DOS) distribution of MnAs. It has been proven that the electronic structures of the MnAs dots grown by the two procedures differ markedly. One corresponds to metallic, ferromagnetic NiAs-type MnAs, the other is similar to that reported for half-metallic zinc-blende MnAs. Both system behave superparamagnetically (as revealed by magnetization measurements), but with both the blocking temperatures and the intra-dot Curie temperatures substantially different. The intra-dot Curie temperature is about 260 K for the former system while markedly higher than room temperature for the latter one. Relations between growth process, electronic structure and other properties of the studied systems are discussed. Possible mechanisms of half-metallic MnAs formation on GaN are considered.Comment: 20+ pages, 8 figure

Strongly Localized Donor Level in Oxygen Doped Gallium Nitride

A classification in terms of localization of donor defects in GaN is performed by Raman spectroscopy under large hydrostatic pressure. We observe a significant decrease of free carrier concentration in highly O doped GaN epitaxial films at 22 GPa, indicating the presence of a strongly localized donor defect at large pressure. Monitoring the phonon plasmon coupled mode, we find similarities with results on highly n-type bulk crystals. We refine the model of localized defects in GaN and transfer it to the AlGaN system

HIGH PRESSURE FREEZE-OUT OF ELECTRONS IN UNDOPED GaN CRYSTAL. PROOF OF EXISTENCE OF RESONANT DONOR STATE (NITROGEN VACANCY)

We investigated free carriers related opticał absorption in GaN in hydrostatic pressures up to 30 GPa. The disappearance of this absorption at pressures close to 18 GPa was explained by trapping electrons resulting from the shift of nitrogen vacancy related donor level into the GaN energy gap at high pressure. We estimated the energetic position of this level at atmospheric pressure to be about 0.8 eV above the conduction band minimum

High Electron Mobility in AlGaN/GaN Heterostructures Grown on Bulk GaN Substrates

Dislocation-free high-quality AlGaN/GaN heterostructures have been grown by molecular-beam epitaxy on semi-insulating bulk GaN substrates. Hall measurements performed in the 300 K–50 mK range show a low-temperature electron mobility exceeding 60 000 cm2/V s for an electron sheet density of 2.4×1012 cm−2. Magnetotransport experiments performed up to 15 T exhibit well-defined quantum Hall-effect features. The structures corresponding to the cyclotron and spin splitting were clearly resolved. From an analysis of the Shubnikov de Hass oscillations and the low-temperature mobility we found the quantum and transport scattering times to be 0.4 and 8.2 ps, respectively. The high ratio of the scattering to quantum relaxation time indicates that the main scattering mechanisms, at low temperatures, are due to long-range potentials, such as Coulomb potentials of ionized impurities