High Temperature Instabilities Research Articles

Detonation simulations in supersonic flow under circumstances of injection and mixing

The unsteady, reactive Navier-Stokes equations with a detailed chemical mechanism of 11 species and 27 steps were employed to simulate the mixing, flame acceleration and deflagration-to-detonation transition (DDT) triggered by transverse jet obstacles. Results show that multiple transverse jet obstacles ejecting into the chamber can be used to activate DDT. But the occurrence of DDT is tremendously difficult in a non-uniform supersonic mixture so that it required several groups of transverse jets with increasing stagnation pressure. The jets introduce flow turbulence and produce oblique and bow shock waves even in an inhomogeneous supersonic mixture. The DDT is enhanced by multiple explosion points that are generated by the intense shock wave focusing of the leading flame front. It is found that the partial detonation front decouples into shock and flame, which is mainly caused by the fuel deficiency, nevertheless the decoupled shock wave is strong enough to reignite the mixture to detonation conditions. The resulting transverse wave leads to further mixing and burning of the downstream non-equilibrium chemical reaction, resulting in a high combustion temperature and intense flow instabilities. Additionally, the longitudinal and transverse gradients of the non-uniform supersonic mixture induce highly dynamic behaviors with sudden propagation speed increase and detonation front instabilities.

Feasible acid-etching method for investigating temperature-dependent domain configurations of ferroelectric ceramics

Domain structure often plays significantly important roles in determining both the piezoelectric properties and the piezoelectric temperature stabilities of ferroelectric ceramics. However, a convenient and reliable technique for studying temperature-dependent domain configurations is greatly desired. Inspired by “burning CDs”, we proposed here a temperature-variable acid-etching method (abbreviated hereafter as VTAE) for such purpose. Utilizing this method, domain configurations of the poled 0.96(K0.48Na0.52)(Nb0.96Sb0.04)O3–0.04(Bi0.50Na0.50)ZrO3 (abbreviated hereafter as KNNS-4%BNZ)lead-free ceramic were investigated at different temperatures. Diversified domain patterns that consist of parallel-stripe domains, herringbone domains and wedge-shaped domains are recognized in different phases, respectively. It is found from the evolution of domain configurations with temperature that nanodomain structure is closely related to the high piezoelectric responses and temperature instabilities in the KNNS-4%BNZ ceramic. The result proves that this VTAE method is a feasible method for studying the temperature-dependent domain configurations of ferroelectric ceramics.

Nanodomain Engineered (K, Na)NbO3 Lead‐Free Piezoceramics: Enhanced Thermal and Cycling Reliabilities

The growing environmental concerns have been pushing the development of viable green alternatives for lead‐based piezoceramics to be one of the priorities in functional ceramic materials. A polymorphic phase transition has been utilized to enhance piezoelectric properties of lead‐free (K, Na)NbO3‐based materials, accepting the drawbacks of high temperature and cycling instabilities. Here, we present that CaZrO3‐modified (K, Na)NbO3 piezoceramics not only possess excellent performance at ambient conditions benefiting from nanodomain engineering, but also exhibit superior stability against temperature fluctuation and electrical fatigue cycling. It was found that the piezoelectric coefficient d33 is temperature independent under 4 kV/mm, which can be attributed to enhanced thermal stability of electric field engineered domain configuration; whereas the electric field induced strain exhibits excellent fatigue resistance up to 107 sesquipolar cycles. These findings render the current material an unprecedented opportunity for actuator applications demanding improved thermal and cycling reliabilities.

High temperature instabilities of ohmic contacts on p‐GaN

AbstractThis paper describes an investigation of the instabilities of ohmic contacts on p‐type GaN submitted to long term high temperature stress. Transfer Length Method (TLM) was used to separately analyze the contribution of contact resistivity and sheet resistance on contact degradation. Before treatment, contacts showed linear behaviour, indicating good ohmic properties. On the other hand, thermal ageing at 250 °C induced the degradation of electrical characteristics: in particular, (i) an increase of the contact resistivity was detected after stress, as well as (ii) a rectifying behaviour in current‐voltage (IV) characteristics of contact appeared. The degradation process has been ascribed to the diffusion of an atomics species from PECVD‐deposited passivation layer to p‐type ohmic contact: the interaction of this species with acceptor dopant can lower acceptor concentration, thus broadening the Schottky barrier at the interface and inducing the observed non‐linearity of the contacts. The square root time dependence of degradation of the electrical characteristics agreed with the hypothesis of diffusion. After 160 hours of aging, the passivation layer was removed and the structures were submitted to a new thermal treatment. This further treatment was found to induce the recovery of the electrical characteristics of the contacts, due to the fact that passivation acts like (i) a source of atomic species responsible of diffusion and/or (ii) a blocking layer for these species: the removal of passivation stopped the diffusion process and make outgassing process possible. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Reversible Degradation of Ohmic Contacts on p-GaN for Application in High-Brightness LEDs

This paper analyzes the high-temperature long-term stability of ohmic contacts on p-type gallium nitride (p-GaN). The contributions of the ohmic contacts and semiconductor material degradation are separated by adopting the transmission line method (TLM). Before stress, the current-voltage (I-V) curves measured at the pads of the TLMs showed a linear shape, indicating a good ohmic behavior of the contacts. Thermal treatment at 250degC was found to induce the worsening of the electrical characteristics of the contacts: identified degradation modes consist of a shift of the I-V curves toward higher voltages and strong nonlinearity of the characteristics around zero. This paper shows that the high-temperature instabilities of ohmic contacts on p-GaN are related to the interaction between the device surface and the plasma-enhanced chemical vapor deposition SiN passivation layer. Hydrogen contained in the passivation layer is supposed to play an important role in the degradation process: the interaction with the acceptor dopant at the metal/semiconductor interface induces the decrease of the effective acceptor concentration. As a consequence, both the ohmic contact characteristics and the semiconductor sheet resistance are worsened.

Thermal Stability of HfN Compounds on HfO2/SiO2 Gate Stacks

The thermal stability of HfN compounds on HfO2/SiO2 gate stack has been investigated. Before annealing HfN and HfSiN were found to have a work function of a band-edge N metal. Films have very good thermal stability as measured by Rutherford backscattering. Gate stack high temperature instabilities were observed to occur at the SiO2/Si interface because of the high oxygen solubility of HfN and HfSiN which tends to reduce the underlying SiO2 layer. Attempts to improve the high temperature gate stack stability by incorporating oxygen into the Hf compounds to reduce its oxygen solubility produced some SiO2 interfacial layer re-growth and a work function shift toward mid gap.

Interface instabilities and electronic properties of ZrO2 on silicon (100)

The interface stability of Zr-based high-k dielectrics with an oxide buffer layer was explored with x-ray (hυ=1254eV) and ultraviolet (hυ=21.2eV) photoemission spectroscopy. Zirconium oxide films were grown and characterized in situ in a stepwise sequence to explore their chemical stability and electronic properties as a function of film thickness and processing conditions. The buffer layers serve to lower the interface state density and to address the high temperature instabilities of ZrO2 in direct contact with Si. This research addresses three issues: (1) the development of the band offsets and electronic structure during the low temperature (T<300°C) growth processes, (2) variations in the band structure as effected by process conditions and annealing (T<700°C), and (3) the interface stability of Zr oxide films at high temperatures (T>700°C). Annealing the as-grown films to 600°C results in an ∼2eV shift of the ZrO2-Si band alignment, giving a band offset that is, favorable to devices, in agreement with predictions and in agreement with other experiments. We propose that the as-grown films contain excess oxygen resulting in a charge transfer from the Si substrate to the internal (ZrO2-SiO2) interface and that annealing to 600°C is sufficient to drive off this oxygen. Further annealing to 900°C, in the presence of excess Si at the surface, results in decomposition of the oxide to form ZrSi2.

Non-perturbative temperature instabilities in N = 4 strings

We derive a universal thermal effective potential, which describes all possible high-temperature instabilities of the known N = 4 superstrings, using the properties of gauged N = 4 supergravity. These instabilities are due to three non-perturbative thermal dyonic modes, which become tachyonic in a region of the thermal moduli space. The latter is described by three moduli, s, t, u, which are common to all non-perturbative dual-equivalent strings with N = 4 supersymmetry in five dimensions: the heterotic on T 4 × S 1, the type IIB on K 3 × S 1, and the type I on T 4 × S 1. The non-perturbative instabilities are analyzed. These strings undergo a high-temperature transition to a new phase in which five-branes condense. This phase is described in detail, using both the effective supergravity and non-critical string theory in six dimensions. In the new phase, supersymmetry is perturbatively restored but broken at the non-perturbative level. In the infinite-temperature limit the theory is topological with an N = 2 supersymmetry based on a topologically non-trivial hyper-Kähler manifold.

Transport equations and QCD collective modes in a self-consistent covariant background gauge

Following an interpretation of high-temperature instabilities of perturbative QCD, a self-consistent covariant background gauge quantization is proposed. Non-perturbative classically gauge invariant gluon, ghost, and quark equations of motion for coupled one- and two-point functions are derived in binary collision approximation (RPA). A stability analysis is described to determine coloured and colourless collective modes. Preliminary results indicate non-perturbative behaviour of the quark-gluon plasma involving self-consistent classical background fields. It leads to mass generation for gluons as well as for collective modes.