Temperature Degradation Mechanisms Research Articles

High Temperature Degradation Mechanism of Concrete with Plastering Layer.

At present, the research on the high temperature degradation of concrete usually focuses on only the degradation of concrete itself without considering the effect of the plastering layer. It is necessary to take into account the influence of the plastering layer on the high temperature degradation of concrete. With an increase in the water/cement ratio, the explosion of concrete disappeared. Although increasing the water/cement ratio can alleviate the cracking of concrete due to lower pressure, it leads to a decrease in the mechanical properties of concrete after heating. It is proved that besides the water/cement ratio, the apparent phenomena and mechanical properties of concrete at high temperature can be affected by the plastering layer. The plastering layer can relieve the high temperature cracking of concrete, and even inhibit the high temperature explosion of concrete with 0.30 water/cement ratio. By means of an XRD test, scanning electron microscope test and thermogravimetric analysis, it is found that the plastering layer can promote the rehydration of unhydrated cement particles of 0.30 water/cement ratio concrete at high temperature and then promote the mechanical properties of concrete at 400 °C. However, the plastering layer accelerated the thermal decomposition of C-S-H gel of concrete with a water/cement ratio of 0.40 at high temperature, and finally accelerate the decline of mechanical property of concrete. To conclude, the low water/cement ratio and plastering layer can delay the deterioration of concrete at high temperature.

Dual resonance phonon-photon-phonon terahertz quantum-cascade laser: physics of the electron transport and temperature performance optimization.

The state of the art terahertz-frequency quantum cascade lasers have opened a plethora of applications over the past two decades by testing several designs up to the very limit of operating temperature, optical power and lasing frequency performance. The temperature degradation mechanisms have long been under the debate for limiting the operation up to 210 K in pulsed operation in the GaAs/AlGaAs material system. In this work, we review the existing designs and exploit two main temperature degradation mechanisms by presenting a design in which they both prove beneficial to the lasing operation by dual pumping and dual extracting lasing levels. We have applied the density matrix transport model to select potential candidate structures by simulating over two million active region designs. We present several designs which offer better performance than the current record structure.

High temperature degradation mechanism of a red phosphor, CaAlSiN3:Eu for solid-state lighting

Thermal properties of a red phosphor CaAlSiN3:Eu (CASN) at elevated temperatures were evaluated. A heat treatment at 800 °C degraded the photoluminescence property of CASN and caused irreversible changes in both the excitation and emission intensities. The heat treatment in air simultaneously decreased the N elements and increased the O elements. Consequently, the Eu2+ luminescence center was oxidized and CASN lost its photoluminescence property. Although the crystal structure of CASN host was stable even after the heat treatments, the local structure change around the Eu2+ ions is the origin of the thermal degradation of CASN. We found that the heat treatment in N2 atmosphere suppresses the thermal degradation. This is due to the suppression of N evolutions and the incorporation of O elements, which sustains the optically active Eu2+ state.

In-Situ Diagnostics of Ageing Mechanisms in Li-Ion Batteries Using Entropy Spectroscopy and Incremental Capacity Analysis

Knowledge and the ability to predict capacity decay, battery state of health and ageing mechanism will be vital information to enable long, safe and profitable operation of large Li-ion battery systems. In this paper a 30 Ah high-energy commercial NMC-graphite pouch cell was aged through accelerated cycling at 3 temperatures and 2 selected current rates. The temperatures were 5, 25 and 45 °C, and the current rates were either 1C or 1.5 C for both charge and discharge. The remaining capacity after accelerated cycling at a C/10 discharge at 25 °C is defined as the battery’s state of health (SoH). Several in-situ techniques were applied to characterize and diagnose the observed ageing, e.g.: Entropy spectroscopy (ES) is a battery’s entropy profile with respect to state of charge (SoC) [1]. The entropy spectrum is specific to a Li-ion battery’s anode and cathode chemistry. The changes in entropy spectra can be used as indicators of a battery’s state-of-health [1-3] and a diagnostic tool for ageing mechanisms. This method has been extended at Institute for Energy Technology to record entropy spectra for cells up to 50 Ah size. Incremental capacity analysis (ICA) [4] involves measuring the differentiated of capacity as a function of voltage during charge and discharge, also known as dQ/dV analysis. Dubarry et al claims that ICA on data during ageing can obtain knowledge (qualitative and quantitive) on the degradation mechanisms [4-6]. The entropy spectra for selected cells cycled at the 3 temperatures are presented in the figure. The cells cycled at 5 °C had fast capacity decay, losing more than 20% capacity after less than 20 cycles. A well-known ageing mechanism explaining this is the plating of Lithium metal on the anode electrode during charge at lower temperatures [7, 8]. We observe a shift of the 13 J/Kmol entropy peak for these cells with approx. 20 and 25 % on the SoC axis correlating to the change in SoH. This shift can correspond to the shift of the Li-ion balance between anode and cathode during Lithium plating. The cells cycled at 45 °C obtained more than 500 normalized cycles before reaching a SoH of 80%. However, at this stage, the SoH was declining fast and reached almost 70% after only 530 normalized cycles. In the graph we observed a different change of the entropy spectra for these cells and the spectrum at 72% SoH has a wider and lower entropy peak at about 9 J/Kmol. This change can possibly be interpreted as changes to the electrode materials during cycling. The spectrum for the cell cycled at 25 °C corresponds more to the change observed for cycling at 5 °C and the smaller shift of the 13 J/Kmol peak correlate to the higher SoH of 85%. This suggests that the possible degradation mechanism for the cycling at 25 °C correlates better to a Li-plating mechanism than the high temperature degradation mechanism occurring at 45 °C. The observed in-situ ES results were correlated to an ICA study of the C/10 characterization data at 25 °C, and the conclusions were sought verified by ex-situ analysis techniques opening the cycled pouch cells. References Viswanathan, V.V., et al., Effect of entropy change of lithium intercalation in cathodes and anodes on Li-ion battery thermal management. Journal of Power Sources, 2010. 195(11): p. 3720-3729.Reynier, Y., R. Yazami, and B. Fultz, The entropy and enthalpy of lithium intercalation into graphite. Journal of Power Sources, 2003. 119: p. 850-855.Osswald, P.J., et al., Fast and Accurate Measurement of Entropy Profiles of Commercial Lithium-Ion Cells. Electrochimica Acta, 2015. 177: p. 270-276.Dubarry, M., et al., Incremental capacity analysis and close-to-equilibrium OCV measurements to quantify capacity fade in commercial rechargeable lithium batteries. Electrochemical and Solid State Letters, 2006. 9(10): p. A454-A457.Dubarry, M. and B.Y. Liaw, Identify capacity fading mechanism in a commercial LiFePO(4) cell. Journal of Power Sources, 2009. 194(1): p. 541-549.Dubarry, M., et al., Identifying battery aging mechanisms in large format Li ion cells. Journal of Power Sources, 2011. 196(7): p. 3420-3425.Broussely, M., et al., Main aging mechanisms in Li ion batteries. Journal of Power Sources, 2005. 146(1-2): p. 90-96.Vetter, J., et al., Ageing mechanisms in lithium-ion batteries. Journal of Power Sources, 2005. 147(1-2): p. 269-281. Figure 1

Oxidation performance of nano-scale multilayer coatings on γ-TiAl

There is a major drive to introduce γ-TiAl into gas turbine engines in order to reduce weight. However, this will require the development of coatings that protect against oxidation at high temperature, but do not adversely affect the mechanical properties. This work reports the high temperature degradation mechanisms of a nanoscale CrAlYN/CrN multilayer coating deposited on γ-TiAl(8Nb) by a combined high power impulse magnetron sputtering / unbalanced magnetron sputtering. Detailed TEM/STEM of FIB prepared specimens from isothermal static oxidation tests at 850°C for up to 1030 hours is presented. The evolution of the complex oxide structure and the implications for future coating development is discussed.

Resonant-Phonon Terahertz Quantum-Cascade Lasers and Video-Rate Terahertz Imaging

We review the development of terahertz quantum-cascade lasers (QCLs) that can be uniquely qualified based on a resonant-phonon depopulation scheme. Record performances in terms of operating temperature and optical power output are reported. The best temperature performance is achieved in the metal-metal (MM) waveguides, which provide near-unity mode confinement and low waveguiding loss at terahertz (THz) frequencies even for cavities with subwavelength dimensions. A pulsed operation up to a heat-sink temperature of 169 K at v ~ 2.7 THz and a continuous-wave (CW) operation up to 117 K at v ~ 3 THz are demonstrated with a five-level design that has a two-well injector region. Some of the key temperature degradation mechanisms for this design are discussed. For operation at lower frequencies (v < 2 THz), a one-well injector design is developed that reduces intersubband absorption losses in the injector region. A QCL operating at v = 1.59 THz (lambda = 188.5 mum) up to a heat-sink temperature of 71 K in cw mode is demonstrated with that design. To obtain high-power output and low beam divergence from the MM waveguides, a lens-coupled scheme is demonstrated. A peak power output of 145 mW, a beam width of 4.8deg, and a maximum lasing temperature of 160 K are obtained from a 4.1 THz QCL in this configuration. In the latter part of the paper, we report on the demonstration of video-rate (20 frames/s) terahertz imaging with QCLs as the source for illumination and a 320 times 240 element room-temperature microbolometer focal plane array as the detector. The QCLs for the imaging system are processed into semiinsulating surface-plasmon waveguides, and are operated in a cryogen-free thermomechanical cooler in quasi-CW mode at a heat-sink temperature of ~30 K. Real-time imaging in transmission mode is demonstrated at a standoff distance of 25 m with a v ~ 4.9 THz QCL in this setup.

Low temperature degradation mechanism of tetragonal zirconia ceramics in water: role of oxygen vacancies

A new degradation mechanism is proposed for tetragonal ZrO 2, in which the degradation is assumed to accomplish by the annihilation of oxygen vacancies in the ZrO 2 lattice, while the degradation is supposed to propagate along the grain boundaries. This new mechanism is supported by an XPS study and the studies of other researchers, and it has also satisfactorily explained most phenomenological observations concerning the degradation.