Degradation Of Thermal Stability Research Articles

Emerging continuous SiC fibers for high-temperature applications

Silicon carbide (SiC) fibers are responsible for the ultimate strength and toughness of SiC-fiber reinforced composites in harsh environments. The development of a new generation of continuous SiC fibers continues to advance the mechanical properties of composite materials. Tyranno™ SA4 fiber was recently released as a successor of Tyranno™ SA3 fiber. Laser-driven chemical vapor deposition (LCVD) has been adopted as an alternative fiber processing route to synthesizing high-strength SiC fiber with tailorable small diameters and chemical compositions. Both Tyranno™ SA4 and laser-driven CVD fibers show very high tensile strength, about 4 GPa in the as-fabricated condition. The degradation of thermal stability and strength due to annealing in an inert environment were similar for Tyranno™ SA3 and SA4 fibers because of their similar carbon-rich, crystalline microstructure. Silicon-rich fibers produced by LCVD possessed heterogeneous crystallinity, which was attributed to laser power distribution and showed microstructural instability at 1500 °C and above. The new SiC fibers demonstrated an increase in as-fabricated strength but faced the same challenges in environmental resistance as the traditional SiC fibers do.

Degradation of thermal stability and micromechanical properties of the C-S-H phase induced by ultra-confined water at elevated temperatures.

Water in the nanometer to micrometer-sized pores of calcium silicate hydrate (C-S-H) is essential for the binding process of cementitious materials. The quantity, location, and physical state of water in C-S-H pores under extreme conditions significantly influence the strength and durability of cementitious materials. The present study employed ReaxFF and molecular dynamics (MD) simulation to evaluate the effects of water ultra-confined in the nanopores on the structure, bonds, dynamics, and tensile mechanism of the C-S-H grains at elevated temperatures. The results indicate that the temperature elevation may interfere with the water molecule's hydrogen-bond network between the C-S-H grains, causing a notable nanometer-scale pore expansion. Simultaneously, the diffusion coefficient of water molecules confined in nanopores gradually increased, and their dynamic characteristics shifted from a glassy nature to free water. Additionally, high temperatures promoted hydrolysis reactions and the breakage of chemical bonds in the C-S-H framework, causing disintegration of the silicate skeleton and a decrease in the mechanical attributes of C-S-H. Moreover, the uniaxial tensile test at high temperatures revealed that the silicate chain groups in the C-S-H substrates underwent thermal curling. In contrast to interlayer-bound water, under the action of tension, water molecules in nanopores are viscous, forming water layers.

Evaluation of Heat Generation and Thermal Degradation of Lithium Ion Batteries by a Calorimetry Method

With the wide application of lithium-ion batteries (LIBs), it is important to understand the internal heating effects and thermal runaway behavior of such batteries to evaluate their thermal safety and improving thermal management systems. The heat generation of LiCoO2/graphite LIBs under various conditions was compared using calorimetry and electrochemical methods. The heat generation results of the two methods in the process of charging/discharging at different current rates were consistent, but the calorimetry method is simple, convenient, and more feasible. The greater the working current, the greater the irreversible heat generation. Through the identification of the thermal runaway behavior, it was found that the change of the onset temperature of self-heating reactions fluctuates in a narrow range. Thermal runaway after battery degradation is more likely to be triggered by the degradation of thermal stability.

Perpendicular Magnetic Anisotropy for CoFeBZr/MgO

Magnetic tunnel junctions (MTJs) using perpendicular magnetic anisotropy (PMA) have been studied for high density magnetoresistive random access memory (MRAM) with the expectations of scalability for driving current and reducing switching anomalies by local magnetization non-uniformity. CoFeB which has been well developed for MTJ using in-plane magnetization, is also a favorable choice for perpendicular magnetization with the stalking structure of Ta/CoFeB/MgO because its interfacial anisotropy results low current density (J c ) for magnetization reversal by spin transfer torque (STT) [1]. However the practical MRAM technology requires further reduction of J c without degradation of thermal stability. The Gilbert damping measurement showed that Zr insertion provides reduction of the damping constant (α) for the Co film with in-plane magnetization [2]. We present the effects of Zr insertion (5%) in Co(65)Fe(20)B(10) for PMA due to interfacial anisotropy with MgO. Perpendicular magnetization is successfully formed up to 1.56 nm of CoFeBZr with the stalking structure of Ta/CoFeBZr/MgO. The damping is determined as α = 0.004 from the line width of ferromagnetic resonance (FMR) which is lower than value (∼ 0.008) for CoFeB [3]. Moreover, the effective anisotropy energy (K eff ) for maximum value shows 0.163 MJ/m3 for CoFeBZr which is comparable to that 0.183 MJ/m of CoFeB [4]. This study results that insertion of Zr in CoFeB suggests a method for low current switching of MTJ using PMA.

Si-SiO 2 interface passivation by plasma NH 3 and atomic H

Plasma ammonia treatment at 400 °C leads to de-passivation of a fully hydrogenated Si-SiO 2 interface, and to passivation of a fully de-hydrogenated Si-SiO 2 interface. Plasma NH 3 exposure causes irreversible Si surface damage and degradation of thermal stability. Atomic hydrogen exposure, although it results in similar effects on the Si-SiO 2 interface, does not introduce additional defects or a decrease of the Si surface thermal stability. The difference between plasma NH 3 exposure and atomic H exposure is speculated to be due to either the nitridation of Si-SiO 2 interface or radiation damage resulting from plasma NH 3 exposure. EPR measurements indicate changes of the paramagnetic defect properties and an increase in the paramagnetic defect density generated by plasma NH 3 exposure.

Recording performance of CoCrPt-(Ta, B)/TiCr perpendicular recording media

The magnetic properties, microstructure and the recording performances of CoCrPt-(Ta, B)/TiCr perpendicular magnetic recording media were studied in conjunction with the effect of thermal agitation. The addition of Ta or B to CoCrPt media significantly improves its low noise performance, mainly due to the reduction of grain size and intergranular magnetic interactions. However, adding Ta or B also reduces the value of perpendicular magnetic anisotropy, K/sub u/, especially in the initial growth layer, resulting in the degradation of thermal stability. In order to achieve high thermal stability without degrading the low noise performance of these media, an M/sub r//M/sub g/ value of nearly 1.0 is required. This may be achieved by suppressing of the reduction of K/sub u/ and the reducing intergranular magnetic interactions, whilst maintaining their small grain size.

Thermal aftereffects in thin film magnetic recording media

Thermal aftereffects in written bits in CoCrPt thin film magnetic recording media are examined using a magnetoresistive (MR) head and a magnetic force microscope (MFM). Decays in signal output over time at room temperature in media with very thin magnetic layers of about 14-nm thickness or less were observed by using an MR head. The decay in signal output was accompanied by an increase in medium noise. Even with a precise examination of cross-track profile using an MFM, no change in track width caused by thermal aftereffects can be detected. Thermal decay of the signal output in a 14-nm thick magnetic layer with small remanence thickness product M/sub r/t/sub mag/ of about 40 G-/spl mu/m was successfully reduced by increasing the platinum content in the CoCrPt magnetic layer or by adding boron to the CrTi underlayer without increasing the medium noise. The reduction of thermal decay is considered to be caused by the increase of magneto crystalline anisotropy and increase in magnetic-crystal-grain size. These results suggest that it is possible to reduce the medium noise without reducing the crystal grain size and without much degradation of thermal stability even for media with small M/sub r/t/sub mag/ which is needed to achieve a recording density of 10 Gb/in/sup 2/.

Effects of thin Cr interlayer on time decay of magnetization and magnetization reversal for CoCrTaPt thin film media

The effect of thin Cr interlayer on the time decay of magnetization and magnetization reversal for CoCrTaPt/Cr/CoCrTaPt media is presented. The time decay of magnetization was measured at various reverse magnetic fields. The maximum magnetic viscosity coefficient which occurs around the remanent coercivity increases with increasing interlayer thickness, which implies the degradation of thermal stability. The activation volume decreases with increasing Cr interlayer thickness. Both torque magnetometer and the law of approach to saturation methods were used to measure the magnetic anisotropy. The results show a decrease in anisotropy with increasing interlayer thickness which may explain the degradation of thermal stability. The distribution of rotational hysteresis losses (W/sub r/) were measured and the rotational hysteresis integral (R/sub h/) was found to decrease with increasing Cr interlayer thickness which may be caused by reduction of the magnetic interaction.

Failure mechanism of a thermal barrier coating system on a nickel-base superalloy

An investigation was carried out to determine the failure mechanism of a thermal barrier coating system on an Ni-base superalloy. The coating system consisted of an outer layer of yttria-stabilized zirconia (top coat), and an inner layer of Pt-aluminide (bond coat). Specimens were exposed at 1010 and 1150 °C with a 24-h cycling period to room temperature. Scanning electron microscopy combined with energy dispersive X-ray spectroscopy as well as X-ray diffraction were used in microstructural characterization. Spallation of the oxide scale developed by the bond coat was found to be the mode of failure. Experimental results indicated that the breakdown of oxide was affected by internal oxidation of Hf diffusing from the alloy substrate into the bond coat surface developing localized high levels of stress concentration at the oxide–bond coat interface. It was concluded that the cause of failure was degradation of thermal stability of the bond coat accelerating its oxidation rate and permitting outward diffusional transport of elements from the substrate.