Real-time High Temperature Research Articles

Real-time high temperature investigations of an individual natural hematite ore particle for chemical looping oxygen exchange

In chemical looping reactors, oxygen carrier’s physical and chemical properties may be continuously modified in response to reduction-oxidation (redox) cycling, affecting the oxygen exchange ability and therefore overall energy efficiency. Reports currently available on hematite’s high temperature redox behaviors have been based primarily on ‘bulk’ material testing in thermogravimetric units or fixed-bed/fluidized bed reactors, followed by post analysis. While ‘bulk’ based analysis effectively provides valuable information on oxygen exchange efficiency as a whole, interpretation of oxygen carrying behaviors occurring in an ‘individual’ particle caused by local particle alterations would be complicated. The present work discusses oxygen exchange behaviors of an ‘individual’ hematite particle based on real-time surface morphology changes subjected to redox cycling. In-operando observations were made possible by utilizing a custom-made high temperature fixed-bed reactor equipped to the confocal scanning laser microscope. The present technique revealed a unique particle expansion behavior that followed a zig-zag (up-down) pattern corresponding to each reduction and oxidation. After 10 redox cycles with 20 consecutive gas exposures, the overall volume was more than doubled, the surface area increased by 25%, and roughness increased by 4 times, resulting in meaningful alterations in the oxygen exchange capability. Reduction-oxidation transformation kinetics was accerelated at each cycle with a directional growth of the product layer over the particle surface; 1 s was long enough for a surface oxidation boundary to travel ≈385 μm after the 1st cycle. The surface transformation kinetics was of 2 dimensional growth in early cycles, which quickly became 3 dimensional growth in later cycles.

Mode I Fracture Toughness Test and Fractal Character of Fracture Trajectory of Red Sandstone under Real-Time High Temperature

To evaluate the stability and compactness of high-temperature underground construction, it is necessary to test the fracture toughness of surrounding rock (red sandstone) under real-time high temperature. In this paper, SCB specimens recommended by the International Society for Rock Mechanics are used to measure the mode I fracture toughness of red sandstone at real-time high temperatures. Also, to reveal its fracture characteristics and fracture mechanism, the fracture morphology observation (SEM experiment), XRD experiment, mercury intrusion porosimetry testing, and fractal measurement of fracture trajectory are carried out on the red sandstone specimens at various temperatures. The results show that (1) temperature may have a significant impact on the fracture toughness and fracture characteristics of red sandstone. On the whole, the fracture toughness values decrease with the increase in temperature, while the fractal dimensions of fractal trajectories increase with the increase in temperature. (2) Temperature has a significant influence on the fracture mode of red sandstone. At relatively low temperatures (20°C–400°C), the main fracture mode is transgranular failure. At relatively high temperatures (400°C–700°C), the fracture mode is mainly intergranular failure. (3) The weakening mechanism of red sandstone is mainly due to the effect of thermal dehydration when the temperature is between 100°C and 400°C. When the temperature is between 400°C and 700°C, the weakening mechanism is mainly due to thermal cracking and theα-βphase transition of quartz.

Enhanced real-time high temperature piezoelectric responses and ferroelectric scaling behaviors of MgO-doped 0.7BiFeO3-0.3BaTiO3 ceramics

The thermal stability of the piezoelectric properties in BiFeO3 based high-temperature piezoceramics is traditionally investigated through room temperature characterization via thermal annealing measurements, which cannot reflect the actual performance at elevated temperatures. In this work, the microstructures, electrical properties, real-time high-temperature piezoelectric responses and ferroelectric scaling behaviors of 0.7Bi1.05(Fe1-xMgx)O3-0.3BaTiO3 (BFMx-BT) ceramics were studied systematically. Doping with 0.6% Mg resulted in enhanced piezoelectric properties with d33 = 170 pC/N and kp = 0.29 along with a high-temperature stability (Td = 440 °C). With a significant jump near the Td, in situ high temperature piezoelectric responses displayed a distinct behavior different from the ex situ high temperature piezoelectricity. Scaling behaviors were presented by studying the field amplitude and frequency dependence of P-E hysteresis loops areas. The behavior of the real-time high temperature piezoelectricity may provide a new insight into the development of new high-temperature piezoceramics.

Fabrication of FBG Strain Gauge Used for High Temperature Strain Monitoring

Fiber Bragg Grating (FBG) sensing technology is widely used in detection of temperature, strain and etc. Now the application of FBG sensor is limited below 200°C. Application over 200°C is still an engineering challenge since no suitable FBG strain gauge. In this paper, FBG strain gauge structure which consists of three FBGs is designed and fabricated based on the theoretical strain and stress analysis. This strain gauge can be used for the real-time high temperature strain monitoring situation. The elastic high-temperature alloy (10MoWVNb) is chosen as the substrate. The three FBGs with a similar performance are fabricated on the substrate by high-temperature glue. Among the three FBGs, FBG1 is used for the horizontal strain monitoring, FBG2 is used for the longitudinal strain monitoring, and FGB3 is used for high temperature cross-sensitive compensation. The fabricated high temperature FBG gauge is demonstrated suitable for high temperature strain monitoring by experiment.

Watching bismuth nanowires grow

We report real-time high temperature scanning electron microscopy observations of the growth of bismuth nanowires via the on-film formation of nanowires (OFF-ON) method. These observations provide experimental evidence that thermally induced-stress on a Bi film is the driving force for the growth of Bi nanowires with high aspect ratios, uniform diameter, and high-quality crystallinity. Our results show that immobile grain boundaries in the Bi film are required for the growth of nanowires so that grain broadening resulting in hillock formation can be prevented. This study not only provides an understanding of the underlying mechanism, but also affords a strategy for facilitating nanowire growth by OFF-ON.

Watching GaN Nanowires Grow

We report real-time high temperature transmission electron microscopy observations of the growth of GaN nanowires via a self-catalytic vapor−liquid−solid (VLS) mechanism. High temperature thermal decomposition of GaN in a vacuum yields nanoscale Ga liquid droplets and gallium/nitrogen vapor species for the subsequent GaN nanowire nucleation and growth. This is the first direct observation of self-catalytic growth of nanowires via the VLS mechanism and suggests new strategies for synthesizing electronically pure single-crystalline semiconductor nanowires.

A nearly real-time high temperature laser–plasma diagnostic using photonuclear reactions in tantalum

A method of measuring the temperature of the fast electrons produced in ultraintense laser–plasma interactions is described by inducing photonuclear reactions, in particular (γ,n) and (γ,3n) reactions in tantalum. Analysis of the γ rays emitted by the daughter nuclei of these reactions using a germanium counter enables a relatively straightforward near real-time temperature measurement to be made. This is especially important for high temperature plasmas where alternative diagnostic techniques are usually difficult and time consuming. This technique can be used while other experiments are being conducted.

Complementary observations of defects in quasicrystals by X-ray topography and electron microscopy

Further information on defects previously [J. Gastaldi et al. Phil. Mag. Lett. 72 (1995) 311.] observed by X-ray topography in quasicrystalline grains, and that show a loop-shaped contrast, was obtained by combining this technique with electron microscopic observations and chemical analyses. The experiments, carried out either at room temperature or during in situ and real-time high temperature annealing, allowed to get clues on the relationships between these loop-shaped defects, the growth process of the quasicrystal grains and the defect behaviour under the annealing treatment.