Thermal Defects Research Articles

In-situ temperature monitoring during rotary ultrasonic-assisted drilling of fiber-reinforced composites

Drilling polyetherimide composite is difficult due to the anisotropic and inhomogeneous structure of fiber and matrix. Drilling of fiber-reinforced composites is a thermo-mechanical friction procedure required in various aerospace and automotive applications. The major reason for component rejection during manufacturing processes is due to thermal damage and temperature elevation defects such as delamination and fiber pullout caused during drilling. This work aims to propose a rotary ultrasonic-assisted drilling technique for composite materials to reduce thermal damage using diamond-impregnated tools. The influence of drilling parameters such as rotational speed, feed rate, abrasive grit size, and ultrasonic power was monitored on temperature elevations. The temperature was monitored using thermocouples located at different distances of 1.0, 2.0, and 3.0 mm from the main drilled hole. The response surface methodology was used for the design of experiments during the drilling of composite material and optimization of process parameters was carried out using analysis of variance. Rotational speed and abrasive grit size were observed to have the highest contribution of 42% and 37% for temperature elevations respectively. It was witnessed that temperature increases with an increase in rotational speed, feedrate, and abrasive grit size. Increasing the ultrasonic power during drilling temperature can be minimized. The temperature elevations at the tool-composite interface can be reduced by leveraging the application of rotary ultrasonic-assisted drilling.

High brightness in-band pumped fiber MOPA with output power scaling to >4.6k W.

An in-band pumping technique that employs low numerical aperture (NA) and high brightness fiber lasers emitting at 1018nm enables outstanding power scaling performance by increasing pump power handling capacity of fiber components, reducing thermal load and quantum defect. Here, we present design criteria for an in-band pumped fiber master oscillator power amplifier (MOPA) structure, pumped with 1018nm fiber lasers, as well as mitigation strategies for nonlinear effects for scalable single-mode laser operation at multi-kilowatt power levels. In addition, we report experimental demonstration of an in-band pumped MOPA with an output power scaling up to 4.63kW and a slope efficiency of ∼88%. The MOPA system is pumped by 18×264W high brightness fiber lasers operating at 1017.8±0.3n m with an NA of <0.075. The laser emits at 1080.4nm wavelength with a stimulated Raman scattering suppression of >36.8d B and has a near-diffraction-limited beam quality of M 2∼1.61. To the best of our knowledge, our laser has the highest brightness of 153G W⋅c m -2⋅s r -1 reported at 1080nm in co-pumping configuration.

Tuning optical and optoelectronic properties of polycrystalline lead selenide via low-energy assisted ion beam

Tuning the optical and optoelectronic properties of materials is attractive for solar energy and infrared sensing applications. Here, we proposed an ion beam co-growth preparation method that can continuously tune the optical band gap and absorption and thus optoelectronic properties of lead selenide. The optical band gap of the material can be effectively tuned by ion energy and beam current, varying continuously from 0.79 to 1.03 eV. The tuning mechanism of ion beam on optical and optoelectronic properties was revealed by orthogonal range analysis. The results of range analysis on optical properties show that ion energy and beam current play the key role in adjusting the optical band gap and absorptivity. Ion beam regulation also reduces the Urbach energy of the material, suggesting low thermal disorder and structural defects of the resulting material. The photoresponsivity and specific detectivity of thin film devices can also be effectively tuned by the ion energy and beam current. The material is sensitive to mid-infrared radiation, and the preliminary photoresponsivity is up to 0.61 A/W at 3 µm, which is better than those of traditional thermal diffusion in oxygen atmosphere.

From Electric Doping Control to Thermal Defect Nucleation in Perovskites

AbstractSimultaneous optical microscopy and electric transport measurements on La0.7Sr0.3MnO3 nanobridges, grown on SrTiO3 (STO), show direct evidence of directional oxygen vacancy migration under large voltage bias. Comparative study on discontinuous structures, with voltages applied across a micron‐scale gap, demonstrates that high electric fields induce electrolytic modifications confined to the anode. Extensive electromigration is shown to induce the formation of linear surface scars on the STO substrate, following well defined crystallographic directions. The reproducible triggering of these surface dislocations is demonstrated, unveiling their thermal rather than electrical origin and it is shown that they do not represent electroformed conduction channels. High‐resolution scanning transmission electron microscopy imaging reveals that the scars correspond to nanometer scale steps caused by (101) gliding planes in the STO caused by the proliferation of edge dislocations and local lattice expansion. These findings shed light on selective electrically‐controlled atom migration and its role in structural modification of functional oxides, opening a unique avenue for ionotronic device design.

Evolution of Gesn Lasers Towards Photonic Integration into Practical Applications

GeSn alloys have emerged as a promising material for group IV light sources because alloying Ge with Sn increases the directness of the bandstructure, thus improving the efficiency of light emission. Despite several years of progress in GeSn lasers, however, the integration of such lasers into practical applications still faces challenges such as high threshold, low operating temperature, and large device footprint. In this report, we address these challenges via each of the studies containing thermal management, defect reduction, and nanowire growth approach. First, we demonstrate improved lasing characteristics including reduced threshold and increased operating temperature in GeSn microdisks directly sitting on Si, which is allowed by the enhanced thermal management over the conventional suspended microdisks. Although there is a concern about poor optical confinement of the sitting approach, we confirm the simultaneous achievement of excellent heat dissipation and superior optical confinement from the microdisk released on Si through experiments and theoretical simulations. We also demonstrate a decreased threshold in microdisk lasers fabricated using a high-quality GeSn-on-insulator (GeSnOI) substrate. Photoluminescence measurements show that the reduction of defects in GeSnOI leads to enhancement of spontaneous emission and reduction of the lasing threshold. Lastly, we present the potential for GeSn nanowire lasers having smaller footprints by observing clear cavity resonances in a single nanowire grown by a bottom-up growth approach. Our demonstrations provide guiding principles to push the performance of GeSn lasers to the limit towards a realization of practical group IV light sources.

Novel detection approach for thermal defects: Study on its feasibility and application to vehicle cables

AbstractIt is important to accurately assess the thermal defect state of equipment to ensure its safe operation. A novel thermal defect detection method called the square even‐order derivative method is proposed. First, the derivative functions of temperature were constructed to investigate the main‐side‐peak characteristics of the derivative curve, and the mathematical expressions for the position, depth, intensity of the defects, and parameters of the derivative curve were deduced, which can be used to quantitatively detect the defects. Then, numerical and experimental analyses of an example case of defected epoxy resin plates were carried out to verify that thermal defects can be effectively and accurately detected by the proposed method. Finally, the method was applied to a vehicle cable to detect its thermal defects in another example case of defected vehicle cable, which preliminarily validated the feasibility of the proposed method in practical applications.

Atomistic simulation on the generation of defects in Cu/SiC composites during cooling

The coefficient of thermal expansion (CTE) mismatch between the reinforcement and the matrix results in thermal residual stresses and defects within metal-matrix composites (MMCs) upon cooling from the processing temperature to ambient temperature. The residual stresses and thermally induced defects play an important role in the mechanical properties of MMCs, it is critical to understand the mechanism of defect formation and evolution. This study provides atomistic simulations to reveal the generation of thermal residual stresses, dislocation and incomplete stacking fault tetrahedron (ISFT) during cooling in the idealized Cu/SiC composites. We found that dislocations are generated explosively in a certain temperature range during cooling, which results in a non-linear relationship between dislocation density and temperature. The combined effect of the stresses induced by CTE mismatch and the thermodynamic state of the metal leads to the rapid generation of dislocations. The Shockley partial and the highly stable stair rod are the two dominant dislocation structures. The immobile stair-rod dislocations and the highly stable ISFTs formed in the initial high temperature stage inhibit further development of plastic deformation. The present results provide new insights into the defect formation mechanism and the dislocation strengthening mechanism of MMCs caused by thermal mismatch between constituents.

Localized Unipolar Shear Deformation Autowaves in a Nonequilibrium Paramagnet

It is shown that a nanosecond unipolar soliton-like pulse of the type of a localized shear deformation autowave propagating perpendicular to the magnetic field can be formed in a cubic paramagnetic crystal subjected to longitudinal static deformation in the direction of external magnetic field. The influx of the energy stored in paramagnetic ions into the pulse due to the nonequilibrium initial population of their stationary quantum states is compensated by irreversible losses caused by pulse damping due to its interaction with thermal lattice vibrations, defects, and microinhomogeneities.

High Speed Grinding of EN31 Steel with Alumina Nanofluid Coolant

Grinding is a choice for obtaining high surface quality and closer dimensional tolerance. For meeting this objective economically, the material removal rate (MRR) must be sacrificed to certain extent. The MRR could be increased by either increase in wheel speed, depth of cut, and feed rate. An increase in MRR increases the surface temperature leading to thermal defects on surfaces. Improving the MRR without compromising the surface quality is a challenging objective. Many investigations are progressing for machining various materials under different cooling environments to meet such challenges. This experimental novel aim is to investigate the performance of Al2O3 nanofluid in high-speed grinding of EN31 steel under flood cooling method for reduction of surface roughness and cutting zone temperature. Taguchi’s full factorial design was employed for experimental investigation. The factors such as feed, depth of cut and cutting fluid environment were considered for analysing the responses of cutting zone temperature and surface roughness. The thermal analysis on the work piece was carried out with experimental values by the finite element analysis method. The nanofluids outperform in terms of reduction of surface roughness and downsizing cutting zone temperature. The proposed nanofluid-based grinding significantly reduced the surface roughness and cutting zone temperature.

Thermography evaluation of defect characteristics of building envelopes in urban villages in Guangzhou, China

In many cities, the building envelope is deteriorating and aging. Understanding the defect characteristics of the envelope is essential to increasing the safety of the house and reducing the loss of energy. A 3-day infrared thermography (IRT) experiment was conducted on 5 buildings (11 rooms) in an urban village in Guangzhou, China. The analysis includes qualitative analysis and quantitative analysis. In qualitative analysis, three types of building defects were found, namely, conduction thermal defects, ventilation defects and moisture. In terms of quantitative analysis, (1) an improved temperature factor framework (ITF) is proposed to evaluate conduction thermal defects, (2) the influence of different influencing factors on the thermal performance of the building envelope is analyzed. Qualitative analysis results showed that conduction thermal defects were the most common thermal defects in Nanting Village and occurred 28 times in five residential buildings. It is mainly distributed in the joints of different walls and the joints of walls and other structures. Ventilation defects and moisture were the other two causes of envelope defects. Quantitative analysis showed that orientation has an important effect on thermal defects. The temperature distribution of walls at different heights is different to some extent. Thermal bridges are more likely to occur on extended floors built on the original building.

Numerical investigation of impact of crystal diameter fluctuations on intrinsic point defects distribution in Si crystal grown by Czochralski method

Intrinsic point defects distribution is primarily controlled by the crystal pulling rate and temperature gradient directly at growth interface in silicon crystals grown using the Czochralski (Cz) method. The fluctuation in crystal diameter during the Cz process is one of the primary features associated with the temporal variation in growth process parameters, such as the pulling rate and heater power in the growth system, and which is expected to affect the thermal stress, defects, and dopant incorporation in a growing crystal. In this study, we numerically investigated the effect of temporal variation in crystal diameter on the intrinsic point defects distribution in a silicon crystal grown using the Cz method. The point defect dynamics in the crystal were numerically evaluated within a two-dimensional axisymmetric unsteady global heat and mass transport model for dynamically pulled silicon crystals, considering the shape of the crystal and diameter variation. Unsteady numerical simulations showed that the distribution of intrinsic point defects near crystal side surface in a growing silicon crystal significantly depends on the temporal variation in the crystal diameter.

Microstructural analysis of the thermal fatigue tested Be/CuCrZr joint with artificial defects between Ti/Cu interlayers

The ITER enhanced-heat-flux (EHF) FW fingers undertook by China are made by hot isostatic pressing (HIP) method to bond beryllium armor tiles to CuCrZr/316L(N) heat sinks with 10 μm-thick Ti coating on beryllium tiles and 0.5 mm-thick oxygen free copper sheet as interlayer. To study its thermal fatigue behavior, small scale mock-up was manufactured with artificial defects (AD) on the beryllium tile surface with Ti coating. Thermal cycling of the mockup was carried out at 4.7 MW/m2 high heat flux (HHF) for 5000 cycles. The interface between Be/Ti/Cu and the fracture surface on beryllium tiles were observed by stereomicroscopy and scanning electron microscopy with energy spectrometer. The Be/Ti/Cu interfacial phase composition and the behaviors of defects after high heat flux test were analyzed. In addition, the generation and propagation mechanism of thermal fatigue defects were preliminarily studied in this paper. The results show that uniform bonding between Be and Cu was achieved and the interfacial phases are Be base/ Be-Ti intermediate/ α-Ti/ Ti2Cu/ TiCu /Cu base in the sequence. The formation of complex brittle phases reported in previous study, such as Ti3Cu4 or α-TiCu4, can be avoided by using Ti single interlayer. When the original defect dimension is only 3 × 3 mm2, the defect will not propagate after 5000 cycles 4.7 MW/m2 HHF testing (HHFT). Besides, decreasing beryllium tile size can reduce the possibility of thermal fatigue failure. In addition, from the research on fracture surface on beryllium tiles, the final fracture made for beryllium detachment mainly happened in Ti layer near the beryllium side. While thermal fatigue cracks preferentially originated at Be/Ti interface and the oxidation of beryllium surface under thermal cycling test would accelerate the generation and propagation of thermal fatigue defects.

Prediction of the grinding wheel specification influence on thermal defects in dry grinding through a hierarchical FEM model

Prediction of the grinding wheel specification influence on thermal defects in dry grinding through a hierarchical FEM model

Variable temperature thermal droop characteristics of 255 nm UV LED

Thermal droop, i.e., the loss of emission efficiency over a certain temperature range, is an important performance bottleneck for the successful commercial application of deep-ultraviolet light emitting diodes. In this study, we examined the mechanism of two thermal droop processes of 255 nm AlGaN quantum well light emitting diodes under temperature stresses in order to obtain steady optical output in a broad temperature range. We discovered that the increase in leakage current in the low forward bias region is accompanied by a decrease in apparent carrier concentration of quantum wells near the p side during the thermal droop process at high temperature (&amp;gt;300 K), indicating that the activation of thermal defects enhances the trap assisted tunneling effect and causes the optical power to decrease more significantly at low current. Compared with normal temperature, the low emission power at low temperatures is attributed to the minority trap H1, which has an activation energy of 0.527 eV at 190 K, according to deep level transient spectrum analysis. At low temperatures above 175 K, the optical power increases as the temperature rises due to enhanced hole injection. By analyzing the droop characteristics, we concluded that the activation of thermal defects is the most probable cause of high temperature thermal droop in 255 nm AlGaN quantum well light emitting diodes, whereas hole trap H1, which is linked to gallium vacancy complexes related defects, is most likely the source of low temperature thermal droop.

Features of recognising the type of defect in oil-filled equipment using the nomogram method

The paper presents the results of analysis of nomograms (graphical areas) of defects in oil-filled equipment of electric networks. The input data were the results of dissolved gas analysis for 2997 transformers and shunt reactors with different types of defects, that is, the training sample. In order to level out possible contradictions in using different criteria, the training sample was split into separate data sets, not only with the same defect type, but also with identical values of characteristic gas ratios, gas percentages and gas-to-gas ratios with maximum concentration. To account for drift of coordinate values of individual nomograms in the obtained arrays, it is proposed to represent defect nomograms in the form of reference regions. The maximum and minimum coordinate values (ratios of each of the gases to the gas with the maximum concentration) obtained for homogeneous arrays of DGA results are used as values of the boundaries of reference regions. The centre of the graphic area coincides with the reference nomogram. As a result, 115 nomograms characteristic of thermal type defects, electrical discharges as well as overheating with different hot spot temperature accompanied by discharges with different energy density and discharges with different energy density accompanied by overheating with different hot spot temperature have been drawn. A brief analysis of the obtained graphic areas is given, the most characteristic damages corresponding to one or another graphic area are considered, and the values of characteristic gas ratios corresponding to the analysed areas are analysed. In the process of analysis it was established that the highest reliability value of defect type recognition can be achieved by simultaneous use of all three diagnostic criteria, namely, values of gas ratios, gas percentages and nomograms (graphic areas) of defects. The obtained results make it possible to significantly increase the number of reference nomograms, which will significantly increase the number of recognisable defects and consequently reduce the risk of accidental damage to oil-filled equipment due to missing defects caused by failure to recognise them.

Recognition of overheating in different temperature ranges, which is accompanied by discharges of different intensity, based on the dissolved gas analysis

In wartime and post-war reconstruction, it is the responsibility of a rather ageing fleet of electricity generation equipment to provide a reliable supply of electricity to the households and industry. The challenge is to ensure operational reliability and to extend the service life of this equipment. Non-destructive testing methods are the most practical in terms of operation. For non-destructive testing of high-voltage power transformers, the dissolved-oil gas analysis is the most common and widely used method. This method, using various diagnostic criteria (value of gas ratios, percentage of gases, etc.), allows most of the defects (both electrical and thermal type) occurring in a transformer to be detected. However, during operation, so-called combined defects can occur, i.e. discharges turning into overheating, or overheating accompanied by discharges. In addition, two or more defects of different types can occur in the same transformer. The values of the criteria used for the recognition of the combined defects differ significantly from the values of the criteria typical for thermal or electrical defects, which significantly complicates the recognition process. The purpose of this article is to analyse the values of diagnostic criteria as well as the recognition reliability of overheating in different temperature ranges, accompanied by discharges with different intensity. For a comprehensive analysis, the results of dissolved gas analysis were used for 471 high-voltage power transformers in which overheating in different temperature ranges, accompanied by discharges of different intensity, were detected. The 471 analysed values were split into 38 arrays with similar gas percentages, gas ratios and defect nomograms. Based on an analysis of the recognition reliability of overheating in different temperature ranges, using the norms and criteria regulated by the best known standards and methods, it was found that the highest recognition reliability is provided by the values of gas ratios, which are regulated by the ETRA Square and also by Nomogram method.

Critical Conditions for Dynamic Recrystallization of S280 Ultra-High-Strength Stainless Steel Based on Work Hardening Rate

Isothermal and constant-strain-rate compression experiments for S280 ultra-high-strength stainless steel were carried out under deformation temperatures of 1000–1150 °C and strain rates of 0.001–10 s−1 with a Thermecmaster-Z thermal simulator. The flow–stress behavior of the alloy was studied and the hot deformation activation energy was calculated. A critical strain model of the dynamic recrystallization (DRX) of the alloy was established using the work hardening rate for the first time. The results show that S280 ultra-high-strength stainless steel was positively sensitive to the strain rate and negatively sensitive to temperature, and its flow–stress curve showed characteristics of flow softening. The hot deformation activation energy corresponding to the peak strain was 519.064 kJ/mol. The DRX critical strain of the steel was determined from the minimum value of the −∂(lnθ)/∂ε − ε curve. The relationship between the DRX critical strain and peak strain could be characterized as εc=0.599εp and the relationship between the DRX critical stress and peak stress could be characterized as σc= 0.959σp The critical strain model of DRX could be expressed as εc=0.010Z0.062. The research results can provide theoretical support for avoiding the generation of actual thermal processing microstructure defects such as coarse grains and for obtaining products with excellent microstructure and properties.

MAIN CHARACTERISTICS OF THE HEAT FLOW METER

The problems and prospects of application of non-destructive testing methods for technical diagnostics of the thermal networks state and various technological objects are discussed. The recording of the temperature state of thermal processes by using a flow meter is due to its sensitivity to the change of thermophysical characteristics and the ability to control without the use of an external energy source, etc. The description of the developed device to measuring of the heat flow using of a thermoelectric heat flow converter of a special design is shown. A distinctive feature of the device is the heating element that installed on an insulating layer serving as a support surface. Calibration of the device is proposed to be carried out by replacing the heat flow from the investigated object with the heat flow released in the heating element when an electric current passes through it. The developed device can register the changes in the heat flux density in the range of (25-100) W/m2, which allows it possible to detect the smallest thermal insulation defects.

A feasibility study on high-efficient laser cutting of SiC particles reinforced aluminum matrix composite using single-pass strategy

Conventional machining of particles reinforced metal matrix composite (PRMMC) is very challenging due to the uniform distribution of abrasive reinforced SiC particles, which easily causes machined surface scratches and severe tool wear. The rapid development of high-power fiber laser has made it possible to cut PRMMCs with high efficiency. In this work, high-power fiber laser was employed to cut 5.0 mm thick SiC particle reinforced aluminum (SiCp/Al) composites. The effects of laser power, cutting speed and assist gas pressure on surface morphology and defect characteristics were investigated. The statistical analysis was conducted to investigate interaction between cutting parameters and kerf geometry. A high-resolution confocal laser scanning microscope was used to further analyze machined surface morphology and microscopic defects. Thermal defects were highly related to cutting parameters, which included cataphracted dross, droplet dross, craters, apophyses, microcracks and combustion. Microcracks were prevalent mainly due to the difference in thermal properties between reinforcement and matrix materials. Three dimensional finite element simulation of two-phase SiCp/Al composites was carried out to predict the temperature and stress distribution, and the striation formation mechanisms were further explained. A new method for predicting thermal stress of SiCp/Al composites was proposed and subsequently compared/validated with experimental results.

Laser-arc hybrid additive manufactured Al[sbnd]Cu alloy for T-shaped structure: Microstructure evaluation and molten pool behavior

T-shaped structure is widely used in the aeronautic and aerospace industries for the development of lightweight applications. However, poor metallurgical bonding and thermal defects by remelting at the intersection is prone to occur in the additive manufacturing of the structure. In this study, T-shaped structure of AlCu alloy was fabricated by laser-arc hybrid additive manufacturing (LAHAM), and the molten pool behavior of the process was analyzed. According to the remelting of intersection, it was divided into the top layer and the remaining layers. The remelting zone of intersection (RZI) was obvious in the top layer. During the solidification of molten pool from the boundary to the center, the ratio of temperature gradient G to solidification rate R (G/R) at the S/L interface decreased from 2.57 × 108 (K·s)/m2 to 9.27 × 107 (K·s)/m2, and promoted the transformation of equiaxed dendrites. In the remaining layers, the recoil pressure, much smaller G/R and larger G∙R made the grains as fine equiaxed along the z direction in the laser zone (LZ), while coarse columnar were observed in the arc zone (AZ). Due to the microstructure changes, the deposited T-shaped structure exhibited a yield strength of 146.5 ± 2.2 MPa and an ultimate tensile strength of 290.3 ± 5.9 MPa.