Papers by Keyword: Thermal Stress

Abstract: The development of integrated circuits (IC) is mainly driven by the advanced processes and increased silicon wafer diameter in the past several decades. It is technically believed that the diameter of 300 mm for SiC wafer is too difficult to be achieved since SiC crystal diameter expansion is a long and tough process, the growth process of which is different from that of silicon crystal. Herein, we demonstrate the diameter expansion process of SiC crystal from 200 mm to 300 mm using physical vapor transportation (PVT) method and show the world’s first 300 mm 4H-SiC single crystal substrate with 100% 4H polytype. The driving force of crystal diameter expansion and resultant thermal stress are discussed in this paper. Based on the successful preparation of 300 mm SiC seed crystal, 12-inch high-purity, conductive n-type & p-type SiC substrates are subsequently fabricated. Quality characterization of the 300 mm SiC substrate shows very low micropipe and threading screw dislocation density below 0.05 cm^-2 and 120 cm^-2, respectively. Furthermore, both 500 μm and 750 μm thickness substrates are fabricated with bow and warp values lower than 10 μm and 30 μm, indicating high quality 300 mm substrates applicable in power devices and other emerging areas.

Abstract: We have demonstrated a novel process that precisely controls the wafer bow, a key parameter of overall warpage, of 4H-SiC wafers to any desired value by integrating a thermal sublimation process, Dynamic AGE-ing® (DA), immediately prior to the CVD epitaxial process. This method achieves atomic-level flatness of the CMP-finished surface independent of the growth and etching amounts, while concurrently eliminating sub-surface damage (SSD). When DA is applied to simultaneously etch the Si-face and grow the C-face, the wafer bow decreases linearly with increasing C-face growth. In wafers with poorer mechanical processing quality, an increase in bow is observed for C-face growth below 120 nm, likely due to the relaxation of SSD on that side. The process also removes SSD from the Si-face, ensuring that both sides are sufficiently cleared of damage. Furthermore, for wafers with an initially negative bow, simultaneous Si-face growth and C-face etching using DA produces a linear increase in bow. By applying these processes, we successfully adjusted the bow to –0.4 µm on an 8-inch wafer that initially measured –12.0 µm. These results indicate that, regardless of the initial bow severity, precise control of the wafer bow can be achieved without adversely affecting subsequent CVD epitaxy processes by appropriately managing the growth layer thicknesses on both faces using DA.

Abstract: A method of thermal structure analysis is developed for supersonic missile rudder. The method calculates the aerodynamic heat based on Eckert reference enthalpy method, and calculates the temperature field by finite element method. The temperature field is taken as the volume load while calculating the thermal stress gradient and time history. Considering the influence of thermal stress, the natural vibration characteristics such as the mode and vibration frequency with time are obtained. The numerical computation results show that the aerodynamic heat is distributed unevenly across the rudder, such as stagnation point, laminar flow, transition and turbulence. The solid blocks of rudder shaft, longitudinal and transverse wall plates cause local low temperature, and large temperature gradient leads to higher thermal stress. Due to the change of material properties and thermal stress, the natural vibration frequency of the rudder has a significant decrease at sometime. The software of ANSYS was used to calculate and output cloud diagrams. The software of Microsoft Office was used to make curve graphics. The developed APDL command flow is very efficient and portable, which is convenient for complex structural model, and can provide technical support for supersonic missile rudder.

Abstract: A temperature stress testing machine (TSTM) was used to investigate effect of cement composition and temperature history on thermal stress of concrete. Results show that the benefit of higher amount of C3S on concrete strength enhancement was compromised by the relatively higher temperature rise at early age, leading to a lower temperature difference. However, by means of a delicate design of cooling history, the deficiency of concrete with higher amount of C3S can be compensated and the cracking sensitivity was closer to that of concrete with lower amount of C3S.

Abstract: Fiber Reinforced Cementitious Matrices (FRCM) represent a very efficient strengthening solution for the improvement of the structural behavior of existing masonry constructions. However, the mechanical performances of these materials could be significantly affected by different environmental conditions, such as the exposure to thermal stresses. This aspect should be properly assessed for a correct design of the strengthening interventions. The Italian Guidelines for the identification, qualification and acceptance control of FRCM systems to be used for the structural strengthening of existing constructions prescribe the execution of direct tensile tests on FRCM coupons to evaluate the behavior of these composite systems subject to thermal stress. Within this framework, the objective of the present research is to evaluate the effect of a thermal stress on the tensile behavior of three different FRCM systems, composed by steel or basalt fibers and lime-based or cement-based mortar matrix. Tensile tests are performed, for each system, on samples at ambient temperature (22°C) and on samples conditioned at 80°C for six hours, according to the indications provided by the Guidelines. The test results show that the effect of the thermal stresses is more evident in the first phases of the tests, i.e. during the elastic phase and the mortar matrix cracking phase, while it is less significant in the last phase, which is related to the fibers behavior. The tensile strength of the investigated FRCM systems is, indeed, within the limits provided by the Italian Guidelines, while the curves of the conditioned samples may deviate from the reference ones at the beginning of the tests. These findings are critically discussed within the paper.

Abstract: As a new type of micro device, the reliability of thin film thermocouple sensor is very important in actual work. Aiming at a thin-film thermocouple sensor with NiCr as the functional layer, three main stresses of the thin-film sensor in actual work are analyzed: thermal stress, vibration stress and centrifugal stress. Various stress analyses have been verified through theoretical calculations or finite element simulations. The results show that thermal stress is the main influence stress, and the critical areas of reliability have been obtained. The prepared thin film sensor samples were tested through high temperature experiments, and the analysis results were verified. When a thermal load of 700 degrees Celsius is applied, the stress environment of the protective layer, the functional layer and the insulating layer of the structure is severe, and it is vulnerable to severe damage, and cracks are found in the insulating layer, which becomes a weak area of reliability.

Abstract: Aiming at the reliability of thin-film thermocouples applied to turbine blades at high temperatures, combined with high-temperature tests and finite element analysis, this paper studies its failure mechanism and thermal stress under thermal load. Multi-layer thin-film thermocouple samples were prepared on ceramic substrate, and high-temperature tests were carried out under different temperature loads, and the phenomenon of film shedding and cracking was observed using electron microscope. This paper analyzes the failure mechanism of the film sensor based on the function and structure, and uses ANSYS to analyze the thermal stress distribution of the film under high temperature load. Combining several existing theoretical models, this paper analyzes the factors affecting the thermal stress of the film and conducts simulation verification.

Abstract: To reduce the cost of silicon carbide (SiC) substrates, we have developed a high-temperature chemical vapor deposition (HTCVD) method for high-productivity crystal growth. We have conducted research using crystals of diameter 4 inches or less. In order to further reduce the cost, development of a 150-mm substrate has been demanded. With increasing crystal diameter, the occurrence of cracks should be suppressed efficiently. The internal structure of the furnace was designed to reduce the distribution of temperature in the radial direction of the crystal, ultimately reducing the stress responsible for the formation cracks. We demonstrated a 150-mm 4H-SiC substrate without cracks using by HTCVD method.

Abstract: Here, we present a numerical approach to analyze the integrity of a vessel that was subject to a weld repair. A Post-Weld Heat Treatment (PWHT) process was implemented to a vessel undergone weld repair due to leakage. Due to the thick wall of the welded bottom head, this welding process must be followed by the PWHT to relieve the residual stress, as well as to improve the material properties. PWHT process was performed by heating the welded area to reach 675 °C temperature. A numerical approach using finite element analysis (FEA) method was performed to analyze the integrity of the vessel. Based on the analysis, the structure is still stable within the applied load. PWHT process does not lead to buckling on the main structure and the load is still lower than the load required for the occurrence of buckling. A sensitivity analysis was also performed with reduced temperatures to 630 °C or reduction of PWHT area width. These changes were found to have negligible effects in reducing the stress and strains in the vessel. After PWHT is completed, the structure is still considered to be safe to be operated, as indicated by its strain that is still below the allowable strain and only relatively small deflection was occurred.

Abstract: This study focuses on the thermo-elastic rolling contact problem of a graded coating/substrate system. The problem is formulated under the plane thermoelasticity framework. Assuming an exponential variation of the shear modulus within the coating, the governing singular integral equations are extracted by means of the Fourier transform. The solution to problem is provided via the Gauss-Chebyshev integration method. The sensitivity of the contact stresses as well as the surface temperature rise to the stiffness ratio, the coating thickness and the non-dimensional speed is investigated. The results indicate that the thermal expansion ratio substantially affects the contact stresses. Also, the softening coatings will result in maximum surface temperature rise. The coating thickness can alter the surface temperature rise such that an increase of the coating by a factor of 1.6 may result in 50% reduction of the maximum surface temperature.