Thermal Shock Temperature Research Articles

Grain boundary effects on thermal shock responses of yttria-stabilized zirconia

The rapid temperature change or thermal shock is inevitable in many engineering applications of polycrystalline yttria-stabilized zirconia (YSZ). Nevertheless, the thermal shock response of YSZ especially its dependence on the grain boundary (GB) structure in YSZ remains almost unknown to date. In this study, the effect of the GB on thermal shock behaviors of YSZ ceramics was investigated by molecular dynamics (MD) simulations in conjunction with experimental characterizations. According to the distribution of misorientation angles of GBs experimentally extracted from our sintered YSZ samples, four most common GBs with a misorientation angle ranging from 35° to 63° were constructed in MD simulations through the annealing technique. The thermal shock simulations on the bi-crystalline YSZ with such GBs show a weakening effect of the GB on the thermo-elastic strain wave, which is found to be the most significant in the [100]/[111] GB and almost vanish in its [110]/[111] counterpart. Our simulations also reveal that the weakening effect of GB on the thermo-elastic strain wave is majorly attributed to the impedance mismatch between its two adjacent grains, although the grain-boundary thermal resistance observed also can weaken the temperature wave after crossing GBs. These findings provide valuable atomic insights into the thermomechanical behaviors of YSZ under thermal shock, which will guide the design of microstructures of YSZ for some special applications in solid oxide fuel cells and thermal barrier coatings.

The millisecond fabrication of medium-entropy alloy as a high-performance bifunctional electrocatalyst for ultralong-term rechargeable zinc–air batteries

Zinc air battery (ZAB) has widely recognized advantages such as high theoretical energy density, environmental friendliness, and safety. However, its cathodic reactions involve sluggish four-electron process oxygen reduction and oxygen evolution reactions (ORR and OER). Therefore, developing a cheap, efficient and durable dual functional oxygen electrocatalyst has important scientific significance and practical value for the development of ZAB. Herein, we applied noble metal free medium-entropy alloy (CoFeNi) nanoparticles which were in embedded nitrogen doped carbon framework (N-PCF) as a highly active and ultra-stable bifunctional electrocatalyst. The CoFeNi@N-PCF was prepared by high temperature thermal shock approach from ion-exchanged ZIF percussor to ensure the uniform dispersion of medium-entropy alloy, hierarchical porosity and strong metal-support interaction. Due to the medium-entropy induced synergistic effects, the as-prepared bifunctional electrocatalyst has a large ORR onset potential (Eonset) of 0.92 V and half-wave potential (E1/2) of 0.85 V (vs. RHE) in 0.1 M KOH. And the overpotential of OER in 1.0 M KOH is only 320 mV at 10 mA cm−2. When used in ZAB, the as-assembled rechargeable ZAB with the self-supported air electrode provides an extremely high-power density of 203 mW cm−2, a superior specific capacity of 812 mAh g−1, and amazing cycled steadily for 600 h. The assembled flexible ZAB with CoFeNi@N-PCF exhibits an excellent stability (over 45 h) and the excellent bending ability under different angles. This work can provide some new insights into the design and preparation of advanced bifunctional oxygen electrocatalysts for next-generation metal-air batteries.

Engineering Mn–O strength in manganese oxide catalyst to enhance propane catalytic oxidation

Developing highly active and thermally stable transition-metal-oxide catalysts for light alkane catalytic oxidation is of great importance but very challenging. In the work, the catalytic performance for propane deep oxidation was significantly enhanced on the Mn1NixOy solid solution catalyst by weakening the Mn–O bond strength of manganese oxide. The characterization results combined with DFT calculations demonstrated that the introduction of Ni could create additional defect sites, resulting in a significant improvement of the adsorption and the activation of both propane and oxygen molecules. The optimal Mn1Ni0.15Oy catalyst exhibits abundant oxygen vacancies with low MnO bond strength, high redox ability and oxygen mobility, which account for its superior activity. Specifically, the specific reaction rate on Mn1Ni0.15Oy is three times higher than that on pure Mn2O3. Moreover, a Mn1Ni0.15Oy/cordierite monolithic catalyst was made by wash-coating method, and exhibited remarkable catalytic performance and stability on catalytic oxidation of propane during the 45 days (1080 h) on stream, even after tens of high temperature thermal shocks (up to 650 ℃), which declared to be satisfied for practical industrial applications. This work will shed light on designing high-efficiency and long-life environmental catalyst for VOCs elimination.

High-specific capacity thermal battery cathode Fe and Ni doped CoS2 by enhanced thermal stability and conductivity

CoS2 can meet the strict requirements of high probability output capacity and high energy output capacity of thermal batteries in modern weaponry thanks to its advantages of low resistivity and high thermal decomposition temperature combined with a full Li+ conductive electrolyte. Nevertheless, CoS2 has the limitations of a low voltage platform and theorical capacity. In this case, Fe and Ni are doped into the CoS2 lattice through low-temperature solid-phase sintering to synthesize Fe0.1Co0.8Ni0.1S2 with a single-phase structure. Fe0.1Co0.8Ni0.1S2 generated by the solid phase method has higher thermal stability, which can reduce the high temperature thermal shock at the immediate start of the thermal battery and assure the safety of the thermal battery in operation. Meanwhile, the mass loss of Fe0.1Co0.8Ni0.1S2 at 615 °C is only 5 %, allowing it to discharge at the maximum effective mass at the normal operating temperature(∼500 °C). Because of the synergistic action of Fe2+, Ni2+, and Co2+ in the discharge process, the discharge voltage of CoS2 increases significantly, giving CoS2 higher specific energy. The simultaneous boost in specific energy and specific capacity indicates that doping has been highly successful in modifying CoS2, making CoS2 more appropriate for the use of high current and long-life thermal cell systems.

VDM 330

Abstract VDM 330 (Nicrofer 3718) is an austenitic iron-nickel-chromium-silicon alloy with good high-temperature strength and good resistance to carburization, thermal shock, and high temperature oxidation. It is used extensively in high-temperature environments where resistance to the combined effects of thermal cycling and carburization is required. This datasheet provides information on composition, physical properties, hardness, elasticity, and tensile properties. It also includes information on high temperature performance and corrosion resistance as well as forming, heat treating, machining, and joining. Filing Code: SS-1364. Producer or source: VDM Metals GmbH (a subsidiary of Acerinox, S.A.).

Study of the influence of ZrB2 content and thermal shock on the elastic modulus of spark plasma sintered ZrB2-B4C composites using a non-destructive ultrasonic technique

Non-destructive ultrasound phase spectroscopy (UPS) was used to study the influence of thermal shock on the longitudinal elastic constant (C11) of ZrB2-B4C composites. C11 of the composites was measured in the as-fabricated condition and after different numbers of thermal shock cycles. The relative change in C11 was used as the thermal shock resistance parameter. Composites with 70 and 50 vol% ZrB2 were subjected to three thermal shock temperature differences (ΔT) of 600 °C, 800 °C, and 1000 °C, respectively. The rate of drop in C11 increased significantly with an increase in ΔT and B4C content. The mechanism of thermal shock damage has been discussed considering the influence of processing-induced thermal residual stresses and the average stress state in both phases during thermal shock. It was shown that the powder particle arrangement plays a major role in the damage mechanism and B4C-B4C particle contacts provide easy paths for crack propagation.

Thermal shock resistance of industrial foam filters-a comparative study

For ceramic foams, there is no thermal shock resistance testing standard available at the moment. This work aims to investigate the influence of the different test conditions. Among the descending thermal shock test conditions, heating mode, thermal shock temperature, and quench medium, the latter has the highest impact on the residual mechanical properties. Reference alumina (Al2O3) foams and commercially-available ceramic foam filters (ZrO2, Al2O3–ZrO2, Al2O3–P, and SiC) were subjected to thermal shock with water and air as quenching medium. Due to the low thermal expansion coefficients, SiC and ZrO2 possess the highest calculated thermal shock resistance Rfoam. SiC and ZrO2 foams also had the highest experimentally evaluated residual strength after being quenched in water. An immersion thermal shock procedure with molten metal was tested on Al2O3 reference test pieces. The residual elastic moduli and strengths decrease with increasing temperature which is a function of immersion duration and depends on the usage of an interlayer between the foams and the melt. In order to evaluate the temperature gradient and its development over time, temperature measurements were conducted using a thermocouple inserted and fixed in the midplane of the foams and bulk test pieces.

Improving the anti-thermal-shock properties of Sc2O3-Y2O3-ZrO2-CaF2-PHB abradable coatings by fabricating the textures on ceramic matrix composites: experiment verification and numerical analysis

The abradable coating is one of the most important coatings for improving the efficiency of aeroengine. To improve the durability of Sc2O3-Y2O3-ZrO2-CaF2-PHB abradable coating under the conditions of high temperature thermal shock, the femtosecond laser processing technology is proposed to fabricate the texture on the surface of SiCf/SiC composites to improve the coating/matrix contact area. The environmental barrier coating (EBC) was prepared by vacuum plasma spraying (VPS) equipment on SiCf/SiC ceramic matrix composites to avoid water-oxygen corrosion. And the abradable coating was prepared by air plasma spraying (APS) equipment on the EBC. The thermal shock test of the abradable coating at (1,250 ± 50)°C was established in the simulated gas environment of the oxygen-propane flame, and the variation of interface stress between the layers during the thermal shock test was analyzed by ANSYS software. The hidden crack between the layers was detected by an infrared thermal imager. The results show that the surface textures have significant influences on the anti-thermal-shock properties of Sc2O3-Y2O3-ZrO2-CaF2-PHB abradable coating by improving the contact area and optimizing the interface stress distribution. The bonding strengths of the coating are increased by 14.6% and 42.2% when the surface textures increase the contact area of the substrate and the coating by 41.3% and 104%, respectively. Compared with the coatings without texture treatments, the coatings with texture treatments can reduce coating thickness by 30% and the coatings do not peel off after the thermal shock tests. Influenced by the cyclic thermal stress, the cracks of abradable coating are initiated at the defect position and gradually propagated a shell-like shape. The textures on the surface of SiCf/SiC composites have deep influences on improving the high-temperature thermal shock life of Sc2O3-Y2O3-ZrO2-CaF2-PHB abradable coating.

The microstructure evolution and failure mechanism of Sn37Pb solder joints under the coupling effects of extreme temperature variation and electromigration

The effect of electromigration and thermal shock on the microstructure evolution and failure mechanism in the Sn37Pb/Cu solder joints were investigated in this study. With increasing thermal shock cycles, Pb atoms acted as the dominant effective charge carrier, which migrated with the electron flow and accumulated at the anode side. As the number of thermal shocks extended to 20 cycles, the dissolution of Cu trace was found at the corner positions where the current was crowded in the solder joints. High and low temperature thermal shock demonstrated the acceleration of solder failure that accompanies an increase in peak temperature and shock frequency during electromigration process. Combining the extreme temperature thermal shock (–196 to 120 °C) with the current density of 2.0 × 104 A/cm2, the cracks were prone to initiate at the corner and then propagate along the IMC layers with thermal shock cycles. The high thermal stress caused by the thermal expansion mismatch between Sn37Pb solder and Cu pad with the large temperature variation (ΔT = 316 °C) significantly aggravated the crack propagation, which induced the failure of Sn37Pb solder joints after 22 cycles.

Thermal shock resistance of Cr/CrN/Cr/CrAlN multilayer anti-erosion coating

In this work, a 12 μm Cr/CrN/Cr/CrAlN erosion-resistant multilayer coating was prepared by arc ion plating. According to the application conditions of high-temperature changing environment, the high-temperature thermal shock performance and erosion resistance of the coating were studied. The results show that the Cr/CrN/Cr/CrAlN coatings show no change after thermal shock up to 700 °C. After thermal shock at 900 °C, the surface shows discolouration and wrinkling, and the nitride phase in the coatings disappears and Cr2O3 and α-Al2O3 phases appear. The hardness of the coating after thermal shock is reduced from 3658.57 Hv to 1930.96 Hv, and the adhesion strength is reduced from HF1 to HF4. The erosion rates of the coatings after thermal shock at room temperature, 300 °C, 500 °C and 700 °C do not vary significantly, and are within 1 mg/g at 30° and 90°. The coating is completely unprotective when the thermal shock temperature reaches 900 °C, reaching 2.731 mg/g at 30° and 4.832 mg/g at 90°. It shows that the coating has an anti-erosion protection effect within 700 °C, but the coating has completely failed after a thermal shock at 900 °C and has no protective effect.

Two-step joining of reaction bonded silicon carbide (RBSC) using borosilicate glass

The borosilicate glass was used to join reaction bonded silicon carbide through two steps, namely: laser cladding and low-temperature heat treatment. The wettability and microstructure of the laser cladding glass layer were examined, while joint microstructure and strength were analyzed at various temperatures and times. The results show that the laser cladding layer improved the wetting of glass on the substrate, formed a good interfacial bond, and reduced the joining temperature. The joints primarily consist of the glass phase, the boron-rich phase, and a few crystals. The high-strength joints obtained after heating the joint interface at 950 °C for 30 min had a flexural strength of 250 ± 20 MPa, a 68% improvement compared with direct connection under the same conditions. The joints also exhibit good thermal shock resistance and high-temperature strength. At 400 °C (thermal shock temperature) and 600 °C (test temperature), the residual flexural strengths were 122 ± 25 and 152 ± 12 MPa, respectively.

Experimental study on acoustic emission characteristics of high temperature thermal shock directional fracturing of granite with different heating conditions

High temperature thermal shock directional fracturing of hard rock has tremendous promise in the mining and tunneling industries, but the utilization mechanism is unclear and must be resolved before taking it to the field application stage. In this study, the granite specimens were subjected to bidirectional horizontal loads using the real triaxial pressure testing machine, and the thermal shock fracturing tests were conducted utilizing the homemade thermal shock system. The AE (acoustic emission) system tracked the whole granite fracture process throughout the test in real time. The results showed that the granite‘s fracture time under the high temperature thermal shock was negatively related to the peak heating temperature and heating rate. The spectral analysis of the AE was employed to consider a large number of 20–40 kHz AE signals as a sign of thermal shock damage. By spatial localization of AE events, it was determined that the thermal shock cracks sprouted at the borehole edge in the specimen’s center. In the horizontal direction, the cracks extended from the center to the specimen edge; in the vertical direction, the cracks extended from the specimen surface to the deeper portion of the specimen. The RA-AF value analysis indicated that the granite thermal shock damage mode was a mixed tensile-shear damage mode dominated by tensile damage. In addition, the higher the heating rate, the higher the percentage of tensile cracks. Increasing the heating temperature promoted the formation of shear damage inside the specimen when the heating rate was equal. Analyzing the b-value and βt-value indicated that at low heating rates (10 and 60 °C/min), many micro-cracks formed inside the granite before merging into macro-cracks. However, with the higher heating rate (110 °C/min), the formation of micro-cracks and the macro-cracks within the granite proceeded almost simultaneously. The research results provided some experimental foundation and theoretical guidance for high temperature thermal shock directional fracturing of hard rocks.

The high temperature oxidation and thermal shock behavior of a dense WSi2–TaSi2 coating on Ta substrate prepared by a novel two-step process

In order to improve the oxidation resistance of the Ta substrate, a novel two-step process including molten salt electrodeposition (Na2WO4-WO3 system) and halide activated pack cementation was adopted to prepare a WSi2–TaSi2 coating on tantalum substrate. During the electrodeposition process, dense tungsten coatings were fabricated at current densities of 30 mA/cm2, 40 mA/cm2 and 50 mA/cm2. It was observed that the grain size exhibited a log-normal distribution. When the current density was 40 mA/cm2, the grain size and flattest surface of the tungsten coating reached 9.50 ± 0.23 μm and 6.792 μm, respectively. When performing the static oxidation test, the WSi2–TaSi2 coating could effectively protect the Ta substrate oxidized at 1600 °C for 30 h. This is attributed to the presence of dense SiO2 and Ta2O5, which acted as a protective layer and suppressed the further penetration of oxygen. Furthermore, due to the matching thermal expansion coefficient between each layer and the sealing ability of semi-molten SiO2, the four-layer SiO2–W5Si3–WSi2–Ta5Si3 coating could successfully pass 721 thermal shock tests from 1600 °C to room temperature.

Qualitative Comparison of Lock-in Thermography (LIT) and Pulse Phase Thermography (PPT) in Mid-Wave and Long-Wave Infrared for the Inspection of Paintings

When studying paintings with active infrared thermography (IRT), minimizing the temperature fluctuations and thermal shock during a measurement becomes important. Under these conditions, it might be beneficial to use lock-in thermography instead of the conventionally used pulse thermography (PT). This study compared the observations made with lock-in thermography (LIT) and pulse phase thermography (PPT) with halogen light excitation. Three distinctly different paintings were examined. The LIT measurements caused smaller temperature fluctuations and, overall, the phase images appeared to have a higher contrast and less noise. However, in the PPT phase images, the upper paint layer was less visible, an aspect which is of particular interest when trying to observe subsurface defects or the structure of the support. The influence of the spectral range of the cameras on the results was also investigated. All measurements were taken with a mid-wave infrared (MWIR) and long wave infrared (LWIR) camera. The results show that there is a significant number of direct reflection artifacts, caused by the use of the halogen light sources when using the MWIR camera. Adding a long-pass filter to the MWIR camera eliminated most of these artifacts. All results are presented in a side-by-side comparison.

Experimental Study on the Effect of Thermal Shock on Physical and Mechanical Properties of Limestone

Thermal shock is the physical process of rapid cooling of a hot object. The physical and mechanical properties of rocks that have undergone thermal shock will change. This variation can be applied to the development of geothermal in limestone reservoirs and serve as an effective means of enhancing heat exchange capacity. The essence of this is rock thermal shock rupture. The reason for the change in rock properties due to thermal shock is distinguished from other factors and is a process of instantaneous impact thermal stress caused by non-constant heat transfer, resulting in rock damage from microscopic damage to macroscopic damage. In order to study the variation of physical and mechanical properties of limestone with heating and cooling temperatures under the effect of thermal shock, a self-developed thermal shock test device was used to test the physical and mechanical properties of limestone heated to 100 °C~600 °C after thermal shock. The results strongly suggest that the heating temperature under the effect of thermal shock is the main factor affecting the evolution of basic physical properties of limestone; with the increase in heating temperature, the color of the specimen changes from off-white to white, the mass decreases and the volume nonlinearly increases, with a maximum reduction of 1.39% in mass and a maximum expansion of 2.79% in volume at 600 °C. Indeed, 500 °C is the temperature of abrupt mass loss. The heating temperature and the cooling medium temperature act together to deteriorate the mechanical properties of limestone. With increasing heating temperature (decreasing cooling temperature), the uniaxial compressive strength of limestone decreased by 39.5% (19.3%), modulus of elasticity by 59.5% (22.9%), tensile strength by 42.9% (7.6%), and cohesion by 43.2% (22.5%). The peak strain increases by 74.2% and the angle of internal friction increases by 27% (25.9%). The above data are average values. The empirical equations of the compressive strength, modulus of elasticity, tensile strength of limestone under the action of thermal shock versus heating temperature were obtained. The differences in the physical and mechanical properties of limestone after two heat treatments (thermal shock and high-temperature heating) were compared and analyzed, and the results showed that the physical and mechanical properties of limestone deteriorated more severely after thermal shock compared to high-temperature heating. The research results can provide technical support for the drilling of geothermal development and wellbore stability assessment in limestone reservoirs, and enrich the theory of high-temperature rock mechanics.

Effect of Heating and Cooling of Aluminium-Based Fibre-Metal Laminates on Their Tensile Strength

The widespread use of composite materials in the construction of machines encourages to better understand their properties and the impact of various external factors on these properties. Fibre metal laminates (FMLs) consist of alternating layers of metal and a polymer matrix laminate reinforced with continuous fibres. The aim of this work was to investigate the effect of cyclical temperature changes and thermal shocks (heating the sample to a high temperature in a short time) on the strength properties of FMLs from AW-1050A aluminium sheet, glass fibre fabric and carbon fibre fabric. The research concerns the determination of how cyclical temperature changes affect the tensile strengths properties of FMLs. The results indicate a small effect of the cycles on the tensile strength of the composites. The composites with glass fibre reinforced laminate also showed high resistance to delamination, moreover, the samples did not delaminate even after they were broken. The carbon fibre reinforced laminate composites showed a tendency to delaminate after heat treatment.

Deuterium erosion and retention properties on MoAlB ceramics by ion irradiation

Mo(W)AlB with outstanding neutron shielding capacities, good resistance to high temperature oxidation and thermal shock, is considered as a promising neutron shielding material for applications in fusion reactors. The deuterium (D) erosion and retention properties on the surface of Mo(W)AlB under D ion irradiation were firstly investigated by experiments and theoretical simulation in this work, and compared with metallic tungsten (W). The results show that the total erosion depth of Mo(W)AlB is comparable to Mo(W) when they were irradiated at 500 and 300 eV, and lower D retention than W was found. There was no D blister or crack occurs on its surface whereas W does under same irradiation because of relatively low anisotropic swelling and very high hardness respectively linked to the stress expansion for MoAlB compared to W. Through this work, Mo(W)AlB is expected to be a good candidate as a neutron shielding material in a compact fusion reactor.

Self healing behavior of enamel incorporated with nickel particles of various sizes during high temperature oxidation process

The self-healing nickel-enamel composite coating can spontaneously heal cracks at a certain temperature, thus ensuring its high-temperature service ability in thermal shock environment. In this study, nickel particles of various sizes (50 nm, 2 µm and 20 µm) were incorporated into a silicate enamel. Flexural strength of the pure enamel and its composites was investigated to analyze their thermal shock and crack healing behavior. Results indicated that nickel particles incorporation enhanced strength and thermal shock resistance of enamel, but such an enhancement depended on the size of particles. When nickel particles were in a scale of 2 µm, the composite performed the highest flexural strength, 34% higher than the enamel free of nickel particles. Moreover, its critical thermal shock temperature (ΔTC) was increased by 100 ℃. In the meantime, the composite was endowed with the optimum ability of cracks self-healing. After cracks nucleation in the process of thermal shock, they were largely healed in subsequent oxidation stage. Flexural strength was recovered to 87.9% of the value before thermal shock.

Large amplitude vibration of annular and circular functionally graded composite plates under cooling thermal shocks

In this paper, large amplitude thermally induced vibration (TIV) of annular and circular plates is analyzed under cooling shocks. A novel comparison of the TIV response is presented under rapid cooling and heating shock loads. This is the first time that TIV study has been presented for any flexural structure under thermal cooling. Structures can experience cooling shock loads in many circumstances, for example when exposed to spills of cryogenic liquids. The functionally graded composite plates are made of 304 grade stainless steel (SUS304) and low-carbon steel (AISI1020) and are graded through the plate thickness to have a continuously varying spatial composition profile in thickness direction. Thermo-mechanical properties are assumed to be temperature dependent. The plates are modeled using first-order shear deformation theory with von Karman geometric nonlinearity. The Generalized Differential Quadrature Method (GDQM) and Newton–Raphson technique are employed to solve the governing equations. A Fourier heat equation is used to evaluate temperature distribution over time using GDQM and Crank–Nicolson approaches. The validation study is conducted based on the literature and FEM software ABAQUS. Detailed parametric study is presented focusing on the effect of thermal shock magnitude, rapidity of the thermal shock application, plate geometry, material distribution, geometrical nonlinearity and temperature dependency of material properties. Differences in the response due to these factors have been shown between cooling and heating thermal loads. Results show that importance of the inertia effect is substantially higher under sudden cooling than it is under sudden heating, for the same thermal load magnitude and rapidity of thermal shock. Moreover, it is shown that stresses induced by thermal cooling can potentially reach yield strength of the material with relatively large amplitudes of fluctuations. Therefore, to estimate realistically the structural behavior of thin/slender plates under rapid cooling, the inertia effect should be taken into the account and TIV analysis needs to be performed.

Thermally induced fracturing in hot dry rock environments - Laboratory studies

In enhanced geothermal system (EGS) wells, where strong thermal gradients exist, hydraulic fracturing is the apparent driver for creating permeability in the reservoirs. Thermal effects, however, have been observed to accompany hydraulic loading and may further enhance the permeability of the reservoir. The mechanism of thermal fracturing in downhole conditions, however, is not well understood, limiting the efficient engineering of thermal effects on EGS stimulation. This laboratory study examines the behavior of thermal shock stimulation and temperature propagation by flowing room-temperature water through a borehole of a hot specimen block without confining stresses or borehole hydraulic loading. This condition isolates the effect of thermal shock from hydraulic pressure and confining stress. We monitored temperatures across the inside of the specimens and assessed fractures by visual inspection, bubble leakage, pressure decay, and acoustics. Thermal loading resulted in an enhancement of permeability by induced macro/microcracks. The profiles of borehole pressure decay obtained before and after each stage of stimulation show increased permeability of the treated specimens. The acoustic measurements indicate the extent of fractures, and the bubble leakage tests on the specimen surface visually indicate localized permeation paths created by newly created cracks. The maximum thermal gradient was achieved near the borehole surface at the start of the water flow. As the flow continued, the magnitude of the thermal gradient near the borehole decreased while that of the thermal gradient away from the borehole increased. Thermally driven fractures, initiated from the borehole walls, propagated perpendicular to the borehole surfaces. These “seed” fractures created during thermal stimulation, although they might be suppressed under very high stress, help reduce the levels of breakdown pressure in concurrent or subsequent pressure-based fracturing methods.