Transformation Thermodynamics Research Articles

Liberalizing the effects of Al and Cr in coatings for enhanced interface stability with Mo-rich Ni3Al-based superalloys

The interfacial behaviors between protective coatings and substrate single-crystal superalloys significantly impact their service performances. This work focuses on the elements interdiffusion and microstructural evolution at the interface between a NiCrAlY coating and a series of high-Mo Ni3Al-based single-crystal superalloys, through the coupled phase-field simulations and DICTRA kinetics calculations of their model systems. The critical roles of Al and Cr played in the microstructural evolution at coating/superalloy interface have been confirmed, which requires the appropriate content decreases of Al and Cr in the coating at 1100 °C. However, the addition of Mo reduces the driving force of element diffusion from coating to substrate by increasing the activities of Al and Cr in superalloy. Moreover, increasing the Mo content in superalloy from 8 wt% to 10 wt% could also counteract the promoting effect of Al on Ni mobility and mitigate the γ′ coarsening and detrimental TCP precipitation. Therefore, the effects of Mo enable the reasonable increase of Al content in coating to better balance the interfacial stability and oxidation resistance of coated superalloy, besides its advantage in reducing the anisotropy of TCP precipitate. The obtained interfacial evolution mechanisms resulting from the phase transformation thermodynamics and element interdiffusion kinetics are expected to aid the coating design for advanced single-crystal superalloys servicing at ultra-high temperature.

Analysis of phase transformation thermodynamics and kinetics and its relationship to structure-mechanical properties in a medium-Mn high strength steel

The difference in transformation mechanisms between the normal and reverse phase transformation in medium Mn steel was revealed using thermodynamic and kinetic analysis. The results indicate that the austenite formation during the reverse phase transformation is a fast reaction, significant amount of 46.9 vol% retained austenite is obtained after subjected partial reversion at 650 °C × 1 h. At the nucleation stage, the lath-like structure, dislocations, martensitic packet boundaries, and grain boundaries provide high-density nucleation sites for austenite formation. The critical temperature of austenite formation is reduced by the enrichment of Mn and C. However, no ferrite transformation occurred during the normal phase transformation (650 °C × 6 h isothermal treatment and subsequent furnace-cooling process). At the nucleation stage, the undersaturated Mn and C in the prior austenite grain interior provide insufficient concentration fluctuation, leading to a low nucleation rate. At the phase growth stage, the atomic mobility of Mn and C in the parent austenite (control the ferrite growth) is 1/69 and 1/282 than that in ferrite (control the austenite growth) at 650 °C. The differences in nucleation site density, driving force, and element diffusion rate lead to significant variations in phase transformation behaviors. The excellent mechanical property with the product of strength and elongation (PSE) exceeding 20 GPa % is achieved via the sustained transformation-induced plasticity (TRIP) effect by metastable retained austenite over a large strain range.

Bilayer thermal metadevices that mold transient heat flows

Thermal manipulation has been widely researched due to its potential in novel functions, such as cloaking, illusion and sensing. To avoid the spatially inhomogeneous and anisotropic heat conductivity tensors (introduced by transformation thermodynamics), several thermal cloaks based on scattering cancellation (SC) approach are designed and experimentally demonstrated. However, the current SC cloak is only effective in the steady-state heat transfer condition as the heat capacity terms are neglected by the requirement of constant external fields. In order to expand the SC cloak to the transient regime, a bilayer scheme is introduced to achieve the accurate solutions of the camouflage equations considering time factors. Indeed, there are two equations to solve with two unknown quantities: first the thermal conductivity and second the product of specific heat capacity by the density. We construct thermal camouflage devices and verify their ability to mold dynamic heat flows both by simulations and experiments. Our proposed method paves an efficient avenue to extend SC approach to dynamic heat transfer regime.

Active Thermal Metasurfaces for Remote Heating/Cooling by Mimicking Negative Thermal Conductivity.

Remote temperature control can be obtained by a long-focus thermal lens that can focus heat fluxes into a spot far from the back surface of the lens and create a virtual thermal source/sink in the background material, around which the temperature field distribution can be remotely controlled by varying the parameters of the thermal lens. However, because of the lack of negative thermal conductivity, existing thermal lenses have extremely short focal lengths and cannot be used to remotely control the temperature field around the virtual thermal source/sink. In this study, a general approach is proposed to equivalently realize materials with negative thermal conductivity using elaborately distributed active thermal metasurfaces (ATMSs). Subsequently, the proposed ATMS is used to implement a novel thermal lens with a long focal length designed using transformation thermodynamics, and finally realize the ATMS with realistic materials and experimentally verify the performance of the designed long-focus thermal lens (measured focal length of 19.8mm) for remote heating/cooling. The proposed method expands the scope of the thermal conductivity and provides new pathways to realize unprecedented thermal effects with effective negative thermal conductivity, such as "thermal surface plasmon polaritons," a thermal superlens, the thermal tunneling effect, and the thermal invisible gateway.

Design via topology optimisation and experimental assessment of thermal metadevices for conductive heat flux shielding in transient regime

The design via density-based topology optimisation (TO) of thermal metadevices for conductive heat flux manipulation in transient regime is introduced in this work. Such approach consists in the solution of a TO problem conceived to achieve simple devices of feasible manufacture, which are made of two macroscopically distinguishable isotropic materials of highly contrasting thermal properties. The objective function to be minimised during the solution of such TO problem is defined as the error in achieving a prescribed behaviour of the conductive heat flux in the region intended for its manipulation, and the well-known solid isotropic material with penalisation (SIMP) technique is used to define the material distribution as a continuous function. The finite element method is used for the numerical analysis of the metadevices performance, and for each finite element there is a single microparameter (artificial density) determining the material from which it is made. The proposed approach has been particularly applied to the design of metadevices to shield the conductive heat flux in a given region during a transient heat conduction process. Although the so-designed metadevices are made of standard isotropic materials, they actually behave as metamaterials allowing the desired manipulation of the heat flux. Such approach has been implemented in the design of an easy-to-manufacture metadevice capable of fulfilling the assigned task better than other design merely based on intuition, whose material distribution was conceived to at least roughly emulate the heterogeneous effective properties dictated by the method of transformation thermodynamics. Since the microparameters directly define the distribution of both isotropic materials, it has also been possible to manufacture a metadevice designed under the proposed approach and to experimentally evaluate its performance.

Effect of Hydroxyapatite on the Migration of Fe(III) Ions: Kinetic, Equilibrium and Thermodynamic Study.

The recycling and regeneration of Fe(III) is the key point for promoting the oxidation reaction of ore to produce acidic mine drainage (AMD). Hydroxyapatite (HAP) has excellent adsorption ability of Fe(III), which has good biocompatibility and is widely distributed in nature. In order to achieve the source treatment of AMD, the migration and transformation of Fe(III) in the presence of HAP were systematically investigated. In this study, the influence of HAP on the migration of Fe(III) was evaluated though the transformation capacity of Fe(III) by HAP. The adsorption transformation kinetic, equilibrium and thermodynamics of Fe(III) using HAP were also systematic investigated. The transformation efficiency of Fe(III) increased with the increasing initial pH value and reached 99.8% at a pH of 5 due to the hydrolysis reaction. The transformation efficiency was also up to 99% at an initial pH of 2 when the reaction temperature increased to 313 K. However, the transformation capability of Fe(III) decreases with reaction temperature. The kinetics of the adsorption of Fe(III) fitted the pseudo second order kinetic model. Experimental results were also analyzed by the Langmuir isotherm equations at room temperatures. RL separation factor for Langmuir isotherm showed that the migration of Fe(III) is successfully hindered by HAP. Various thermodynamic parameters such as enthalpy (ΔH), Gibbs free energy (ΔG) and entropy (ΔS) changes were computed, which showed that the transport lag of Fe(III) caused by HAP is spontaneous and endothermic.

A Thin-Film Thermal Meta-Device With Dual Function of Thermal Shield and Generation Based on an Artificially Tilted Structure

A thin-film thermal meta-device with a dual-function of thermal shield and generation is proposed and fabricated in this work. The tilted layered structure composed of thermoelectric materials with low thermal conductivity and metal materials with high thermal conductivity is designed to realize the orientation of the incident heat flux and the utilization of heat energy based on the Seebeck effect of the materials. The constructed transverse thin-film thermoelectric device with a Bi2Te3/Cu-layered structure can achieve a maximum output voltage of 20.4 mV and a maximum output power of 7.39 μW at a temperature difference of 80 K. On this basis, the dual-functional film meta-device based on transformation thermodynamics is capable of realizing a heat flux shielding in the central area and generating a transverse output voltage. The experimental results also indicate that there exists a well linear relationship between the output voltage of the meta-device and the temperature difference, which can be extended to accurately monitor the temperature of the central area. This work provides a certain reference for the development of a new class of thermoelectric devices with thermal protection and temperature detection capability.

Three-dimensional patterned distribution of thermal conductivity in the volume for effective thermal concentration

For thermal devices and electronic systems, heat flow manipulation is critical to meet various needs. In this article, based on transformation thermodynamics, three-dimensional (3D) structures in the volume for patterned thermal conductivity were designed and fabricated. The 3D structures in the volume acting as heat flow routes can be achieved as pre-designed, providing a promising method to manipulate heat flow applied to the localized thermal concentration, thermal cloaking, etc. Simulation and experiments show that, by the 3D structures for thermal concentration, the heat flow could be concentrated to the pre-designed region, 7.0 K higher compared to the isotropic sample without any internal structures, and with a temperature gradient of over 1.3 K mm−1 to the surroundings (the heat source is 361 K), indicating an effective heat concentration. It also reveals that the heat flow could be accurately manipulated by carefully designing the geometric and material parameters of the internal structures, providing an alternative method to construct a 3D matrix for heat flow manipulation in the volume.

A Short Review on the Effect of Cr on the fcc–hcp Phase Transition in Fe–Mn-Based Alloys

The effect of Cr on the fcc–hcp martensitic transformation in the Fe–Mn–Cr system has been discussed considering different aspects: (a) the relative phase stabilities, (b) the magnetic order of the fcc phase, (c) the structural parameters and volume change between fcc and hcp, (d) the driving force of the martensitic transformation and relevant thermodynamics quantities, (e) the thermal cycling behavior, and (f) the pseudoelastic effect. Particularly, in this work it has been found that when Cr content increases, the effect of cycling on the energy barrier decreases. This may be explained by a small volume change, which could lead to a slighter introduction of plastic deformation during thermal cycling through the martensitic transition.

Environmental Response of 2D Thermal Cloak under Dynamic External Temperature Field.

As a typical representative of transformation thermodynamics, which is the counterpart of transformation optics, the thermal cloak has been explored extensively while most current research focuses on the structural design instead of adaptability and practicability in a dynamic environment. The evaluation of energy processes involved in the thermal cloak under dynamic conditions are also lacking, which is essential to the engineering application of this functional structure. In this paper, based on the dynamic environment of a sinusoidal form with ambient amplitude, distribution density, phase, and temperature difference as variables, we evaluated the cloaking performance and environmental response of a 2D thermal cloak. Considering the heat dissipation and energy loss in the whole procedure, local entropy production rate and response entropy were introduced to analyze the different influences of each environmental parameter on the cloaking system. Moreover, we constructed a series of comprehensive schemes to obtain the fitting equation as well as an appropriate scope to apply the thermal cloak. The results are beneficial to the novel use of the concept of entropy and valuable for further improving the working efficiency and potential engineering applications of the thermal cloak.

Anisotropic elastic and thermodynamic properties of the HCP-Titanium and the FCC-Titanium structure under different pressures

Stress induced phase transformation from hexagonal close-packed titanium (HCP-Ti) to face-centered cubic titanium (FCC-Ti) is believed to be a reason for the pronounced work hardening of carbon nanotube-reinforced titanium (CNT/Ti) composites prepared by high-pressure torsion (HPT). Here, the correlation between the phase transformation from the HCP-Ti to the FCC-Ti structure in Ti and the improved mechanical properties of CNT/Ti composite is revealed by investigating the structural transformation mechanism, the stability, electronic properties, anisotropic elasticity and thermodynamics of the FCC-Ti and HCP-Ti crystals under pressure of 0–15GPa by means of first-principles calculations and comparing with the experimental findings. The results show that the formation enthalpies △HTi, the bulk modulus B, the shear modulus G and the Young's modulus E of the FCC-Ti and HCP-Ti structures gradually increase with increasing pressure, and the hybridization between the electronic orbitals of the atoms becomes stronger. The Young’s modulus of the cubic FCC-Ti structure shows strong anisotropy along the [0 1 0] and [1 1¯ 0] directions, while the HCP-Ti structure exhibits an obvious anisotropy of E in the (1 0 0) crystal plane. The thermodynamic stability of the HCP-Ti and FCC-Ti structures decreases under high pressure. The different relative stability of the two structures results in a high propensity of structural transformation from the HCP-Ti to the FCC-Ti structure. A large number of FCC-Ti structures are prepared, which can effectively improve the mechanical properties of CNT/Ti composites. Our results may help to better understand the phase transition from HCP-Ti to FCC-Ti under high pressure, and may reveal the structure-property relationship of CNT/Ti composites.

Design of a bifunctional thermal device exhibiting heat flux concentration and scattering amplification effects

By performing a two-step coordinate transformation, an N-sided polygonal bifunctional thermal device with nonsingular homogeneous material parameters is proposed based on transformation thermodynamics. The presented device could function as not only a concentrator with heat flux concentration effect, but also an illusion device with a scattering amplification effect. Moreover, to further eliminate the anisotropy of thermal conductivity tensors, multilayer realization of the device through isotropic materials is also investigated. The effectiveness of the design method and the performance of the devices are validated by the full-wave simulations. This work paves a new way for the design of easy-to-implement bifunctional devices and has broad application prospects in such fields as thermal-energy harvesting and thermal camouflage.

Cylindrical thermal invisibility cloak based on transformation thermodynamics

The paper provides a theoretical proof that the methods of transformation optics can be used for thermodynamics. Thermal conductivity tensor is calculated for the development of the thermal invisibility cloak, which directs the heat flux around a certain area. The finite element method was used to simulate the cloak with parameters close to real. The simulation was performed by the finite element method for two cases: with gradient k and with copper/mica layered structure.

Thermal surface transformation and its applications to heat flux manipulations.

A new method to control heat flux, called thermal surface transformation (TST), is introduced from transformation thermodynamics. Compared with transformation thermodynamics, TST has many advantage. First, there is no mathematical calculation during the whole process in TST (novel thermal devices can be designed graphically in a surface-to-surface way). Second, all thermal devices of various functions, shapes and sizes designed by TST only require one homogenous anisotropic thermal medium, i.e., thermal-null medium (TNM). With the help of the effective medium theory, TNM can be realized by layered copper and expanded polystyrene, whose performance on controlling heat flux by TST is verified by numerical simulations. Many examples are given, including thermal imaging devices, thermal unidirectional cloak, concentrator, rotator and thermal focusing devices.

Tunable control of extremely concentrated heat flux through a thermal manipulator

It is crucial to effectively control the heat flux in thermal functional devices and electronic systems. Here, we numerically simulate and experimentally design a simple but effective thermal manipulator based on transformation thermodynamics, which is capable of realizing an extreme heat flux concentration in a small area and guiding the heat flux to the region we desired. The experimental results show that the thermal manipulator has compressed the heat flux from the line-shaped thermal source with an extremely high concentration efficiency of 91.1% and a region concentration ratio of 39.7. The temperature gradient in the small area is 4.42 K/mm, which is about 115% of the applied temperature gradient, indicating a very strong heat concentration capability. Furthermore, the thermal manipulator can physically rotate the heat flux under a constant temperature gradient through a diagonal thermal conduction vector as well. By concentrating and deflecting the heat flux, the extreme concentration and tunable control of the heat flux have been demonstrated, providing an effective thermal manipulating method for the heat flux control in the small area.

Numerical Examination of Metamaterial Concentrator Design for High Efficient Thermal Energy Harvester

Nowadays, research on energy harvesting is highly interest to harvest more waste thermal energy which produced by any kind of thermal engine in our daily life. Thermal metamaterials, designed by transformation thermodynamics are artificial structure that can actively control heat flux at a continuum scale. However, right now the problem of using metamaterial on thermal harvesting is to know what is the best shape design, configuration, and best material to obtain highly efficient on thermal energy harvesting because of the number of research on this field is limited. Flower-shaped metamaterial thermal concentrator has developed and it showed that the design can performed well on directing heat flux to specific location. However, the Flower-shaped metamaterial design that has been conducted before not optimized yet. In the current paper, the numerical simulation and analysis for several metamaterial and shape construction variations were performed using Ansys Workbench™ to characterize the best configuration to optimizing flower-shaped thermal concentrator. A number of interesting numerical computation presented on this research manuscript including total heat flux, temperature distribution, and thermal concentrator efficiency on the surface of the constructed concentrator. The aim of this paper is to provide more reference on metamaterial design for thermal harvesting.

Converging heat transfer in completely arbitrary profiles with unconventional thermal concentrator

Heat transfer manipulation is always an attractive theme in the field of thermal technique and utilization. The proposition of transformation thermodynamics presents great potentials on innovating the conventional thermal techniques with metamaterial. However, only simple designs with uniform profiles have been investigated, as completely arbitrary structures would lead to extreme transformations and singular parameters. These non-negligible problems restrict practical applications of metamaterial concentrators. In this paper, we focus on the heat transfer concentration in “complete” arbitrary profiles through rotatory linear maps. Four schemes, classified into two categories, are demonstrated with homogeneous media configurations. Both distinctive behaviors of internal concentration and external robustness on the thermal fields are significantly observed. Such designs not only efficiently avoid extreme parameters but also provide an alternative way to design metamaterial concentrators under different conditions. The findings may provide new ideas of manipulating heat transfer process, and promoting the novel thermal mate-devices to potential utilization.

Three-dimensional thermal illusion devices with arbitrary shape**Project supported by the National Natural Science Foundation of China (Grant No. 11702069) and the Fundamental Research Funds for the Central Universities, China (Grant No. HEUCFM180203).

Since the concept of invisible cloak was proposed by Pendry and Leonhardt in 2006, many researchers have applied the theory of coordinate transformation to thermodynamics and overcome the complexity of inhomogeneous and anisotropic of material parameters. However, only two-dimensional (2D) thermal illusion devices are researched recently. According to this situation, our study focuses on three-dimensional (3D) thermal illusion devices including shrinker (or invisible cloak), concentrator, amplifier, reshaper, and rotator with arbitrary shape in a general way. In this paper, the corresponding material parameters of thermal illusion devices mentioned above are derived based on the theory of transformation thermodynamics and the simulated results agree well with the theoretical derivations. In addition, the conventional invisible cloak just controls the temperature gradient rather than the temperature value which is more concerned in physical applications. Here, we find that the temperature value of the cloaked object can be controlled by adjusting the location of the original point of the coordinate system.

High temperature shape memory behavior of Ni47.3Ti29.7Hf20Pd3 alloys

The shape memory properties of Ni47.3Ti29.7Hf20Pd3 alloys were investigated through stress-free thermal cycling, constant-stress thermal cycling and constant-temperature stress cycling experiments. The alloys showed reversible phase transformations above 180 °C, which makes them suitable for high temperature shape memory applications. Constant-stress thermal cycling experiments revealed that recoverable strains of about 2% were possible under compressive stress levels higher than 700 MPa and both the recoverable strain and thermal hysteresis values were strong functions of applied stress. Additionally, two-way shape memory strain of 0.7% was observed. Unlike most of the shape memory alloys that have Class I transformation, Class II transformation was observed in these alloys and its relationship to the elastic energy storage was discussed through thermo-elastic martensitic transformation thermodynamics approach.

Thermal cloaking of complex objects with the neutral inclusion and the coordinate transformation methods

We explore the cloaking of a complex shape by either the neutral inclusion or the transformation thermodynamics (TT) methods. Thin cloaks are built and the heat cloaking efficiency is investigated for both the steady-state and the transient regimes. We show that the neutral inclusion cloak is more efficient in both regimes, though it has the drawback that the thermal conductivity of the cloaked shape must be known. In practice, the neutral inclusion method is more flexible and easier to implement than the coordinate transformation method, especially for complex shapes.