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A novel model of two-stage thermoelectric cooler with inhomogeneous thermal conductivity in steady-state operating condition is established. The modification of the constant properties model allows controlling the distribution of Joule heat. Considering internal irreversibilities of the thermoelectric cooler, expressions for the cooling capacity, coefficient of performance and exergy efficiency are derived. By utilizing numerical methods, the temperature profile along the thermoelectric legs is presented. The optimal operating regions are explored. The coefficient of performance versus cooling capacity describing optimal operating regions in inhomogeneity materials are plotted. Mean while, the influence of the main parameters such as the variation of thermal conductivity distribution, cold-end temperature and the number of thermoelectric modules on the cooling performance is discussed in detail. Results indicate that the cooling capacity, coefficient of performance and exergy efficiency are improved compared to those of homogeneous two-stage thermoelectric coolers when an appropriate inhomogeneous property parameter is applied. The work can provide guidance on design of actual two stage thermoelectric coolers with inhomogeneous materials.

Ultraviolet photodetectors (UV PDs) have attracted extensive attention owing to their wide applications, such as optical communication, missile tracking, and fire warning. Wide-bandgap metal-oxide semiconductor materials have become the focus of high-performance UV PD development owing to their unique photoelectric properties and good stability. Compared with other wide-bandgap materials, studies on indium oxide (In

Low-density polyethylene (LDPE) has excellent dielectric properties and is extensively used in electrical equipment. Hexagonal boron nitride (h-BN) is similar to a graphite-layered structure, and alumina fiber (Al

Ga2O3 is a good candidate for deep ultraviolet photodetectors due to its wide-bandgap, good chemical, and thermal stability. Ga2O3-based photoelectrochemical (PEC) photodetectors attract increasing attention due to the simple fabrication and self-powered capability, but the corresponding photoresponse is still inferior. In this paper, the oxygen vacancy (Vo) engineering towards α-Ga2O3 was proposed to obtain high-performance PEC photodetectors. The α-Ga2O3 nanorods were synthesized by a simple hydrothermal method with an annealing process. The final samples were named as Ga2O3-400, Ga2O3-500, and Ga2O3-600 for annealing at 400 ℃, 500 ℃, and 600 ℃, respectively. Different annealing temperatures lead to different Vo concentrations in the α-Ga2O3 nanorods. The responsivity is 101.5 mA W−1 for Ga2O3-400 nanorod film-based PEC photodetectors under 254 nm illumination, which is 1.4 and 4.0 times higher than those of Ga2O3-500 and Ga2O3-600 nanorod film-based PEC photodetectors, respectively. The photoresponse of α-Ga2O3 nanorod film-based PEC photodetectors strongly depends on the Vo concentration and high Vo concentration accelerates the interfacial carrier transfer of Ga2O3-400, enhancing the photoresponse of Ga2O3-400 nanorod film-based PEC photodetectors. Furthermore, the α-Ga2O3 nanorod film-based PEC photodetectors have good multicycle, long-term stability, and repeatability. Our result shows that α-Ga2O3 nanorods have promising applications in deep UV photodetectors.

Low-density polyethylene (LDPE), as an excellent dielectric insulating material, is widely used in electrical equipment insulation, whereas its low thermal conductivity limits its further development and application. Hexagonal boron nitride (h-BN) filler was introduced into LDPE to tailor the properties of LDPE to make it more suitable for high-voltage direct current (HVDC) cable insulation application. We employed melt blending to prepare h-BN/LDPE thermally conductive composite insulation materials with different contents. We focused on investigating the micromorphology and structure, thermal properties, and electrical properties of h-BN/LDPE composites, and explained the space charge characteristics. The scanning electron microscope (SEM) results indicate that the h-BN filler has good dispersibility in the LDPE at a low loading (less than 3 phr (3 g of micron h-BN particles filled in 100g of LDPE)), as well as no heterogeneous phase formation. The results of thermal conductivity analysis show that the introduction of h-BN filler can significantly improve the thermal conductivity of composites. The thermal conductivity of the composite samples with 10 phr h-BN particles is as high as 0.51 W/(m&middot, K), which is 57% higher than that of pure LDPE. The electrical performance illustrates that h-BN filler doping can significantly inhibit space charge injection and reduce space charge accumulation in LDPE. The interface effect between h-BN and the substrate reduces the carrier mobility, thereby suppressing the injection of charges of the same polarity and increasing the direct-current (DC) breakdown strength. h-BN/LDPE composite doped with 3 phr h-BN particles has excellent space charge suppression effect and high DC breakdown strength, which is 14.3% higher than that of pure LDPE.

We investigate the validity of using the low-dissipation model (LD model) to describe the maximum power regime of the endoreversible model under Newton's law of cooling. We find it valid only when the temperature difference of heat reservoirs (${T}_{h}\ensuremath{-}{T}_{c}, {T}_{h}g{T}_{c}$) is small. Thus the efficiency at maximum power derived from the LD model is valid to the first order of Carnot efficiency when describing endoreversible heat engines. We conclude that the LD model produces the Curzon-Ahlborn efficiency (${\ensuremath{\eta}}_{\text{CA}}=1\ensuremath{-}\sqrt{{T}_{h}/{T}_{c}}$) in the maximum power regime with no dependence on dissipation ratios.

The multi-objective optimization of the hybrid system consisting of molten carbonate fuel cell and absorption refrigerator is carried out based on the nondominated sorting genetic algorithm II (NSGA-II). The operating current density, temperature, pressure of the molten carbonate fuel cell and the irreversibility parameter of the absorption refrigerator are selected as the decision variables, and the equivalent power, efficiency and ecological performance of the hybrid system are treated as the comprehensive optimization objective functions. The Pareto optimal solution set under the constraint conditions of decision variables is get. Two kinds of effective decision-making tools are used: TOPSIS and LINMAP. They are used to obtain relative optimum solution as well as the optimized operation condition. At last, the results of the multi-objective optimization are compared with those of the single objective optimization.

Channel geometry has a great influence on the performance of proton exchange membrane fuel cell (PEMFC). In order to optimize the channel geometry of PEMFC, a 3-D, non-isothermal, steady-state model of a PEMFC with a single straight channel is established. Multi-objective genetic algorithm is employed as the optimization method. Two conflicting objectives, power consumption (Econ) and output power (Ecell) of the PEMFC are chosen as objective functions. The length and width of the channel are selected as the optimization variables. Three decision-making methods including TOPSIS, LINMAP and fuzzy Bellman-Zadeh are employed to select the decision solutions from the Pareto optimal frontiers. After comparing these three kinds of decision solutions, the final optimal solution is ascertained. The results show that the optimal channel has a shorter length and a wider width, and the optimal PEMFC has higher energy efficiency and more even distributions of reactants. In addition, the structure of the optimal PEMFC improves the performance of guarding against cathode flooding.

To evaluate the potential of inhomogeneous thermoelectric materials, the hybrid system model comprised of a molten carbonate fuel cell, a thermoelectric generator with inhomogeneous heat conduction and a regenerator is established. In particular, thermal conductivity of thermoelectric materials is regarded as a spatial dependence coefficient. Taking electrochemical and thermodynamic irreversible losses into account, the expressions of hybrid system’s equivalent output power and efficiency are deduced and optimal operating current density interval of proposed system is determined. The maximum output power density and efficiency of the hybrid system is 35.4% and 6.9% higher than stand-alone molten carbonate fuel cell, respectively. It is proved that inhomogeneous thermoelectric materials are capable of significantly enhancing the hybrid system performance, since the efficiency can be improved by 25.4% and the optimal operating interval is shifted to a lower current density direction compared with that with homogeneous materials. Finally, several critical parameters on the system performance are analyzed through further discussions of the established model. The results obtained can sever as a theoretical guidance for the optimization of thermoelectric integrated system.

For further recovery of waste heat, alkali metal thermoelectric converter (AMTEC), thermally regenerative electrochemical cycles (TRECS), and two-stage thermoelectric generator (TTEG) are incorporated into a novel thermoelectric coupling system. The waste heat from AMTEC is split into TRECS and TTEG simultaneously. A general mathematical model including the main irreversible losses inside the system and the external heat leakage is built. With the optimization of the current density of AMTEC, the electrolyte thickness, and the fraction of heat dumped into each subsystem, the maximum modeling efficiency and power output are about twice as much as the single AMTEC system. They are 11% and 14% higher than the efficiency and power output obtained if the heat provided to the TRECS and TTEG are equal. Proper distribution of waste heat is necessary. The parametric selection criteria for the optimum states of the hybrid system operation are achieved. Furthermore, the effects of a set of critical factors relevant to TRECS and TTEG are also evaluated. The optimal dimensionless current of TTEG can maximize the power output of the hybrid system, while the internal resistance of TRECS decreases the power output. The conclusions obtained here are general and involve several special cases.

A new concept of inhomogeneous thermoelectric materials is advanced recently. In the present study, a novel two-stage thermoelectric cooler (TEC) in electrically parallel configuration is investigated. It is made of the inhomogeneous materials with space-dependent electrical conductivity. The internal irreversibilities of the thermoelectric cooler are taken into account. The temperature profile and heat flux in the thermoelectric legs are derived. Thermal rectification can be realized through the modification of the constant properties model which can result in higher cooling capacity and coefficient of performance (COP). The optimal operating regions are explored. Furthermore, the ratio of thermocouples between two stages, the area-length ratio of legs and cold side temperature have significant effect on some important performance characteristics. In the case of optimal ratio of thermocouples between two stages and optimal area-length ratio of legs, coefficient of performance and cooling capacity rise 22.6% and 28.9% respectively by using inhomogeneous materials.

Two-dimensional (2D) materials are building blocks for the next generation of electronic and optoelectronic devices. Tuning band gap in 2D materials over a broad range from ultraviolet to infrared is of scientific and technological importance for a wide range of applications, but its execution remains a challenge. Herein, tuning the band gap from 5.27 eV to 0.69 eV has been realized by utilizing an external electric field. Interestingly, under external electric field the MgZnO monolayer remains a direct band gap semiconductor, which has clear advantage for applications in optical devices. Moreover, the external electric field significantly leads to a red shift of the optical absorption peaks. The absorption coefficients and reflectivity decrease with increase in the external electric field in MgZnO monolayer. These findings should render these materials suitable for future applications in electronic and optoelectronic devices.

The local asymptotic stability of an irreversible heat pump subject to the total thermal conductance constraint is investigated based on the stability criteria for almost linear systems. An irreversible heat pump system that is modeled by the differential equation expresses the rate of change of the temperatures of the working fluid in the heat addition and heat rejection processes. From the local stability analysis, we find that a critical point of an almost linear system is asymptotically stable. After an arbitrarily small perturbation, the system state exponentially decays to an asymptotically stable critical point with either of two characteristic relaxation times that are a function of the total thermal conductance UA , thermal conductance allocation ratio ϕ , heat capacity C , internal irreversible factor I , temperature of the heat reservoir T H and heating load Q . The behavior of solutions of the system is presented qualitatively by sketching its phase portrait. One eigendirection in a phase portrait is the nonzero constant vector, and the other is a function of UA , ϕ , I , Q H , T H and T L . Finally, we discuss the local asymptotic stability and steady state energetic properties of the irreversible heat pump.

The local stability of a non-endoreversible heat engine working between the maximum power output and the maximum efficiency is analyzed based on stability criteria for almost linear systems. A non-endoreversible heat engine system that is modeled by the differential equation may depend on the numerical values of certain parameters that appear in the equation. From the local stability analysis we find that a critical point of an almost linear system is a stable node. After a small perturbation the system state exponentially decays to steady-state with either of two characteristic relaxation times that are a function of the thermal conductance α, heat capacity C, temperatures of the heat reservoirs TH and TL, internal irreversible factor I, and a parameter θ measuring the degree of lying close to the maximum efficiency. The behavior of solutions of the system is exhibited qualitatively by sketching its phase portrait. Several special cases are discussed in detail so that some important conclusions relative to local stability of an irreversible heat engine in the literature may be derived from the present paper. Finally, we discuss the local stability and energetic properties of the non-endoreversible heat engine.

The purpose of this paper is to present a local stability analysis of a non-endoreversible refrigerator operating at the minimum input power P for given cooling load R absorbed from the cold reservoir, in the isothermal couplings of the working fluid with the heat reservoirs T H and T L ( T H > T L ) through a couple of thermal conductors, both having the same heat conductance α and using I to describe internal dissipations of the working fluid. A non-endoreversible refrigerator system that is modeled by the differential equation may depend on the numerical values of certain parameters that appear in the equation. From the local stability analysis we find that a critical point of an almost linear system is a stable node. After a small perturbation the system state exponentially decays to steady-state with either of two relaxation times that are a function α , I , R , T L and the heat capacity C . We can exhibit qualitatively the behavior of solutions of the system by sketching its phase portrait. One eigenvector in a phase portrait is the non-zero constant vector, and the other is a function of α , I , R , T H and T L . Finally, we discuss the local stability and energetic properties of the non-endoreversible refrigerator.

The purpose of this paper is to present a local stability analysis of an endoreversible refrigerator operating at the minimum input power P for given cooling load R absorbed from the cold reservoir, for different thermal conductances α and β , in the isothermal couplings of the working fluid with the heat reservoirs TH and TL ( T H > T L ) . An endoreversible refrigerator system that is modeled by the differential equation may depend on the numerical values of certain parameters that appear in the equation. From the local stability analysis we find that a critical point of an almost linear system is a stable node. After a small perturbation the system state exponentially decays to steady state with either of two relaxation times that are a function of α , β , T L , R and the heat capacity C . We can exhibit qualitatively the behavior of solutions of the system by sketching its phase portrait. One eigenvector in a phase portrait is the nonzero constant vector, and the other is a function of α , β , R , T H and T L . Finally, we discuss the local stability and energetic properties of the endoreversible refrigerator.

The optimal performance of an absorption heat pump operating between four temperature levels with the losses of heat resistance and internal irreversibility is analyzed first by taking the total heat-transfer area of heat exchangers as an objective function. The minimum total heat-transfer area is described in terms of the rate of the entropy changes of four heat reservoirs, and the generally optimal relation among the heating load, the coefficient of performance and total heat-transfer area is achieved. Then, an ecological optimization criterion is proposed for the best mode of operation of the absorption heat pump. We investigate the ecological optimization performance and derive the optimal heating load, coefficient of performance and entropy production rate at the maximum ecological criterion for the absorption heat pump. Effects of thermal reservoir temperature and the internal irreversibility on the ecological function have been discussed.