The last decade has seen the rapid growth of research on two-dimensional (2D) materials, represented by graphene, but research on their thermophysical properties is still far from sufficient owing to the experimental challenges. Herein, we report the first measurement of the specific heat of multilayer and monolayer graphene in both supported and suspended geometries. Their thermal conductivities were also simultaneously measured using a comprehensive Raman optothermal method without needing to know the laser absorption. Both continuous-wave (CW) and pulsed lasers were used to heat the samples, based on consideration of the variable laser spot radius and pulse duration as well as the heat conduction within the substrate. The error from the laser absorption was eliminated by comparing the Raman-measured temperature rises for different spot radii and pulse durations. The thermal conductivity and specific heat were extracted by analytically fitting the temperature rise ratios as a function of spot size and pulse duration, respectively. The measured specific heat was about 700 J (kg K)−1 at room temperature, which is in accordance with theoretical predictions, and the measured thermal conductivities were in the range of 0.84–1.5 × 103 W (m K)−1. The measurement method demonstrated here can be used to investigate in situ and comprehensively the thermophysical properties of many other emerging 2D materials.

A precision H-type sensor method has been developed to measure the thermoelectric performance of individual single-crystalline CdS nanowires for the first time. A nanomanipulation probe was used to directly pick up an individual nanowire from the array and place it on the sensor. Our method is generally applicable to any nanowire synthesized in either array or powder form. By simply changing the external electrical circuits, the Seebeck coefficient, thermal conductivity, and electrical conductivity have been measured on the same nanowire sample to ensure high accuracy and reliability. CdS nanowires have a large Seebeck coefficient over 300 μV K−1 due to their wide band gap, while their thermal conductivity is only one-tenth of that of the bulk material owing to the significant phonon-surface scattering. The figure of merit, ZT, of the CdS nanowire is 0.01 at 320 K, which is larger by two orders of magnitude than the value for a Bi2S3 nanowire, showing a trend of rapid increase above 300 K.

The thermal conductivity of individual suspended single-walled carbon nanotubes (SWCNTs) has been theoretically predicated to increase with length but this has never been verified experimentally. This then leads to the question of whether the thermal conductivity saturates to a finite constant value in ultra-long SWCNTs. This paper reports on experimental measurements of the thermal conductivity of individual suspended SWCNTs as a function of the characteristic thermal transport length using the same individual suspended SWCNT sample. Interestingly, at around 360 K, the thermal conductivity first increases with increasing characteristic length and then saturates to a finite constant value at a characteristic length of ∼10 μm. These experimental results provide a fundamental understanding of the phonon transport characteristics in suspended, pristine SWCNTs.

•Novel microelectronic devices with free-standing, high quality single layer graphene (SLG).•The measurement principle is simple and robust.•The experimental result has been proved valid by repeated measurements on four different SLG samples.•The width effect on thermal conductivity of SLG is well supported by both experimental result and theoretical analysis.•The new findings in this work provide useful guidelines for the future SLG-based thermal applications.Size dependence is one of the most important unique features of thermal conductivity in two-dimensional materials. Suspended single-layer graphene (SLG) provides a perfect platform for studying the size dependent phonon transport. Here we report measurement and theoretical analysis of heat conduction in suspended SLG as a function of width and temperature. The thermal conductivity of graphene was larger for wider SLG. This width effect was smaller at higher temperatures. In suspended SLG, the long wave-length phonons tend to be more scattered at the lateral boundaries of narrow SLG ribbon, in which the mean free path of phonons is close to the sample width. This behavior can be understood as a mode selectivity of phonon-boundary scattering for suspended SLG. The result revealed the unique width dependence of thermal conductivity in suspended SLG and provided useful guidelines for the future SLG-based thermal applications.

•We present an experimental study of temperature dependent thermal conductivities of SWCNTs using Micro-Raman spectroscopy.•Spatially resolved temperature profiles in electrically heated individual short SWCNTs (60, 20 μm) were measured, with all the temperature profiles being highly symmetric parabolas.•By comparing the temperature profiles with that predicated by Fourier’s law, the results indicated that thermal transport in SWCNTs obeys Fourier’s law at extremely high heat fluxes (1.4 × 1011 W/m2) above 300 K.In recent years many reports have indicated that Fourier’s heat conduction law is violated at size scales comparable to the mean free path of the heat transfer carriers or time scales of femtoseconds. Another open question is whether Fourier’s heat conduction law still holds at extremely high heat fluxes. Here, we present an experimental study of temperature dependent thermal conductivities and spatially resolved temperature profiles in electrically heated single-walled carbon nanotubes (SWCNTs) using Raman spectroscopy. The experimental results yield evidence that thermal transport in SWCNTs obeys Fourier’s empirical law at high heat fluxes (q = 1.4 × 1011 W/m2) above 300 K in vacuum.

•The universal uniformity factor is proposed.•The uniformity principle of TDF in heat exchanger is rigorously proved.•The principle applies to the heat exchangers with variable thermal conductivity.In this paper, the uniformity principle of temperature difference field (TDF) in heat exchanger has been proved with deductive reasoning. A new parameter which is used to measure the uniformity of TDF in heat exchangers is defined as the universal uniformity factor instead of the two-dimensional factor. An infinitesimal change of flow arrangement is studied, and the corresponding variations of the uniformity factor and the total thermal conductance are calculated. By analyzing the relation between the variations, it is proved that the more uniform the distribution of TDF, the smaller the total thermal conductance of heat exchanger for given heat transfer rate and inlet parameters of fluid. The deduction proves that the uniformity principle of TDF is strictly true in heat exchangers. An optimization problem is analyzed to show the better applicability of the universal uniformity factor compared with the spacial uniformity factor, and it illustrates that the uniformity principle of TDF applies to the heat exchangers with variable thermal conductivity.

A comprehensive method to evaluate the thermoelectric performance of one-dimensional nanostructures, called the T-type method, has been first developed. The thermoelectric properties, including the Seebeck coefficient, thermal conductivity and electrical conductivity, of an individual free-standing single crystal Bi2S3 nanowire have been first characterized by applying the T-type method. The determined figure of merit is far less than the reported values of nanostructured bulk Bi2S3 samples, and the mechanism is that the Seebeck coefficient is nearly zero in the temperature range of 300–420 K and changes its sign at 320 K.

As a solid-state membrane with only one-atom thickness, nano-porous graphene has attracted intense attention in many critical applications. Here, the key challenge is to suspend a single-layer graphene (SLG) and drill nanopores with precise dimensions. Here, we report a simple and reliable route for making suspended fluorinated SLG with size-tunable nanopores. Our method consists of two steps: 1. a free-standing SLG ribbon was created between two gold pads after deep dry etching of silicon substrate by xenon difluoride. The SLG was fluorinated by 5–13%. Superior to the normal wet etching method, the dry etching process is much simpler and results in less hole-defect and edge deformation. A large area fluorinated SLG can be suspended due to the sufficient etch depth. 2. a focused ion beam was introduced to drill nanopores in graphene with an initial diameter around 20 nm. Followed by an electron beam induced carbon deposition, the diameter of nanopore was gradually decreased to sub-10 nm. By changing the deposition time, the size of nanopore can be precisely controlled. High-cost transmission electron microscope is no longer needed. Our method provides a simple and effective way for preparing free-standing fluorinated SLG ribbon suitable for single-molecule detection.

•A general method for fabricating suspended 2D material devices is proposed.•The electrical and thermal properties of suspended 2D material can be measured simultaneously.•Arbitrary-shaped electrodes can be prepared with the suspended 2D membrane for realizing different functions.We demonstrate a general process for fabricating graphene nanoelectronic devices that have next several features: free-standing, micrometer-sized monolayer graphene with high quality, arbitrarily-shaped metallic electrodes or sensors. In contrast to the normal routes, a gas etching process is used to create a deep trench in silicon for suspending the whole graphene device in a much larger area. User-designed electrodes or sensors are fabricated on the suspended graphene at the same time for realizing multiple functions. In this work, a suspended gold nanofilm sensor is designed to measure the intrinsic electrical and thermal properties of graphene on site. The sensor serves as both electrode and precise resistance thermometer at the same time. By simply changing the metallic electrode shape and electrical circuit, the free-standing graphene can be made into different devices, such as single-molecule detector or nano-resonator. In order to test the robustness of graphene device, a high electrical current is applied to heat the graphene in vacuum until it breaks. The breakdown current density is measured to be 1.86 mA/μm. More importantly, this method is not only limited to graphene, but also can be applied to any other two-dimensional materials.

Three-dimensional integration with through silicon vias offers a promising solution for future technology nodes. However, the heat accumulation and thermal strain may seriously affect performance, leakage, and reliability of circuits. The cross-plane thermal transport, generation, and propagation of coherent acoustic-phonon wave in thin Pt film-glass substrate have been comprehensively studied by applying the picosecond laser pump–probe method with different configurations. Significantly different time-dependent reflectance signals have been obtained in different configurations and an effect superposition model is proposed to account for cross plane thermal transport inducing ipsi- and contralateral temperature change in thin Pt film, propagation of coherent acoustic-phonon wave in thin Pt film and in glass substrate. The corresponding theoretical predictions match well with the experimental data in the whole delay time range.

Vertically-aligned carbon nanotube array is expected to inherit high thermal conductivity and mechanical compliance of individual carbon nanotube and serve as thermal interface material. In this paper, vertically-aligned multi-walled carbon nanotube arrays have been directly grown on Pt film and the thermal performance has been studied by using laser flash technique. The determined thermal diffusivity decreases from 0.187 to 0.135 cm2 s−1 and the thermal conductivity increases from 1.8 to 3.1 W m−1 K−1 as temperature increases from 243.2 to 453.2 K. The fracture surface of the array peeled off the Pt film was characterized by scanning electron microscopy. It has been illustrated that the tearing surface is not smooth but fluffy with torn carbon nanotubes, indicating strong interfacial bonding and consequent small interface resistance between carbon nanotube array and Pt film. According to Raman spectra and transmission electron microscopy image, the possible mechanisms responsible for the thermal transport degradation are low packing density, twist, and the presence of impurities, amorphous carbon, defects and flaws. The influence of intertube van der Waals interactions has been studied by comparing the phonon dispersion relations and is expected to be not significant.

•We develop a non-contact method to measure micro/nano fibers’ thermal properties.•We realize combined measurement of three properties using one sample.•Laser absorption is accurately determined by the nanosensor temperature rise.•Thermal contact resistance is easily eliminated by moving the laser spot.•The air heat transfer coefficient is determined with high sensitivity and accuracy.The accurate thermal property characterization of micro/nano fibers is crucial for precise micro/nano technology. We developed a non-contact T-type Raman spectroscopy method for combined determination of micro/nano fibers’ laser absorption, thermal conductivity and air heat transfer coefficient. The accuracy of laser absorption measurement is independent of the fiber properties or thermal contact resistance. When determining the thermal conductivity, the thermal contact resistance can be easily eliminated. Moreover, the air heat transfer coefficient is determined based on the accurate laser absorption and thermal conductivity data of the same sample. Case studies show that this method is sensitive and accurate for micro/nano fiber characterization.

In this article, the random walking method is used to solve the steady linear convection-diffusion equation (CDE) with disc boundary condition. The integral solution corresponding to the random walking method is deduced and the relationship between the diffusion coefficient of CDE and the intensity of the random diffusion motion is obtained. The random number generator for arbitrary axisymmetric disc boundary is deduced through the polynomial fitting and inverse transform sampling method. The proposed method is tested through two numerical cases. The results show that the random walking method can solve the steady linear CDE effectively. The influence of the parameters on the results is also studied. It is found that the error of the solution can be decreased by increasing the particle releasing rate and the total walking time.

In this paper, the effect of the computational grids on the wind turbine positioning optimization is studied. The linear wake flow model is used to calculate the turbine wake flow. The power law is used to model the power curve of wind turbine. Greedy algorithm with repeated adjustments is introduced to solve the wind turbine positioning problem, with the target of maximizing the total power output of wind farm. Square grid and triangle grid with various orientations are used to discretize the area of wind farm. Three numerical cases are introduced to study the effect of the computational grids on the optimized results. The results show that the optimized power output can be improved through choosing appropriate grid orientation. The suggested grid orientations for single direction wind case and multi-direction wind case are given.

The electrical and thermal conductivities of polycrystalline gold and platinum nanofilms have been measured simultaneously using a direct current heating method from 60 to 300 K. The measured electrical and thermal conductivities are greatly decreased from the corresponding bulk values. And it is found that the reduction increases as the temperature decreases. The deviation from the bulk value is due to the effect of grain boundary scattering. Furthermore, the experimental results indicate that the grain boundary scattering effect imposes greater influence to the charge transport than to the heat transport. Consequentially, the Lorentz number is several times larger than that of bulk materials, leading to the violation of the Wiedemann–Franz law. The reflection coefficient R (0.86 in platinum, 0.42 in gold) at grain boundaries is obtained based on the Mayadas-Shatzkes theory and Matthiessen’s rule, which agrees well with the previous experiments.

Incorporating both the scattering mechanisms of the rough film surfaces and the microscopic grain boundaries, we report on a realistic simulation of the in-plane thermal conductivity of nanoscale gold, copper and aluminum films on the basis of non-interacting electron and kinetic theory. The results are consistent with previous theoretical analyses and molecular dynamic simulations, as well as available experimental data. The thermal conductivity of metallic films is found to drop much below the bulk value, even showing dielectric effects. As with the size effect on the electrical conductivity of metallic films, the fine-grain structure has a greater effect on the thermal conductivity than the film surface properties. The influence of temperature on the thermal conductivity is also investigated.

The thermal conductivity of individual pitch-derived carbon fibers was measured in the temperature range 100–400 K by a T type method, in which a hot wire served both as a heating source and thermometer, and the electrical and thermal properties of the hot wire were measured by direct current heating. When a tested carbon fiber was attached to the center position of the hot wire, the thermal conductivity of the fiber was determined by a comparison of the average temperature rise of the hot wire with and without the fiber. Results show that the thermal conductivity of the fiber was limited by boundary scattering below 300 K, and saturated around 350 K at a value of about 800 W/(m·K). An unexpectedly high thermal conductivity of about 920 W/(m·K) was observed at around 400 K. The effect of the thermal contact resistance on the measurement was estimated by changing the length of the fiber in the same contact conditions and the radiation effect was also discussed. The uncertainty of the thermal conductivity was estimated to be ± 13%.

Highlights•The turbine height matching for micro-siting in wind farm is studied.•An iteration method is developed to obtain the optimized tower height.•Numerical cases over flat terrain and complex terrain are considered.•The iteration method needs less computation to obtain the optimized height.•The error of the results by the iteration method is less than previous studies.This paper studies the tower height matching problem in wind turbine positioning optimization. Various models are introduced, including the power law wind speed model with height in the wind farm, the linear wake flow model for flat terrain, the particle wake flow model for complex terrain and the power curve model with power control mechanisms. The greedy algorithm is employed to solve the wind turbine positioning optimization at a specified tower height. The optimization objective is to maximize the Turbine-Site Matching Index (TSMI), which includes both the production and the cost of wind farm. Assuming that the optimized layout for each tower height is the same, an iteration method is developed to obtain the approximated optimal height. The convergence of the proposed iteration method is discussed through the mathematical analysis. The proposed iteration method is validated through the numerical cases over both flat terrain and complex terrain. The results indicate that the proposed method can obtain better optimized height in shorter computational time than previous studies.

The tower height of the turbines should match the potential site to achieve maximum power output per unit cost when constructing wind farm. In this paper, the tower height matching problem in wind turbine positioning optimization is studied, based on the wind speed characteristics of the site, the wind turbine power curve, the linear turbine wake flow model and the cost model. The global greedy algorithm with repeated adjustment is employed to solve the wind turbine positioning optimization problem. The Turbine-Site Matching Index (TSMI) is introduced as the objective function, with the consideration of the height effects both on the capacity factor (CF) and the initial capital cost (ICC). A normalized power output (L) is defined to analyze the matching problem. The optimal tower height is obtained through modeling L. The power curve model with and without power control mechanisms are studied. The computational results indicate that the proposed method can obtain the approximated optimal height in short computational time. The height effects on the wake flow and the distances among turbines reduce the optimal height. For the whole turbine layout, the higher tower heights are not always desirable for optimality. There exists an optimal tower height when maximizing TSMI.Highlights► The turbine height matching optimization for micro-siting in wind farm is studied. ► The normalized power output is defined to analyze the height matching problem. ► The fitting method is developed to obtain the optimized turbine height. ► It needs less computation to obtain the optimized height by the fitting method.

Highlights•Flow field similarity is deduced for high Reynolds number airflow of wind farm.•Anemometer Phase Graph technique is presented for matching boundary velocity.•The present method can assess wind resource based on only a single anemometer.•Numerical results and measurements of real wind farm have good agreements.A method for numerical wind energy assessment of wind farm based on observations of a single anemometer is presented. Utilizing computational fluid dynamics, the present method establishes a rough relation between the boundary wind velocity and the wind velocity at the anemometer, guided by which a feedback process is conducted to search for the boundary velocity matching the measurement of the anemometer. The present method is able to provide reliable wind resources distribution for wind farm on complex terrain without applying any mesoscale models. The present method is validated through measurements of anemometers installed within a certain wind farm in China.

Highlights•A statistical model is proposed to merge wind cases for wind power assessment.•The wind turbine power curve is modeled by power law or section power law.•The error of energy assessment by the statistical method is less than 0.1%.•The computational time for wind turbine positioning optimization is reduced.Wind power assessment of wind farm is a critical stage in wind energy utilization. This paper presents a statistical method to model the wind speed distribution for wind power assessment of wind farm. The number of wind cases is reduced and the wind rose is simplified through merging the wind speeds. This method is applied to wind power assessment combined with the linear wake model and the wind turbine power curve, and also can be used in wind turbine positioning optimization. The real coding genetic algorithm is employed to evaluate the performance of the statistical method for wind turbine positioning optimization problem. Two numerical cases are used to test the method. The results show that the proposed method can maintain the accuracy of the wind power assessment and reduce the computational time. This statistical method can effectively accelerate the process of wind power assessment and wind turbine positioning optimization in wind farm.

•Samples are prepared using plastic deformation.•Integrating measurements are carried out to obtain thermal and electrical properties.•Plastic deformation samples have high thermal conductivity.•Thermal and electrical conductivities are sensitive to composition.Amorphous alloys are a kind of material that possess high Young's modulus and preferable material forming ability. Among all the amorphous alloys, palladium-based amorphous alloys form a system that is highly representative. In order to study the impact of preparation process and changes in composition on the thermal transfer properties of the palladium-based amorphous alloys, we concentrated on two kinds of palladium-based amorphous alloys Pd40Ni10Cu30P20 and Pd43Ni10Cu27P20 prepared by the plastic deformation method in this research. The thermal conductivities, electrical conductivities and Seebeck coefficients of these palladium-based amorphous alloys have been measured using the steady state T-type method, the standard four-probe method and the AC heating-DC detecting T-type method respectively. The results showed that in the palladium-based amorphous alloys of the same preparation method, the thermal conductivities and the electrical conductivities are highly sensitive to the change in composition, and the Seebeck coefficients are less sensitive to change in composition. The results also showed that in the palladium-based Pd40Ni10Cu30P20 amorphous alloys of the same composition, the electrical conductivity of the sample prepared by plastic deformation is almost identical to the sample prepared by melt cooling, and the thermal conductivity of the sample prepared by plastic deformation is significantly greater than the sample prepared by melt cooling.