Required Temperature Gradient Research Articles

An Automatic Apparatus for Simultaneous Measurement of Seebeck Coefficient and Electrical Resistivity

A fully automated experimental system was designed for simultaneous measurement of the Seebeck coefficient and electrical resistivity of thermoelectric materials in bulk form. The system incorporates a straightforward and easily fabricated sample holder along with commercially available electronic instrument components. The sample holder showcases a compact design that utilizes two Peltier module heaters to induce sample heating and generate the required temperature gradient. System automation and control are achieved through the implementation of a LabView program. The Seebeck voltage and resistance of the sample (under specified temperature conditions) are determined using I–V measurements. The Seebeck voltage and resistance of the sample correspond to the intercept and slope of the I–V characteristic diagram in the four-point probe method, respectively. To verify the accuracy and reliability of the developed apparatus, a variety of experiments were performed on N-type and P-type bismuth telluride samples. The measurement results closely matched those obtained from commercial systems, with an overall data difference of less than 10% for both the Seebeck coefficient and resistivity measurements.

Patterning of Metallic Nanoparticles over Solid Surfaces from Sessile Droplets by Thermoplasmonically Controlled Liquid Flow.

Optically controlled assembly of suspended particles from evaporating sessile droplets is an emerging method to realize on-demand patterning of particles over solid substrates. Most of the reported strategies rely either on additives or surface texturing to modulate particle deposition. Though dynamic control over the assembly of microparticles is possible, limited success has been achieved in nanoparticle patterning, especially in the case of metallic nanoparticles. This work demonstrates a simple light-directed patterning of gold (Au) nanoparticles based on the thermoplasmonically controlled liquid flow. Excitation at the plasmonic wavelength (532 nm) generates the required temperature gradient, resulting in the particle assembly at the irradiation zone in response to the thermocapillary flow created inside the droplet. Particle streak velocimetry experiments and analysis confirm the existence of a strong thermocapillary flow, which counteracts the naturally occurring evaporative convection flows. By modulating the illumination conditions, we could achieve patterns with various morphologies, including center deposit, off-center deposit, multi-spot deposit, and lines. We successfully applied the developed strategy for realizing closely packed hybrid particle assembly containing different particles: Au and polystyrene particles (PS). We performed optical microscopy, 3D profilometry, and SEM analysis to characterize the particle deposit. We analyzed the periodicity of Au-PS hybrid assembly using fast Fourier transform and radial distribution function analysis. PS particles formed a hexagonal close-packed arrangement at the irradiation zone, with Au NPs residing inside the voids. We believe that the presented strategy could significantly enhance the applicability of the evaporative lithography from sessile droplets for the programmable patterning of metallic nanoparticles.

Mean Operating Temperature (MOT) of Commercial Roof Assembly and Its Impact on the Energy Performance

In the thermal design of low sloped roofing assemblies, two parameters are overlooked, one is the surface temperature of the roof assemblies which provides the required temperature gradient for heat flow, and the other is the mean operating temperature of the roof assembly, which has direct implications on the thermal performance of the insulation. An in situ field study was conducted in collaboration with Alberta Roofing Contractors Association (ARCA) on their headquarters building located in Calgary, to generate data on the mean operating temperature of the roof assemblies and to determine whether the thermal design of roofing assemblies using conventional methods is an accurate portrayal of in-service thermal performance. For the present study, two roof assemblies insulated with polyisocyanurate insulation, one with a white reflective roof membrane and the other with the black membrane were selected and instrumented. During the monitoring period, the mean operating temperature (MOT) of the roof assembly whether it is calculated as the average of interior and exterior ambient (MOTAIR,) or the average of surface temperatures (MOTSurface), was found to be below 24 °C (75 °F), which opposes the current roof thermal designs that are being designed using label R-value (thermal resistance) of the insulation reported at the mean temperature of 24 °C (75 °F) rather than temperature-dependent thermal resistance. The comparison of two energy transfer theoretical models, QConvention and QMOT, with the measured data indicated that the conventional approach of roof thermal design underestimates the energy performance of the roof assembly on average by 30%. The use of roof surface temperatures and the corresponding temperature-dependent thermal resistance of the insulation as in QMOT has been demonstrated to improve predictions of the energy performance. In addition the loss in thermal resistance due to blowing agent diffusion in polyisocyanurate was evaluated after two years of in situ installation.

Technological Methods of Forming Thin Semiconductor Layers Part 3. Сathode Sputtering

The peculiarities and the basic technological ways of forming thin layers of semiconductor materials in vacuum by the method of thermal sputtering in quasi-closed volume are analyzed. The disadvantages of thermal deposition of thin films in open vacuum for multicomponent semiconductor compounds are indicated. We present designs of special collapsible evaporation chambers for quasi-closed volume thermal vacuum deposition, which contain the source material and the substrate in quasi-insulated conditions, and allow to create and control the necessary temperature ratios between the evaporator, substrate, walls and other structural elements. It is shown that the spatial distribution of temperature in the mentioned above chambers provides the evaporation (sublimation) of the starting material, the reflection of vapors from the heated walls, the intense exchange interaction between the gas phase and the condensation surfaces, which contributes to the diffusion mechanism of the transfer of matter and thermodynamically balanced process of film growth. It is shown that for all modifications the conditions of isolation of the localized volume and equilibrium of the condensation process are fulfilled to a greater or lesser extent by creating the required temperature gradient. However, it is problematic to use such designs for mass production because of their complexity and technological features. That is why, the method of thermal deposition of thin films in a quasi-closed volume is most often used in the manufacture of epitaxial single crystalline layers for research and experimental development, and for industrial production it is very effective to develop methods for obtaining thin films that combine on the one side universality of open vacuum, and on the other allow to approximate the processes of evaporation (sublimation) – condensation to thermodynamic equilibrium, for example, different variants of “hot walls”. It is stated that the analyzed methods or their modifications are today a necessary means of creating thin-film semiconductor structures with predetermined properties. It is stated that the analyzed methods or their modifications are today a necessary means of creating thin-film semiconductor structures with predetermined properties.

Cost-Effective Experimental Setup for Studies of Spin Seebeck Effect and Electrical Transport in Thermoelectric Materials

In this article, we report on the design of a low-cost, accurate, and easy-to-implement room-temperature experimental setup to comprehensively study the spin Seebeck effect (SSE) in ferrimagnetic insulators (FIs). Neodymium permanent magnets (NdPMs) are used to generate a fixed uniform magnetic field while the sample is staged in a custom-designed vacuum chamber containing thermal baths (for generating the required temperature gradient) and a complete 360° sample rotation mechanism (for studying the magnetic field angle dependence). Our experiments reveal excellent magnetic field uniformity (±1%) formed between the magnet poles, a highly accurate temperature gradient stability (±1%), and excellent agreement of the longitudinal SSE (LSSE) response for the Pt/YIG structure studied here with those reported in the literature. We also measure the anomalous Nernst effect (ANE) exhibited in a graphite sample, demonstrating the capability of our setup to accurately measure this parameter. Finally, we demonstrate that our setup can also be used to measure the conventional Seebeck effect (SE) and the electrical resistivity of commercially obtained Bi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Te <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> samples, making it a versatile tool for the broad characterization of thermoelectric materials.

Dynamic and Reversible Accumulation of Plasmonic Core‐Satellite Nanostructures in a Light‐Induced Temperature Gradient for In Situ SERS Detection

AbstractA surface‐enhanced Raman spectroscopy (SERS) detection method that allows dynamic on‐demand generation of SERS substrates at locations of interest for in situ molecular sensing is demonstrated. Thermal convection and thermophoresis, which are both generated in a laser‐induced temperature gradient, are used to accumulate suspended plasmonic nanostructures to form 3D SERS substrate. Raman signals of melamine, which is used as a model analyte, increase to ≈117‐fold within 2 min of laser irradiation because of the accumulation. In addition, it is demonstrated that the accumulation of the nanostructures is reversible, and that reproducible SERS effects can be obtained during a repeated heating and cooling process. Because of the capability of on‐demand generation of a high density of SERS hot spots at different locations in solution, this particle manipulation and SERS detection method is applicable to monitor temporal and spatial variations of the concentrations of molecules. The complexity of the detection system remains the same when using this method since all the measurements are performed with a conventional Raman system and simple fluid channels. The required temperature gradient is generated by the laser used to excite Raman signals, and no nanofabricated substrates and complicated microfluidic or optical components are needed.

Cooling vests with phase change material packs: the effects of temperature gradient, mass and covering area

Phase change material (PCM) absorbs or releases latent heat when it changes phases, making thermal-regulated clothing possible. The objective of this study was to quantify the relationships between PCM cooling rate and temperature gradient, mass and covering area on a thermal manikin in a climatic chamber. Three melting temperatures (24, 28, 32°C) of the PCMs, different mass, covering areas and two manikin temperatures (34 and 38°C) were used. The results showed that the cooling rate of the PCM vests tested is positively correlated with the temperature gradient between the thermal manikin and the melting temperature of the PCMs. The required temperature gradient is suggested to be greater than 6°C when PCM vests are used in hot climates. With the same temperature gradient, the cooling rate is mainly determined by the covering area. The duration of the cooling effect is dependent on PCM mass and the latent heat. Statement of Relevance: The study of factors affecting the cooling rate of personal cooling equipment incorporated with PCM helps to understand cooling mechanisms. The results suggest climatic conditions, the required temperature gradient, PCM mass and covering area should be taken into account when choosing personal PCM cooling equipment.

On the Limb Darkening, Spectral Energy Distribution, and Temperature Structure of Procyon

We have fit synthetic visibilities from three-dimensional ((COBOLD)-B-5+PHOENIX) and one-dimensional (PHOENIX, ATLAS 12) model stellar atmospheres of Procyon (F5 IV) to high-precision interferometric data from the VLT Interferometer (K band) and from the Mark III interferometer (500 and 800 nm). These data sets provide a test of theoretical wavelength-dependent limb-darkening predictions. The work of Allende Prieto et al. has shown that the temperature structure from a spatially and temporally averaged three-dimensional hydrodynamic model produces significantly less limb darkening at 500 nm relative to the temperature structure of a one-dimensional MARCS model atmosphere with a standard mixing-length approximation for convection. Our direct fits to the interferometric data confirm this prediction. A one-dimensional ATLAS 12 model with approximate provides the required temperature gradient. We show, however, that one-dimensional models cannot reproduce the ultraviolet spectrophotometry below 160 nm with effective temperatures in the range constrained by the measured bolometric flux and angular diameter. We find that a good match to the full spectral energy distribution can be obtained with a composite model consisting of a weighted average of 12 one-dimensional model atmospheres based on the surface intensity distribution of a three-dimensional granulation simulation. We emphasize that one-dimensional models with overshooting may realistically represent the mean temperature structure of F-type stars such as Procyon, but the same models will predict redder colors than observed because they lack the multicomponent temperature distribution expected for the surfaces of these stars. (Less)

On the effects of the lithosphere on mantle convection and evolution

An idealized model is discussed for plate motion and mantle convection. It consists of a “numerical” square box filled with a Newtonian-Boussinesq fluid heated from below. At the surface, the fluid has a high viscosity, i.e. it contains a stiff lid which may or may not be laterally decoupled by weakness zones. Lid stiffness and coupling were varied systematically. Only if the lid is effectively decoupled laterally, does it move as a quasi-rigid plate. It is then driven by the lateral pressure differential from the rising and descending plumes in the fluid and it moves against viscous shear at its bottom. Gravitational sliding and sinking may also drive the plate if allowed for in the model. A decoupled freely-moving stiff plate effectively removes heat from the interior. A strongly-coupled stiff lid, on the other hand, or one with a very large aspect ratio inhibits heat transfer through the convecting layer. If heating occurs by a given flux through the bottom or internal heat generation the internal cell temperatures become very high, because the “stagnant” surface lid can only conduct the heat by assuming the required temperature gradient throughout its thickness. Since the Earth is partly covered by conducting continental lids which in addition contain much of the heat sources, the thermal history may be affected; caution must be exercised in parameterized convection modelling in this respect. The overall effect cannot easily be predicted because mantle flow and continental plates will interact. The importance of mechanical decoupling for plate motion strongly calls for thorough studies of the decoupling mechanisms, particularly at convergent plate boundaries, bending, stick-slip processes, and the role of volatiles.

Lunar thermal anomalies and internal heating

The evidence for the existence of the so-called linear thermal anomaly recently reported on the western margin of Mare Humorum is examined critically. Whether the anomalous cooling of Mare Humorum considered as a whole can be caused by internal heating is rejected on the basis of (1) the measured temperature differential compared to the environs observed during the lunar day, (2) the required temperature gradient, and (3) energy considerations compared to terrestrial heat flow. It is concluded that the observed hot spots are unlikely to be caused by internal heating.

The Theory of Geological Thermal Cycles

IN a review of the second edition of Prof. J. Joly's “Surface History of the Earth” (NATURE, Feb. 14, p. 227) Prof. A. Holmes makes the rather sweeping statement that the proposed mechanism for the alternating accumulation and discharge of heat seems to be physically unacceptable. He appears to justify this view from Dr. Jeffreys' opinion, who originally based his condemnation of the theory on the fact that there is no periodic solution to the equation giving the heat flow in a solid medium. When, however, change of state occurs in the medium, we have to allow both for the motion of the medium itself and also for the latent heat of fusion, and obviously the original differential equation will have very little bearing on the result. Dr. Jeffreys, in his second edition of “The Earth”, has simply solved this problem by assuming that if at any depth the radioactive materials were sufficient to raise the medium above the melting point, then the resulting convection currents would exactly carry off the excess heat and a steady state would again be attained. Mathematically, this is equivalent to assuming that the effective conductivity of the medium would adjust itself to give the required heat flow at the required temperature gradient. It is not often that Nature is so accommodating.