Direct Joule Heating Research Articles

Novel Experimental Approach to Determine Elastocaloric Latent Heat

One of the most important parameters of superelastic shape memory alloys to be used in elastocaloric cooling and heating processes is their specific latent heat. Usually, the latent heat of a material is determined by differential scanning calorimetry (DSC) where the material phase transformation is induced thermally under zero stress. During elastocaloric processes however, the latent heat becomes accessible by stress-induced transformation under tensile or compression loading and unloading of the sample. In recent elastocaloric experiments, we observed drastic differences between latent heat values determined in DSC experiments and the ΔT values observed in nearly adiabatic elastocaloric cycles, which reflect the latent heat; in fact, the DSC experiments predicted rather pessimistic values and thus poor cooling performance. Based on these observations we developed and tested a novel experimental approach to determine the latent heat of superelastic materials directly during the elastocaloric process. By comparing or combining direct Joule heating with the strain based process we are able to accurately determine the latent heat for both tensile loading and unloading for any elastocaloric heating or cooling process. Furthermore, the influence of applied mechanical parameters as well as material conditioning on the latent heat can be observed in the elastocaloric experiment.

A Hollow Polyethylene Fiber-Based Artificial Muscle

Recently, researches on artificial muscles for imitating the functions of the natural muscles has attracted wide attention. The fiber-shape actuators, shape-memory materials or deforming devices, which are similar to human muscle fiber bundles, have extensively studied and provided more possibilities for artificial muscles. Herein, we develop a thermal responsible fiber-shaped actuator based on the low-cost hollow polyethylene fiber. The sheath-core structured fibrous actuators and the stainless-steel conductive yarn winded pre-stretched polyethylene actuators are fabricated with the heating assisted pre-stretching procedure. The actuation mechanism of the thermal-responsive orientation change of molecular chains driving the actuation is discussed and demonstrated by 2D XRD patterns. These polyethylene-based fibrous actuators displayed three significant advantages including (i) color-turning and shape-changing bifunctional response, (ii) direct joule heating actuation and (iii) effective contraction (18% shrinkage of the pristine length) and lifting ability (the ratio of lifting weight to self-weight is up to 50).

In Situ Transmission Electron Microscopy Analysis of Copper-Germanium Nanowire Solid-State Reaction.

A promising approach of making high quality contacts on semiconductors is a silicidation (for silicon) or germanidation (for germanium) annealing process, where the metal enters the semiconductor and creates a low resistance intermetallic phase. In a nanowire, this process allows one to fabricate axial heterostructures with dimensions depending only on the control and understanding of the thermally induced solid-state reaction. In this work, we present the first observation of both germanium and copper diffusion in opposite directions during the solid-state reaction of Cu contacts on Ge nanowires using in situ Joule heating in a transmission electron microscope. The in situ observations allow us to follow the reaction in real time with nanometer spatial resolution. We follow the advancement of the reaction interface over time, which gives precious information on the kinetics of this reaction. We combine the kinetic study with ex situ characterization using model-based energy dispersive X-ray spectroscopy (EDX) indicating that both Ge and Cu diffuse at the surface of the created Cu3Ge segment and the reaction rate is limited by Ge surface diffusion at temperatures between 360 and 600 °C. During the reaction, germanide crystals typically protrude from the reacted NW part. However, their formation can be avoided using a shell around the initial Ge NW. Ha direct Joule heating experiments show slower reaction speeds indicating that the reaction can be initiated at lower temperatures. Moreover, they allow combining electrical measurements and heating in a single contacting scheme, rendering the Cu-Ge NW system promising for applications where very abrupt contacts and a perfectly controlled size of the semiconducting region is required. Clearly, in situ TEM is a powerful technique to better understand the reaction kinetics and mechanism of metal-semiconductor phase formation.

Nonlinear chiral refrigerators

We investigate a mesoscopic refrigerator based on chiral quantum Hall edge channels. We discuss a three-terminal cooling device in which charge transport occurs between a pair of voltage-biased terminals only. The third terminal, which is to be cooled, is set as a voltage probe with vanishing particle flux. This largely prevents the generation of direct Joule heating which ensures a high coefficient of performance. Cooling operation is based on energy-dependent quantum transmissions. The latter are implemented with the aid of two tunable scattering resonances (quantum dots). To find the optimal performance point and the largest temperature difference created with our refrigerator, it is crucial to address the nonlinear regime of transport, accounting for electron-electron interaction effects. Our numerical simulations show that the maximal cooling power can be tuned with the quantum dot couplings and energy levels. Further, we provide analytical expressions within a weakly nonlinear scattering-matrix formalism which allow us to discuss the conditions for optimal cooling in terms of generalized thermopowers. Our results are important for the assessment of chiral conductors as promising candidates for efficient quantum refrigerators with low dissipation.

Investigation of functional characteristics of a synergistically configured parallel-type shape memory alloy variable stiffness actuator

Variable stiffness actuator is the essential element for force control and motion generation of most mechanical systems that are used for soft robotic, biomimetic and biomedical applications. Mechanical systems when integrated with smart materials enable controlled variation of stiffness in addition to control of force and displacement, thus exhibiting superior characteristics. Active compliant/variable stiffness actuator using shape memory alloy allows simple and compact design, exhibits comparable actuator efficiency and offers directional controllable actuation, self-sensing and a wide range of variable stiffness. The functional characteristics of a synergistically operated translational/linear compliant variable stiffness actuator, configured synergistically with shape memory alloy wire(s) and passive compression spring, are determined experimentally under no load without control to offer translational actuation. Repetitive actuation is made possible through electrical activation (direct Joule heating) of shape memory alloy wire(s) and the aiding force (the restoring (bias) force) of the passive spring. Actuation with variable stiffness is obtained from the inherent stiffness variation of shape memory alloy during its phase transformation, along with the fixed stiffness of the passive compliant element. The scope of improving the range of stiffness through varied forms and connections of the elements is recognized, while retaining the basic structure without compromising its simplicity.

Measurement of heat transfer coefficient in two phase flows of radiation-resistant zeotropic C2F6/C3F8 blends

We have measured the flow boiling heat transfer coefficient (HTC) of saturated fluorocarbon blends in a horizontal copper-nickel tube with a diameter of 4mm. Direct (Joule) heating of the tube wall was used to obtain heat fluxes from 5 to 13.7kWm−2. Two different injection capillaries were used, permitting measurements at mass fluxes varying between 94 and 164kgm−2s−1. The evaporation pressure was approximately 0.2MPa for the mass fluxes close to 164kgm−2s−1 and 0.15MPa for the lower mass fluxes. The same tube dimensions and material are used in the on-detector cooling channels of the silicon micro-strip charged particle tracker (“SCT”) of the ATLAS experiment at the CERN Large Hadron Collider. The range of heat flux and mass flow studied encompasses the operating conditions of the tracker. When operating in the high radiation environment near to the proton beam collisions radiation tolerant coolants are essential. Saturated fluorocarbons (CnF(2n+2)) are radiation resistant and allow thermodynamic “tailoring” by blending saturated molecules of different orders. Measurements were made with pure C3F8 (R218) and zeotropic blends containing 5, 10, 15, 20 and 25% (molar) C2F6 (R116). This work is a continuation of a previous study which showed that the operating temperature of the ATLAS SCT could be reduced by around 10°C with the admixture of 25% (molar) C2F6, with no changes needed to the existing on-detector and circulatory pipework. The data analysis revealed multiple flow boiling regimes of the two-phase flow that varied as a function of coolant flow rate, heat flux, vapour quality and mixture composition. As expected, the HTC in pure C3F8 was higher than in blends with increasing C2F6 content. Nevertheless, the study confirmed that the ATLAS SCT could be operated at full power dissipation with cooling tube temperatures up to 10°C colder than in pure C3F8 with C3F8/C2F6 blends having relatively modest values of HTCs in the range 1500–4000Wm−2K−1.

Ohmic Heating: An Emerging Concept in Organic Synthesis.

The ohmic heating also known as direct Joule heating, is an advanced thermal processing method, mainly used in the food industry to rapidly increase the temperature for either cooking or sterilization purposes. Its use in organic synthesis, in the heating of chemical reactors, is an emerging method that shows great potential, the development of which has started recently. This Concept article focuses on the use of ohmic heating as a new tool for organic synthesis. It presents the fundamentals of ohmic heating and makes a qualitative and quantitative comparison with other common heating methods. A brief description of the ohmic reactor prototype in operation is presented as well as recent examples of its use in organic synthesis at laboratory scale, thus showing the current state of the research. The advantages and limitations of this heating method, as well as its main current applications are also discussed. Finally, the prospects and potential implications of ohmic heating in future research in chemical synthesis are proposed.

NiMnGa/Si Shape Memory Bimorph Nanoactuation

The size dependences of thermal bimorph and shape memory effect of nanoscale shape memory alloy (SMA)/Si bimorph actuators are investigated in situ in a scanning electron microscope and by finite element simulations. By combining silicon nanomachining and magnetron sputtering, freestanding NiMnGa/Si bimorph cantilever structures with film/substrate thickness of 200/250 nm and decreasing lateral dimensions are fabricated. Electrical resistance and mechanical beam bending tests upon direct Joule heating demonstrate martensitic phase transformation and reversible thermal bimorph effect, respectively. Corresponding characteristics are strongly affected by the large temperature gradient in the order of 50 K/µm forming along the nano bimorph cantilever upon electro-thermal actuation, which, in addition, depends on the size-dependent heat conductivity in the Si nano layer. Furthermore, the martensitic transformation temperatures show a size-dependent decrease by about 40 K for decreasing lateral dimensions down to 200 nm. The effects of heating temperature and stress distribution on the nanoactuation performance are analyzed by finite element simulations revealing thickness ratio of SMA/Si of 90/250 nm to achieve an optimum SME. Differential thermal expansion and thermo-elastic effects are discriminated by comparative measurements and simulations on Ni/Si bimorph reference actuators.

Direct Resistance Joule Heating of Al-10 pct Si-Coated Press Hardening Steel

Various rapid heating methods have been developed to increase the productivity of press hardening steel. One of these methods is direct resistance Joule heating. This heating method results in the melting of the surface coating and the formation of a persistent liquid trail as a result of the high thermal conductivity and low melting temperature of the Al-10 pct Si alloy coating. This can be addressed by an alloying preheating treatment prior to the press hardening process.

Investigation of the effects of thermal gradients present in Gleeble high-temperature tensile tests on the strain state for free cutting steel

A common feature of uniaxial high-temperature tension tests, and to a certain extent compression tests, performed using a Gleeble thermomechanical testing system that employs direct resistance heating for the characterisation of the rheological behaviour of materials is longitudinal and radial thermal gradients. The aim of this article is to experimentally quantify the axial thermal gradients for a given tensile specimen geometry of free cutting steel during heating and deformation, and design a modelling methodology to simulate their influence on the strain distribution as compared to the assumption of isothermal heating and deformation. For this purpose, a feedback algorithm was developed to control the electric current input in a similar manner to that applied by the Gleeble testing system, which implemented via the UAMP user subroutine in ABAQUS for use in electro-thermal simulations of the direct resistance Joule heating used by the Gleeble testing system. The predicted temperature fields were compared with the temperature distributions recorded experimentally along the gauge section of the tensile specimens. Finite element simulations of Gleeble tensile tests were carried out under isothermal conditions and using the temperature distributions calculated by the feedback algorithm for a range of strain rates and temperatures in order to evaluate the difference in predicted stress state. The results show that an isothermal assumption should only be used conservatively in finite element simulation of the Gleeble thermomechanical test employing direct resistance heating to avoid significant errors.

Liquid single crystal elastomer/conducting polymer bilayer composite actuator: modelling and experiments

In order to integrate electroconductive properties in a Liquid Single Crystal Elastomer (LSCE) and to test direct actuation of the LSCE by Joule heating, we present a new bi-layered all-organic composite actuator based on the coupling of a nematic LSCE with a conductive polymer. The bending actuator is fabricated by depositing a thin conductive polymer layer of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) over the surface of a polysiloxane-based monodomain nematic LSCE film. Mechanical properties of PEDOT:PSS, better matched with LSCE ones compared with metals or inorganic nanoparticles used in other approaches, allowed us to develop an all-organic reliable millimetre-scale actuating composite. The thermally induced elongation/compression of the LSCE over 30% is exploited for the fabrication of bending actuators with curvature up to κ = 0.64 mm−1. The LSCE and the composite material are characterized as regards their thermo-mechanical and electrical properties. A model is introduced to describe bending of the composite as a function of the thermo-mechanical properties of the LSCE, and the model is assessed by comparing the model results with the experimental findings. Bending actuation via direct Joule heating of the composite is also assessed by supplying the necessary current (50 mA at 1.3 V) through wires connected to the composite. These results open new possibilities for the application of LCEs in the micro and soft robotics fields, as well as in the biomedical field.

Investment casting of carbon tubular structures

Carbon tubular structures (CTSs) were prepared using metal nanowires/nanotubes, either Ag or Te, as templates. The CTSs are formed by electron beam-induced carbon deposition on the nanowires/nanotubes and the subsequent removal of the core material by direct heating or Joule heating. We call this process “investment casting” of CTSs. The length of the fabricated CTSs varies from sub-micron to tens of microns, while the diameter and wall-thickness may vary from less than ten nanometers to over a hundred nanometers and few nanometers to tens of nanometers, respectively. These parameters can be controlled by the templates and the carbon deposition. With Te nanotubes as templates, it is possible to form tube-in-tube hollow CTSs. By designed construction of Ag nanowires, various CTS-based seamless structures can also be obtained. Through thermal annealing or Joule heating, the amorphous CTSs can be converted to graphitized structures, which show excellent mechanical performance on nano-manipulation. The technique provides a new method of constructing carbon-based complex and multifunctional structures.

Thermoelectric Properties of Pulsed Electric Current Sintered Samples of AgPb m SbSe17 (m = 16 or 17)

Lead chalcogenide materials have drawn attention in recent years because of their outstanding thermoelectric properties. Bulk n-type materials of AgPbmSbTe2+m have been reported to exhibit high figure of merit, ZT, as high as 1.7 at 700 K. Recent reports have shown p-type lead selenide-based compounds with comparable ZT. The analogous material AgPbmSbSe17 shares a similar cubic rock-salt structure with PbTe-based compounds; however, it exhibits a higher melting point, and selenium is more abundant than tellurium. Using solid solution chemistry, we have fabricated cast AgPb15SbSe17 samples that show a peak power factor of approximately 17 μW/cm K2 at 450 K. Increasing the strength of such materials is commonly achieved through powder processing, which also helps to homogenize the source materials. Pulsed electric current sintering (PECS) is a hot-pressing technique that utilizes electric current through the die and sample for direct Joule heating during pressing. The mechanisms present during PECS processing have captured significant research interest and have led to some notable improvements in sample properties compared with other densification techniques. We report the thermoelectric properties of PECS samples of AgPbmSbSe17 along with sample fabrication and processing details.

About the relative importance of compressional heating and current dissipation for the formation of coronal X-ray bright points

Context. The solar corona is heated to high temperatures of the order of 10^{6} K. The coronal energy budget and specifically possible mechanisms of coronal heating (wave, DC-electric fields, ..) are poorly understood. This is particularly true as far as the formation of X-ray bright points (BPs) is concerned. Aims. Investigation of the energy budget with emphasis on the relative role and contribution of adiabatic compression versus current dissipation to the formation of coronal BPs. Methods. Three-dimensional resistive MHD simulation starts with the extrapolation of the observed magnetic field from SOHO/MDI magnetograms, which are associated with a BP observed on 19 December 2006 by Hinode. The initial radially non-uniform plasma density and temperature distribution is in accordance with an equilibrium model of chromosphere and corona. The plasma motion is included in the model as a source of energy for coronal heating. Results. Investigation of the energy conversion due to Lorentz force, pressure gradient force and Ohmic current dissipation for this bright point shows the minor effect of Joule heating in comparison to the work done by pressure gradient force in increasing the thermal energy by adiabatic compression. Especially at the time when the temperature enhancement above the bright point starts to form, compressional effects are quite dominant over the direct Joule heating. Conclusions. Choosing non-realistic high resistivity in compressible MHD models for simulation of solar corona can lead to unphysical consequences for the energy balance analysis, especially when local thermal energy enhancements are being considered.

Modeling of Shape Memory Alloy Actuators Using Likhachev’s Formulation

This article presents a simplified version of Likhachev’s micromechanical model and its integration in ANSYS, a commercial finite element software package used to simulate the response of a structure equipped with shape memory alloy wire actuators controlled by direct Joule heating. The original Likhachev’s formulation is adapted to obtain a model that is easy to characterize, numerically efficient, and general in the sense that it can simulate all the shape memory-related features using the same formulation (superelasticity, shape memory effect, stress generation, etc.). The adapted Likhachev’s formulation is coupled to an electro-thermal model to simulate temporal response of actuators heated by an electrical current. An experimental result obtained with a deformable airfoil powered by shape memory actuators is used to validate the proposed electro-thermo-mechanical model.

Electrical property changes of liquid carbon under high pressures

The homogeneous formation of liquid carbon without the partial melting of carbon specimen is very important to evaluate the physical properties of liquid carbon. The pyrolytic graphite was melted by the direct joule heating with current passed through the specimen along the direction parallel to c-plane. The resistivity of the liquid carbon at the melting point decreases from 630 to 470μΩcm with increase of pressure of 1.4 to 4.0GPa and then increases 490 to 565μΩcm with increase of 5.6 to 9.4GPa. The magnitude of resistivity of the liquid carbon at the melting point within this experiment indicates that the liquid carbon is a "poorly conductive metal". On the other hand the resistivity of liquid carbon tends to change from metal like behaviour (the temperature coefficient of resistivity dρ/dT> 0) to non-metal (or semiconductor) like one (dρ/dT<0) at the pressure of around 4GPa-5.6GPa. These results are considered to fortify our previous conclusion about the existence of the structural transformation of carbon in the liquid state [1].

Intense Atomic Oxygen Emission from Incandescent Zirconia

Particle emissions from an oxide-ion conducting solid electrolyte (tetragonal zirconia polycrystal, 3% Y2O3-doped ZrO2) have been examined using a high vacuum evaluation system. Oxygen gas with a pressure range from 10 to 3 × 103 Pa was fed into a 2 mm outer diameter zirconia tube, whose central region of 10 mm length was heated to 1400−1800 °C by direct Joule heating. The emission species were identified by appearance potential spectroscopy using a quadrupole mass spectrometer. Atomic oxygen (AO) is the dominant species that is radiantly emitted from the high-temperature region. Temperature dependence of the AO emission intensity exhibits far less activation energy (∼0.3−1.0 and 0.7 eV, on average) than electric field-assisted thermal O− ion (∼2 eV) and electron (∼6 eV) emission from zirconia reported before. High AO emission flux (in the order of 1017 atoms·cm−2·s−1) is promising for use as practical AO sources.

Experimental study of ignition of magnesium powder by electrostatic discharge

Ignition of non-aerosolized powders by electrostatic discharge (ESD) is investigated. A spherical powder of Mg, for which thermal ignition kinetics was described in the literature, was used in experiments. The experimental setup was built based on a commercially available apparatus for ESD ignition sensitivity testing. Additional diagnostics enabled measurements of electrical current, voltage, and spark and ignited powder emission in real time. The spark duration was of the order of a few μs. The spark current and voltage were always observed to have significant AC components. The electrical impedance of the spark discharge was determined experimentally using the recorded current traces and assuming that the spark and powder could be represented as a series LRC circuit. The optical emission was filtered to separate the signals produced by the spark plasma and by the heated and igniting powder. The radiation signal produced by the igniting powder was always delayed after the spark. The delay time decreased from about 3.5 to 0.5 ms as the spark energy increased from 10 to 60 mJ; the delay remained nearly constant when the spark energy continued to increase to over 100 mJ. For experiments where the powder volume decreased or where binder was introduced delay times were reduced. For the powder with a small amount of binder, the ignition delay continued to decrease for spark energies exceeding 60 mJ. Interpretation of the obtained experimental data suggests that the ignition is primarily due to direct Joule heating of the powder by the spark current.

Visualization of convective boiling heat transfer in single microchannels with different shaped cross-sections

Convective boiling in transparent single microchannels with similar hydraulic diameters but different shaped cross-sections was visualized, along with simultaneous measurement of the local heat transfer coefficient. Two types of microchannels were tested: a circular Pyrex glass microtube (210 μm inner diameter) and a square Pyrex glass microchannel (214 μm hydraulic diameter). A 100-nm-thick semi-transparent ITO/Ag thin film sputtered on the outer wall of the microchannel was used for direct joule heating of the microchannel. The flow field visualization showed semi-periodic variation in the flow patterns in both the square and circular microchannels. Such variation was because the confined space limited the bubble growth in the radial direction. In the square microchannel, both the number of nucleation bubbles and the local heat transfer coefficient increased with decreasing vapor quality. The corners acted as active nucleation cavities, leading to the higher local heat transfer coefficient. In contrast, lack of cavities in the smooth glass circular microchannel yielded a relatively smaller heat transfer coefficient at lower vapor quality. Finally, the heat transfer coefficient was higher for the square microchannel because corners in the square microchannel acted as effective active nucleation sites.

Magnetospheric energy inputs into the upper atmospheres of the giant planets

Abstract. We revisit the effects of Joule heating upon the upper atmospheres of Jupiter and Saturn. We show that in addition to direct Joule heating there is an additional input of kinetic energy – ion drag energy – which we quantify relative to the Joule heating. We also show that fluctuations about the mean electric field, as observed in the Earth's ionosphere, may significantly increase the Joule heating itself. For physically plausible parameters these effects may increase previous estimates of the upper atmospheric energy input at Saturn from ~10 TW to ~20 TW. Keywords. Ionosphere (Electric fields and currents; Planetary ionosphere) – Magnetospheric physics (Auroral phenomena)