Showing papers on "Thermoelectric effect published in 2000"
TL;DR: The material CsBi(4)Te(6) has been synthesized and its properties have been studied, and at cryogenic temperatures, the thermoelectric properties appear to match or exceed those of Bi(2-x)Sb(x)Te (3-y)Se(y) alloys.
Abstract: Thermoelectric (Peltier) heat pumps are capable of refrigerating solid or fluid objects, and unlike conventional vapor compressor systems, they can be miniaturized without loss of efficiency. More efficient thermoelectric materials need to be identified, especially for low-temperature applications in electronics and devices. The material CsBi(4)Te(6) has been synthesized and its properties have been studied. When doped appropriately, it exhibits a high thermoelectric figure of merit below room temperature (ZT(max) approximately 0.8 at 225 kelvin). At cryogenic temperatures, the thermoelectric properties of CsBi(4)Te(6) appear to match or exceed those of Bi(2-x)Sb(x)Te(3-y)Se(y) alloys.
795 citations
TL;DR: In this article, a polycrystalline skutterudite partially filled with ytterbium ions was shown to have a relatively high dimensionless figure of merit (ZT) in the presence of Yb atoms.
Abstract: We present evidence of a relatively high dimensionless figure of merit (ZT) in a polycrystalline skutterudite partially filled with ytterbium ions. The small-diameter yet heavy-mass Yb atoms partially filling the voids of the host CoSb3 system exhibit low values of thermal conductivity while the quite favorable electronic properties are not substantially perturbed by the addition of Yb. This combination is ideal for thermoelectric applications exemplifying the “phonon-glass electron-crystal” concept of a thermoelectric material, resulting in ZT=0.3 at room temperature and ZT∼1 at 600 K for Yb0.19Co4Sb12.
518 citations
TL;DR: In this paper, a theoretical model for the transport properties of cylindrical Bi nanowires is presented, which shows that the trigonal axis is the most favorable wire orientation for thermoelectric applications.
Abstract: We report here a theoretical model for the transport properties of cylindrical Bi nanowires. Based on the band structure of Bi nanowires and the semiclassical transport model, the thermoelectric figure of merit ${Z}_{1\mathrm{D}}T$ is calculated for Bi nanowires with various wire diameters and wire orientations. The results show the trigonal axis is the most favorable wire orientation for thermoelectric applications, and ${Z}_{1\mathrm{D}}Tg1$ is predicted for n-type trigonal wires with diameters ${d}_{w}l10$ nm. The effect of the T-point holes on ${Z}_{1\mathrm{D}}T$ is also investigated. It is found that ${Z}_{1\mathrm{D}}T$ can be significantly enhanced, especially for p-type Bi nanowires, if the T-point holes are removed or suppressed.
497 citations
TL;DR: An oxide single-crystalline whisker with high thermoelectric properties at temperatures (T) higher than 600 K in air has been discovered as mentioned in this paper, which has a layered structure in which Co-O layers of two different kinds alternate in the direction of the c-axis.
Abstract: An oxide single-crystalline whisker with high thermoelectric properties at temperatures (T) higher than 600 K in air has been discovered. This whisker is assigned to Ca2Co2O5 phase (abbreviated to Co-225 whiskers) and has a layered structure in which Co–O layers of two different kinds alternate in the direction of the c-axis. Seebeck coefficient of the whiskers is higher than 100 µVK-1 at 100 K and increases with temperature up to 210 µVK-1. Temperature dependence of electric resistivity shows a semiconducting-like behavior. These results indicate that the electric carriers are transported via hopping conduction. Using thermal conductivity of a Co-225 polycrystalline sample, figure of merit (ZT) of the Co-225 whiskers is estimated 1.2–2.7 at T≥873 K. This compound is characterized with regard to low mobility and high density of carriers, which contradicts the conventional materials with high thermoelectric properties.
495 citations
TL;DR: In this paper, the authors investigated the thermoelectric properties of PbSexTe1−x/PbTe quantum-dot superlattices for possible improved thermoe-lectric materials.
Abstract: Following the experimentally observed Seebeck coefficient enhancement in PbTe quantum wells in Pb1−xEuxTe/PbTe multiple-quantum-well structures which indicated the potential usefulness of low dimensionality, we have investigated the thermoelectric properties of PbSexTe1−x/PbTe quantum-dot superlattices for possible improved thermoelectric materials. We have again found enhancements in Seebeck coefficient and thermoelectric figure of merit (ZT) relative to bulk values, which occur through the various physics and materials science phenomena associated with the quantum-dot structures. To date, we have obtained estimated ZT values approximately double the best bulk PbTe values, with estimated ZT as high as about 0.9 at 300 K.
280 citations
TL;DR: In this article, a polycrystalline sample of the composite crystal [Ca2CoO3.34]0.614 [CoO2], also known as Ca3Co4O9, has been measured below 300 K. The resulting dimensionless figure of merit becomes ZT300 = 3.5×10-2.
Abstract: Electric resistivity, thermoelectric power and thermal conductivity of a polycrystalline sample of the composite crystal [Ca2CoO3.34]0.614[CoO2], also known as Ca3Co4O9, have been measured below 300 K. Metallic conductivity accompanied by large thermoelectric power has been observed down to 50 K. At 300 K, the sample exhibits a thermoelectric power of S = 133 µVK-1, resistivity of ρ= 15 mΩcm and thermal conductivity of κ= 9.8 mWK-1cm-1. The resulting dimensionless figure of merit becomes ZT300 = 3.5×10-2, which is comparable to the value reported for a polycrystalline sample of NaCo2O4, indicating that the title compound is a potential candidate for a thermoelectric material.
240 citations
TL;DR: In this paper, X-ray powder diffraction and differential thermal analysis of polycrystalline samples of clathrate compounds were used to characterize the Seebeck coefficient and electrical resistivity of the clathrates.
Abstract: Polycrystalline samples of clathrate compounds Ba8Ga16Si30, Ba8Ga16Ge30, Ba8Ga16Sn30, and Sr8Ga16Ge30 were prepared by direct melting and characterized using X-ray powder diffraction and differential thermal analysis. The Ge- and Si-based clathrates melt congruently, whereas Ba8Ga16Sn30 melts incongruently. At room temperature the Ge- and Si-based clathrates possess a moderate negative Seebeck coefficient and a high electron concentration in the range of 7×1020–9×1020 cm−3 while Ba8Ga16Sn30 exhibits substantially lower electron concentration of 2.2×1019 cm−3. The Seebeck coefficient and electrical resistivity were measured over the range 100–870 K. The temperature dependence of transport properties of the clathrates is typical for heavily doped semiconductors. The transport properties were analyzed using a standard semiconductor transport model. There is a good agreement between the assumed model and experimental temperature dependence of the Seebeck coefficient in the extrinsic conductivity range for all...
237 citations
TL;DR: In this paper, a p-type polycrystalline material with a layered structure has been prepared by partial melting and the chemical compositions of the samples are Bi2Sr2Co2O9 (BC-2202), Bi1.8Sr 2Co2Ox (Bi-1.6), and Bi 2Sr 1.8Co2 Ox (Sr-1 8.8).
Abstract: Bi2Sr2Co2O9 (BC-2202) polycrystalline materials with a layered structure have been prepared by partial melting. The chemical compositions of the samples are Bi2Sr2Co2Ox (2202), Bi1.8Sr2Co2Ox (Bi-1.8), and Bi2Sr1.8Co2Ox (Sr-1.8). All three samples are p-type conductors. The electric properties, namely, the Seebeck coefficient (S) and electric resistivity (ρ), of the samples are dependent on chemical composition. The S values increase with temperature at T>673 K and, at 973 K, reach 100, 110, and 150 μV K−1 for the 2202, the Bi-1.8, and the Sr-1.8 samples, respectively. Thermal conductivity (κ) for all samples is lower than for ordinary conducting oxides. The figure of merit (Z) increases with temperature for all samples. Z values at 973 K are 0.77×10−4, 0.61×10−4, and 2.0×10−4 K−1 for the 2202, Bi-1.8, and Sr-1.8 samples, respectively. The thermoelectric properties depend on the chemical composition of the BC-2202 phase. The BC-2202 material thus appears to be a promising thermoelectric material due to its...
231 citations
TL;DR: The p-type behavior was confirmed by the Seebeck effect as discussed by the authors, which indicated that the p-layer conductivity of the transparent CuScO2+x films is a function of the temperature dependence of the conductivity.
Abstract: Transparent films of CuScO2+x have been prepared which show p-type electrical conductivity. The temperature dependence of the conductivity indicates semiconducting behavior with an apparent room temperature activation energy of 0.11 eV. The highest room temperature conductivity observed was 30 S cm−1. Films 110 nm thick show 40% transparency in most of the visible spectrum and become much more transparent in the infrared spectrum. The p-type behavior was confirmed by the Seebeck effect.
227 citations
TL;DR: The thermoelectric power (TEP) of single-walled carbon nanotubes (SWNTs) is extremely sensitive to gas exposure history, leading to apparently "variable" TEP for a given sample at a given temperature.
Abstract: The thermoelectric power (TEP) of single-walled carbon nanotubes (SWNTs) is extremely sensitive to gas exposure history. Samples exposed to air or oxygen have an always positive TEP, suggestive of holelike carriers. However, at fixed temperature the TEP crosses zero and becomes progressively more negative as the SWNTs are stripped of oxygen. The time constant for oxygen adsorption/desorption is strongly temperature dependent and ranges from seconds to many days, leading to apparently "variable" TEP for a given sample at a given temperature. The saturated TEP can be accounted for within a model of strong oxygen doping of the semiconducting nanotubes.
215 citations
TL;DR: In this article, the performance of a thermoelectric module is determined experimentally and the results are then used in the system analysis of a thermal network model, where the thermal resistance of the heat sink is chosen as one of the key parameters.
Abstract: A system design method of thermoelectric cooler is developed in the present study. The design calculation utilizes the performance curve of the thermoelectric module that is determined experimentally. An automatic test apparatus was designed and built to illustrate the testing. The performance test results of the module are used to determine the physical properties and derive an empirical relation for the performance of thermoelectric module. These results are then used in the system analysis of a thermoelectric cooler using a thermal network model. The thermal resistance of heat sink is chosen as one of the key parameters in the design of a thermoelectric cooler. The system simulation shows that there exists a cheapest heat sink for the design of a thermoelectric cooler. It is also shown that the system simulation coincides with experimental data of a thermoelectric cooler using an air-cooled heat sink with thermal resistance 0.2515°C/W. An optimal design of thermoelectric cooler at the conditions of optimal COP is also studied. The optimal design can be made either on the basis of the maximum value of the optimal cooling capacity, or on the basis of the best heat sink technology available.
TL;DR: In this article, the effect of Sb doping on the half-Heusler alloys (MgAgAs type) with the general formula MNiSn where M is a group IV transition metal was investigated.
Abstract: Half-Heusler alloys (MgAgAs type) with the general formula MNiSn where M is a group IV transition metal (Hf, Zr, or Ti) are currently under investigation for potential thermoelectric materials. These materials exhibit a high negative thermopower (−40 to −250 μV/K) and low electrical resistivity values (0.1–8 mΩ cm) both of which are necessary for a potential thermoelectric material. Results are presented in this letter regarding the effect of Sb doping on the Sn site (TiNiSn1−xSbx). The Sb doping leads to a relatively large power factor of (0.2–1.0) W/m K at room temperature for small concentrations of Sb. These values are comparable to that of Bi2Te3 alloys, which are the current state-of-the-art thermoelectric materials. The power factor is much larger at T≈650 K where it is over 4 W/m K making these materials very attractive for potential power generation considerations.
TL;DR: In this article, a new series of oxides Ca3-xBixCo4O9+δ, (x = 0.0−0.75) with Ca2Co2O5-type structures were synthesized, and their structures, electrical properties, Seebeck coefficients, and thermal conductivities were measured.
Abstract: A new series of oxides Ca3-xBixCo4O9+δ, (x = 0.0−0.75) with Ca2Co2O5-type structures were synthesized, and their structures, electrical properties, Seebeck coefficients, and thermal conductivities were measured. The values of Seebeck coefficients of the new oxides are all positive, showing that they are p-type conductors. Both the electrical conductivity and Seebeck coefficients increase with the increasing Bi contents which can be attributed to the increase of carrier mobility due to the larger size of Bi ion. The electrical conductivity, Seebeck coefficient, and the calculated value of the power factor of Ca3-xBixCo4O9+δ (x = 0.5) are 105 S cm-1, 160 μV K-1, and 2.7 × 10-4 W K-2 m-1 at 700 °C, respectively. The thermal conductivity of Ca3-xBixCo4O9+δ (x = 0.5) at room temperature is 1.14 W m-1 K-1 and increase slightly with the increasing temperature. At 700 °C, the figure of merit of Ca3-xBixCo4O9+δ (x = 0.5) is 2.0 × 10-4 K-1.
TL;DR: In this paper, the Hall coefficient, electrical resistivity, and Seebeck coefficient of n-type specimens of β-Ag2Se, the low temperature polymorph of silver selenide, were measured over the temperature range from 70 to 300 K.
Abstract: The Hall coefficient, electrical resistivity, and Seebeck coefficient of n-type specimens of β-Ag2Se, the low temperature polymorph of silver selenide, were measured over the temperature range from 70 to 300 K. The results showed maxima in both Hall coefficient and the electrical resistivity just below the onset of the intrinsic conduction range. This anomaly was qualitatively explained by the deviation of the Coulomb scattering from the usual assumption of independence due to the degenerate nature of the samples. The estimated energy gap for different samples of about 160 meV seems to confirm the existence of the second low temperature phase β2. This second phase is a probable reason for the relatively high thermoelectric figure of merit observed.
TL;DR: In this article, the thermoelectric properties have not been optimized in these multinary alloys and the lowest thermal conductivity obtained in the substituted alloys is ∼3.0 W/m
Abstract: Unlike semiconducting TiCoSb, ZrCoSb and HfCoSb half-Heusler phases are semimetallic below room temperature and exhibit small Seebeck coefficients of ∼−10 μV/K at 300 K. However, upon substituting (doping) the Co and Sb sites with Pt and Sn, respectively, much larger thermopowers (S) are obtained. For ZrCoSb, S reaches −110 and +130 μV/K while resistivity ρ decreases from ∼5×104 μΩ cm in the undoped phase to 1–2×103 μΩ cm in the substituted phases at 300 K. The lowest thermal conductivity obtained in the substituted alloys is ∼3.0 W/m K at 300 K, which is among the lowest reported for this class of structural phases. There are indications that the thermoelectric properties have not been optimized in these multinary alloys.
TL;DR: In this paper, the tin-filled skutterudites were synthesized under high pressure and temperature conditions; they cannot be synthesised under normal synthesis approaches; they exhibit n-type semiconducting behavior with relatively high Seebeck coefficients for compounds whose electronic properties have not been optimized.
Abstract: Thermal conductivity, resistivity, Seebeck coefficient, and structure measurements of CoSb3 with tin interstitially placed in the voids are reported. These tin-filled skutterudites were synthesized under high pressure and temperature conditions; they cannot be synthesized under “normal” synthesis approaches. The tin atoms exhibit very large atomic displacement parameters indicating a large “rattling” motion inside their atomic “cages.” The disorder induced by the Sn atoms is a very good phonon scattering mechanism. The thermal conductivity of these compounds is very low with a temperature dependence that is atypical of simple solids. The tin-filled compounds exhibit n-type semiconducting behavior with relatively high Seebeck coefficients for compounds whose electronic properties have not been optimized. The potential for thermoelectric applications is discussed.
01 May 2000
TL;DR: In this paper, an electrical model for a Peltier cell based on the analogy between thermal and electrical variables is proposed, which allows the global performance of thermocooling circuit and signal system to be checked by using electrical circuit analysis programs such as SPICE.
Abstract: An electrical model for a Peltier cell, based on the analogy between thermal and electrical variables, is proposed. The use of thermal models allows the global performance of thermocooling circuit and signal system to be checked by using electrical circuit analysis programs such as SPICE. The maximum error in the steady state between measured and simulated temperatures are less than 0.3/spl deg/C for hot temperature and less than 0.2/spl deg/C for cold temperature with 31.7/spl deg/C of temperature change.
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11 Jul 2000TL;DR: In this article, the Peltier effect thermoelectric modules and wire heating elements embedded along the edges of the block can be energized in such a manner as to achieve a constant temperature throughout the array or a temperature gradient.
Abstract: Temperature control in a rectangular array of reaction vessels such as a thermal cycler such as is used for PCR procedures is achieved by use of a temperature block that is in contact with a combination of Peltier effect thermoelectric modules and wire heating elements embedded along the edges of the block. The elements can be energized in such a manner as to achieve a constant temperature throughout the array or a temperature gradient. Further control over the temperature and prevention of condensation in the individual reaction vessels is achieved by the use of a glass (or other transparent material) plate positioned above the vessels, with an electrically conductive coating on the upper surface of the glass plate to provide resistance heating.
TL;DR: In this article, a set of single crystals of the layered cobaltite Bi2.3-xPbxSr2.6Co2Oy (0 ≤x≤0.44) were prepared by a flux technique.
Abstract: A set of single crystals of the layered cobaltite Bi2.3-xPbxSr2.6Co2Oy (0 ≤x≤0.44) were prepared by a flux technique. The resistivity ρ and the thermopower S along the in-plane direction were measured, and the thermoelectric properties were compared with those of other oxides. The Pb substitution causes a decrease in ρ and an increase in S below 100 K, which improves the power factor S2/ρ by 3–4 times. The transport parameters exhibit an anomaly near 25–50 K, such as a plateau in S and an upturn in ρ, which we ascribe to the reduction of the density of states owing to a pseudogap.
TL;DR: The thermoelectric properties of the pure and doped half-Heusler compounds FeVSb and FeNbSb are reported in this paper, and the electrical resistivities are between 0.2 and 20 mΩ cm at room temperature.
Abstract: The thermoelectric properties of the pure and doped half-Heusler compounds FeVSb and FeNbSb are reported. The electrical resistivities are between 0.2 and 20 mΩ cm at room temperature. Thermoelectric power measurements indicate that FeVSb is an n-type material with moderate Seebeck coefficients near −70 μV/K at 300 K. The thermal conductivity at room temperature is large, approximately 0.1 W/cm K, and increases with decreasing temperature. Chemical substitutions, which have a dramatic effect on the transport properties, were performed in an effort to enhance the thermoelectric performance. Band-structure calculations are presented for the pure materials.
TL;DR: In this paper, p-type single phase thermoelectric materials with high densification (>99% theoretical density) were prepared via bulk mechanical alloying (BMA) and hot pressing (HP).
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11 Oct 2000TL;DR: In this paper, a temperature control device with excellent temperature uniformity and thermal response, which can be manufactured easily, and a manufacturing method for the same, is presented, where a thermoelectric device 21 is arranged between a substrate mounting plate 1 and a cooling plate 3.
Abstract: The invention provides a temperature control device with excellent temperature uniformity and thermal response, which can be manufactured easily, and a manufacturing method for the same. A thermoelectric device 21 is arranged between a substrate mounting plate 1 and a cooling plate 3. Copper foil electrodes 5, 5, . . . on the upper side of the thermoelectric device 21 are adhered to a lower surface of the substrate mounting plate 1 with an adhesive sheet 17 covering substantially the entire lower surface of the substrate mounting plate 1, and copper foil electrodes 7, 7, . . . on the lower side of the thermoelectric device 21 are adhered to an upper surface of the cooling plate 3 with an adhesive sheet 19 covering substantially the entire upper surface of the substrate mounting plate 1. The total thickness of the adhesive sheet 17 and the copper foil electrodes 5 adhering to it, and the total thickness of the adhesive sheet 19 and the copper foil electrodes 7 adhering to it, is each set approximately 25 to 1000 μm thin, so as to make the thermal resistance sufficiently low. Thermoelectric conversion elements 9 and 13 are distributed across a large area covering at least the entire area corresponding to the substrate 2.
TL;DR: In this article, a two-stage solar energy concentrator with a primary one axis parabolic trough concentrator and a second stage compound parabolic concentrator mounted at the focus of the primary was designed to tolerate tracking misalignment, maintain temperature gradients to suit thermoelectric generation and minimize heat losses.
31 Jan 2000-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this article, the applicability of extrusion process to fabricate Bi 2 Te 3 thermoelements was discussed and high quality products were obtained by hot-extrusion method and their texture and the thermoelectric properties measured.
Abstract: This paper concerns about the applicability of extrusion process to fabricate Bi 2 Te 3 thermoelements. High quality products were obtained by hot-extrusion method and their texture and the thermoelectric properties measured. The hot-extruded material had very fine grains derived from dynamic recrystallization. It was found that hot-extruded material has a ring-like texture in which the normal to the basal plane orients to the radial direction and 〈110〉 orients to the extrusion direction. Figure of merit of the extrudate increases with decreasing extrusion temperature and the value of the product extruded at 693 K showed a value corresponding to about 88% of unidirectionally solidified elements.
IBM1
TL;DR: A review of thermoelectric cooling and its application to the cooling of electronic equipment is provided in this article, with a background discussion of early history, current developments, and the defining temperature-pumping equations.
Abstract: This paper provides a review of thermoelectric cooling and its application to the cooling of electronic equipment. A background discussion of thermoelectric cooling is provided briefly citing early history, current developments, and the defining thermoelectric heat pumping equations. Several examples are provided of early IBM applications of thermoelectric cooling. An analysis to assess thermoelectric cooling enhancement in terms of increases in allowable power dissipation or chip temperature reduction is described along with results.
Patent•
04 Feb 2000
TL;DR: In this paper, a semiconductor device is fabricated by forming an integrated circuit on a front side of the substrate, and forming a thermoelectric cooler on a back side of a substrate.
Abstract: Method and apparatus for thermal management of an integrated circuit A semiconductor device includes an integrated circuit and an integrated thermoelectric cooler formed on a common substrate A semiconductor device is fabricated by forming an integrated circuit on a front side of the substrate and forming an integrated thermoelectric cooler on a back side of the substrate A first thermal sink of semiconductor material capable of absorbing heat from the integrated circuit is formed on the back side of the substrate N-type thermoelectric elements are formed on contacts formed on the first thermal sink P-type thermoelectric elements are formed on contacts formed on a second thermal sink of semiconductor material capable of dissipating heat The p-type and n-type thermoelectric elements are bonded to the contacts on the first and second thermal sinks, respectively, by a flip-chip soldering process Using this method, semiconductor devices including an integrated circuit and integrated modules of thermoelectric elements are formed having cooling capacities corresponding to heat dissipated from different portions of the integrated circuit As a result, substantially uniform temperature distribution across the integrated circuit can be achieved
TL;DR: In this article, X-ray powder diffraction analysis of stoichiometric ternary compounds in the quasi-binary systems GeTe, Bi2Te3, SnTe, and PbBi2Te4 has been performed.
TL;DR: In this paper, the authors report the results of a study of thermoelectric properties of chemically substituted quasiternary materials related to the recently discovered filled skutterudite compound YbFe4Sb12.
Abstract: We report the results of a study of thermoelectric properties of chemically substituted quasiternary materials related to the recently discovered filled skutterudite compound YbFe4Sb12 The study explored partial filling at the Yb site as well as chemical doping with Sn at the Sb site in an attempt to optimize the thermoelectric figure of merit ZT in the system YbyCo4SnxSb12−x Our measurements of these physical quantities from room temperature down to T=10 K indicate that, in our study, only the alloy Yb044Co4Sb12 possessed thermoelectric properties that are improved over the parent compound YbFe4Sb12, attaining a value of ZT=01 at T=300 K
TL;DR: In this paper, the authors theoretically investigated the in-plane lattice thermal conductivity of a quantum-dot superlattice and obtained the results for the most recently suggested applications of SiGe quantum dot super-lattices for thermoelectric devices.
Abstract: We have theoretically investigated the in-plane lattice thermal conductivity of a quantum-dot superlattice. The calculations were carried out for a structure that consists of multiple layers of Si with randomly distributed Ge quantum dots separated by wetting layers and spacers. Our model takes into account scattering of acoustic phonons on spherical quantum dots, and corresponding modification of the phonon dispersion relation. The finite acoustic mismatch between Si and Ge is also taken into account. The obtained results are important for the most recently suggested applications of SiGe quantum-dot superlattices for thermoelectric devices.
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23 Feb 2000
TL;DR: A thermoelectric-cooling temperature control apparatus for a semiconductor device fabrication facility maintains a temperature condition, such as the temperature of a wafer, stable during the fabrication process.
Abstract: A thermoelectric-cooling temperature control apparatus for a semiconductor device fabrication facility maintains a temperature condition, such as the temperature of a wafer, stable during the fabrication process. The apparatus includes a heat exchanger having a thermoelectric cooling element which produces a Peltier Effect to cause the element to absorb and radiate heat according to current flowing through the element