Combination Of Low Thermal Conductivity Research Articles

MAX Phase Ti2AlN for HfO2 Memristors with Ultra‐Low Reset Current Density and Large On/Off Ratio

AbstractA Ti2AlN MAX phase layered thin film electrode and oxygen getter layer for HfO2‐based two‐terminal memristors is presented. The Ti2AlN/HfOx/Ti memristor devices exhibit enhanced resistive switching performance, including an ultra‐low reset current density (< 10−8 MΩ cm2), substantial on‐off ratio (≈ 6000), excellent multi‐level functionality (≈ 9 distinct states), impressive retention (up to 300 °C), and robust endurance (>200 million) as compared to conventional TiN and other alternative materials based memristors. Experimental measurements and modeling suggest that the distinctive combination of low thermal conductivity, high electrical conductivity, and unique ultra‐thin layer‐by‐layer structure of the Ti2AlN MAX phase thin film contribute to this exceptional performance with good reproducibility and stability. The results demonstrate for the first‐time the potential of this innovative sputtered MAX phase material for engineering energy‐efficient, high‐density non‐volatile digital, and analog memory devices aimed toward next‐generation sustainable artificial intelligence.

Medium-entropy (Me,Ti)0.1(Zr,Hf,Ce)0.9O2 (Me = Y and Ta): Promising thermal barrier materials for high-temperature thermal radiation shielding and CMAS blocking

With continuous enhancement of gas-turbine inlet temperature and rapid increase of radiant heat transfer, thermal barrier coating (TBC) materials with a combination of low thermal conductivity and good high-temperature thermal radiation shielding performance play vital roles in ensuring the durability of metallic blades. However, yttria-stabilized zirconia (YSZ), as the state-of-the-art TBC and current industry standard, is unable to meet such demands since it is almost translucent to high-temperature thermal radiation. Besides, poor corrosion resistance of YSZ to molten calcia-magnesia-alumina-silicates (CMAS) also impedes its application in sand, dust, or volcanic ash laden environments. In order to improve the high-temperature thermal radiation shielding performance and CMAS resistance of YSZ and further reduce its thermal conductivity, two medium-entropy (ME) oxide ceramics, ME (Y, Ti)0.1(Zr, Hf, Ce)0.9O2 and ME (Ta, Ti)0.1(Zr, Hf, Ce)0.9O2, were designed and prepared by pressureless sintering of binary powder compacts in this work. ME (Y, Ti)0.1(Zr, Hf, Ce)0.9O2 presents cubic structure but a trace amount of secondary phase, while ME (Ta, Ti)0.1(Zr, Hf, Ce)0.9O2 displays a combination of tetragonal phase (81.6 wt.%) and cubic phase (18.4 wt.%). Both ME (Y, Ti)0.1(Zr, Hf, Ce)0.9O2 and ME (Ta, Ti)0.1(Zr, Hf, Ce)0.9O2 possess better high-temperature thermal radiation shielding performance than YSZ. Especially, the high-temperature thermal radiation shielding performance of ME (Ta, Ti)0.1(Zr, Hf, Ce)0.9O2 is superior to that of ME (Y, Ti)0.1(Zr, Hf, Ce)0.9O2 due to its narrower band gap and correspondingly higher infrared absorbance (above 0.7) at the waveband of 1 to 5 μm. The two ME oxides also display significantly lower thermal conductivity than YSZ and close thermal expansion coefficients (TECs) to YSZ and Ni-based superalloys. In addition, the two ME oxides possess excellent CMAS resistance. After attack by molten CMAS at 1250 °C for 4 h, merely ∼2 μm thick penetration layer has been formed and the structure below the penetration layer is still intact. These results demonstrate that ME (Me, Ti)0.1(Zr, Hf, Ce)0.9O2 (Me = Y and Ta), especially ME (Ta, Ti)0.1(Zr, Hf, Ce)0.9O2, are promising thermal barrier materials for high-temperature thermal radiation shielding and CMAS blocking.

Rapid preparation of Palygorskite/Al2O3 composite aerogels with superhydrophobicity, high-temperature thermal insulation and improved mechanical properties

Al2O3 aerogels are widely employed in heat insulation and flame retardancy because of their unique combination of low thermal conductivity and exceptional high-temperature stability. However, the mechanical properties of Al2O3 aerogel are poor, and the preparation time is considerably long. In this study, we present a simple and scalable approach to construct monolithic Pal/Al2O3 composite aerogels using solvothermal treatment instead of traditional solvent replacement, which remarkably shortened the preparation time. Subsequently, to obtain stable superhydrophobicity (θ > 152°), the Pal/Al2O3 aerogel was modified by gas-phase modification method. The obtained Pal/Al2O3 composite aerogels demonstrate the integrated properties of low density (0.078–0.106 g/cm3), low thermal conductivity (1000 °C, 0.143 W/(m·K)), good mechanical properties (Young's modulus, 1.6 MPa), and good heat resistance. The monolithic Pal/Al2O3 composite aerogels with improved mechanical performance and improved thermal stability can show great potential in the field of thermal insulation.

Substrate Modification for High‐Performance Thermoelectric Materials and Generators Based on Polymer and Carbon Nanotube Composite

AbstractPolymer and carbon nanotube composites have aroused extensive attention for thermoelectric materials owing to the combination of low thermal conductivity of polymer and high electrical conductivity of carbon nanotubes. Surface properties of the substrate are of great importance for the charge transport behaviors of semiconducting thin films, which are less explored in thermoelectric applications. Herein, self‐assembled monolayers (SAMs) are used to modify the substrate for thermoelectric polymer composites. The trifluoromethyl (CF3)‐terminated SAM is beneficial for an improved electrical conductivity; while the SAM with amino group is found to improve their Seebeck coefficient and decrease the electrical conductivity. As a result, polymer composites on CF3‐SAM‐modified substrate show a high room‐temperature power factor of 285 µW m−1 K−2 and a large output power of 2.36 µW for thermoelectric generator at a temperature gradient of 50 K. This work demonstrates that surface modification by SAMs is a promising strategy for improving performance of thermoelectric materials and devices.

Insulating and Robust Ceramic Nanorod Aerogels with High-Temperature Resistance over 1400 °C.

Ceramic aerogels, which present a unique combination of low thermal conductivity and excellent high-temperature stability, are attractive for thermal insulation under extreme conditions. However, most ceramic aerogels are constructed by oxide ceramic nanoparticles and thus are usually plagued by their brittleness and structural collapse at elevated temperatures (less than 1000 °C). Despite great progress achieved in this regard recently, it still remains a big challenge to design and fabricate intriguing ceramic aerogels with enhanced mechanical strength and remarkable thermal stability at ultrahigh temperature up to 1400 °C. To this end, we herein report a facile and scalable strategy to manufacture ceramic nanorod aerogels (CNRAs) with hierarchically macroporous and mesoporous structures by the controllable assembly of Al2O3 nanorods and SiO2 nanoparticles. Subsequently, the high-temperature annealing treatment of CNRAs significantly maximizes mechanical strength and promotes thermal tolerance. The obtained CNRAs demonstrate the integrated properties of super-strong heat resistance (up to 1400 °C), low thermal conductivity (0.026 W/m·K at 25 °C and 0.089 W/m·K at 1200 °C), high mechanical robustness (compressive strength 1.5 MPa), and low density (0.146 g/cm3). We envision that this novel nanorod-assembled ceramic aerogels offer considerable advantages than most of the state-of-the-art ceramic aerogels for thermal superinsulation upon exposure to extremely harsh environments.

Low-temperature specific heat capacity measurements and application to Mars thermal modeling

Data returned from Martian missions have revealed a wide diversity of surface mineralogies, including in geological structures interpreted to be sedimentary or altered by liquid water. These terrains are of great interest because of their potential to document the environment at a time when life may have appeared. Intriguingly, Martian sedimentary rocks show distinctly low thermal inertia values (i.e. 300 - 700 Jm−2K−1s−1/2, indicative of a combination of low thermal conductivity, specific heat capacity, and density). These low values are difficult to reconcile with their competent bedrock morphologies, whereas hundreds of bedrock occurrences, interpreted as volcanic in origin, have been mapped globally and display thermal inertia values > 1200 Jm−2K−1s−1/2. Bedrock thermal inertia values are generally assumed to be driven by their bulk thermal conductivity, which in turn is controlled by their micro- and macro-physical properties (i.e., degree and style of cementation in the case of detritic rocks, horizontal fractures and layering, etc.), and not by their density (well-known from terrestrial analog measurements, and with modest variability) or specific heat capacity (generally uncharacterized for non-basaltic materials below room temperature). In this paper, we demonstrate that specific heat capacity cannot be a potential cause for the differential thermophysical behavior between magmatic and sedimentary rocks through a series of experimental Cp(T) measurements at 100–350 K using differential scanning calorimetry. The results on 20 Martian-relevant minerals investigated in this work indicate that these materials exhibit very similar specific heats, ranging from 0.3–0.7 Jg−1K−1 at 100 K to 0.6–1.7 Jg−1K−1 at 350 K. When used in a Martian thermal model, this range of Cp values translate to very small surface temperature differences, indicating that uncertainty in composition (and its effect on the specific heat) is not a noticeable source of thermal inertia variability for indurated units on Mars. We therefore conclude that the low thermal inertia value of sedimentary rocks compared to magmatic/volcanic rocks is likely due to their low apparent bulk conductivity, which bears information on their internal physical structure. Future work combining the analysis of thermal observations acquired at various local times and seasons will help further characterize this heterogeneity.

Optical and thermal filtering nanoporous materials for sub-ambient radiative cooling

Emitting heat to Space is a passive, dry, and solid-state cooling approach with potential applications in thermal management of buildings, solar cells, and vehicles. Emitting in the 8–13 μm atmospheric window while preventing heat from reaching the cold emitter is important to the performance. Here, we investigate the design of infrared-transparent thermally-insulating materials for radiative cooling at deep sub-ambient temperatures. Unlike surface-based designs that rely on selective emission, selective radiative heat transfer with Space is achieved via selective transmission through an insulating cover. We model the radiative and thermal transport characteristics of several nanoporous materials that are transparent in the 8–13 μm range. We also study the effects of physical morphology and material composition on the scattering and absorption properties of the insulating cover, and its radiative cooling performance. By tailoring the nanostructure and material composition, one can define regions for short-wavelength scattering and long-wavelength absorption, and in turn, block solar and atmospheric heat. The combination of low thermal conductivity and selectively high transparency in the atmospheric window enable high-performance radiative cooling down to 35 K below ambient temperature.

Theoretical study on electronic structure and thermoelectric properties of PbSxTe1−x(x = 0.25, 0.5, and 0.75) solid solution**Project supported by the National Natural Science Foundation of China (Grant Nos. 11647010 and 11704020), the Higher Education and High-quality and World-class Universities (Grant No. PY201611), and the Fund for Disciplines Construction from Beijing University of Chemical Technology (Grant No. XK1702).

The electronic structure and thermoelectric (TE) properties of PbSxTe1−x(x = 0.25, 0.5, and 0.75) solid solution have been studied by combining the first-principles calculations and semi-classical Boltzmann theory. The special quasirandom structure (SQS) method is used to model the solid solutions of PbSxTe1−x, which can produce reasonable electronic structures with respect to experimental results. The maximum ɀT value can reach 1.67 for p-type PbS0.75Te0.25 and 1.30 for PbS0.5Te0.5 at 800 K, respectively. The performance of p-type PbSxTe1−x is superior to the n-type ones, mainly attributed to the higher effective mass of the carriers. The ɀT values for PbSxTe1−x solid solutions are higher than that of pure PbTe and PbS, in which the combination of low thermal conductivity and high power factor play important roles.

High potential thermoelectric figure of merit in ternary La3Cu3X4 (X\u2009=\u2009P, As, Sb and Bi) compounds

We investigate the thermoelectric properties of the relatively unexplored rare-earth ternary compounds La3Cu3X4 (X = Bi, Sb, As, and P) using first principles electronic structure and Boltzmann transport calculations. These compounds, of which the La3Cu3Sb4 and La3Cu3Bi4 have previously been synthesized, are all predicted to be semiconductors and present a wide range of bandgaps varying from 0.24 eV (for the Bi compound) to 0.87 eV (for the P compound). We further find a mixture of light and heavy bands, which results in a high thermoelectric power factor. In addition, as discussed in our previous study (Phys. Rev. B 95 (22), 224306, 2017) at high temperatures of 1000 K these compounds exhibit lattice thermal conductivity less than 1 W/mK. The combination of low thermal conductivity and good transport properties results in a predicted ZT as high as ~1.5 for both La3Cu3P4 and La3Cu3As4, under high p-type doping. This predicted high performance makes these compounds promising candidates for high temperature thermoelectric applications and thus merits further experimental investigation.

Surface Grinding of Ti-6Al-4V Alloy with SiC Abrasive Wheel at Various Cutting Conditions

Ti-6Al-4V alloy is mostly used in biomedical and aerospace industries, as well as in automotive and cutting implements, like scissors and knives due to its high strength-to-weight ratio and excellent resistance to corrosion in many environments. However, Ti-6Al-4V alloy is referred as difficult-to-cut material due to its unique combination of low thermal conductivity and high chemical reactivity with most cutting tools, especially with ceramics, and rapid work hardening during machining,. This will accelerate tool wear and generally adversely affects surface quality of the component. This becomes more critical in grinding operation due to conventional abrasive wheels that have poor thermal conductivity and small dimension of chips, which in turn contributes to more heat to be concentrated in the cutting zone. Depending on the temperature gradient and workpiece, this heat can cause damage to the component surface. So, it is important to control the amount of heat entering the workpiece and prevent damages like burning, surface cracks and other metallurgical alterations in the workpiece. In this context, this work investigates the surface quality of the Ti-6Al-4V alloy in terms of surface roughness e microhardness, after surface grinding with silicon carbide wheel under various cutting conditions. The morphology of the machined was also analyzed in a Scanning Electron Microscope to understand the cutting mechanisms. Conventional and MQL coolant delivery techniques were tested. A specially designed nozzle was tested in the experiments with the MQL. Results showed that surface roughness is dependent on both radial depth of cut and coolant system; and the lowest results were recorded after machining with the combination of the lowest depth of cut and MQL technique. With respect to microhardness, little variation was observed after machining with the MQL technique, unlike when machining with conventional method, irrespective the depth of cut employed.

Thermal stability and enhanced thermoelectric properties of the tetragonal tungsten bronzes Nb8−xW9+xO47 (0 < x < 5)

Engineering of nanoscaled structures allows to control the electrical and thermal transport in solids for thermoelectric applications where a combination of low thermal conductivity and low electrical resistivity is required.

High thermopower and ultra low thermal conductivity in Cd-based Zintl phase compounds.

By combining first principles density functional theory and electronic as well as lattice Boltzmann transport calculations, we unravel the excellent thermoelectric properties of Zintl phase compounds ACd2Sb2 (where, A = Ca, Ba, Sr). The calculated electronic structures of these compounds show charge carrier pockets and heavy light bands near the band edge, which lead to a large power factor. Furthermore, we report large Grüneisen parameters and low phonon group velocity indicating essential strong anharmonicity in these compounds, which resulted in low lattice thermal conductivity. The combination of low thermal conductivity and the excellent transport properties give a high ZT value of ∼1.4-1.9 in CaCd2Sb2 and BaCd2Sb2 at moderate p and n-type doping. Our results indicate that well optimized Cd-based Zintl phase compounds have the potential to match the performance of conventional thermoelectric materials.

Facile General Route toward Tunable Magnéli Nanostructures and Their Use As Thermoelectric Metal Oxide/Carbon Nanocomposites

Engineering nanoscale interfaces is a requisite for harnessing electrical and thermal transports within nanostructured materials, especially those destined for thermoelectric applications requiring an unusual combination of low thermal conductivity and electrical resistivity. Nanocomposites open up possibilities in this area, but are still bound to a very narrow range of materials. Here, we report a new approach combining the sol-gel process toward hybrid materials with spark plasma sintering (SPS) to yield functional nanocomposites based on substoichiometric titanium oxides Ti(n)O(2n-1), so-called Magnéli phases. The potential of this new approach is demonstrated by three results. First, multiple Ti(n)O(2n-1) compounds (n = 3, 4, 5, 6, 8) are obtained for the first time as sole nano-Magnéli crystalline phases with controlled specific surface areas from 55 to 300 m(2)·g(-1), classified as potential thermoelectric n-type metal oxides and paving the way toward advanced systems for energy-harvesting devices and optoelectronics. Second, this work combines the use of sol-gel and SPS processes to yield percolated nanocomposites based on metal oxide nanoparticles embedded in a carbon matrix with low electrical resistivity (2 × 10(-4) Ω·m for a Ti(4)O(7) compound) and reduced thermal conductivity (1 W·m(-1)·K(-1)) with respect to bulk phases. Finally, the discovered materials are reliable with thermoelectric figures of merit (ZT = 0.08) relatively high for n-type Ti-O-based systems and metal oxides. Thereby this study represents a proof of concept for the development of promising, cheaper, and more efficient thermoelectric conversion devices.

High Temperature Thermoelectric Properties of Nano-Bulk Silicon and Silicon Germanium

AbstractPoint defect scattering via the formation of solid solutions to reduce the lattice thermal conductivity has been an effective method for increasing ZT in state-of-the-art thermoelectric materials such as Si-Ge, Bi2Te3-Sb2Te3 and PbTe-SnTe. However, increases in ZT are limited by a concurrent decrease in charge carrier mobility values. The search for effective methods for decoupling electronic and thermal transport led to the study of low dimensional thin film and wire structures, in particular because scattering rates for phonons and electrons can be better independently controlled. While promising results have been achieved on several material systems, integration of low dimensional structures into practical power generation devices that need to operate across large temperature differential is extremely challenging. We present achieving similar effects on the bulk scale via high pressure sintering of doped and undoped Si and Si-Ge nanoparticles. The nanoparticles are prepared via techniques that include high energy ball milling of the pure elements. The nanostructure of the materials is confirmed by powder X-ray diffraction, transmission electron microscopy, scanning electron microscopy, and dynamic light scattering. Thermal conductivity measurements on the densified pellets show a drastic 90% reduction in the lattice contribution at room temperature when compared to doped single crystal Si. Additionally, Hall effect measurements show a much more limited degradation in the carrier mobility. The combination of low thermal conductivity and high power factor in heavily doped n-type nanostructured bulk Si leads to an unprecedented increase in ZT at 1275 K by a factor of 3.5 over that of single crystalline samples. Experimental results on both n-type and p-type Si are discussed in terms of the impact of the size distribution of the nanoparticles, doping impurities and nanoparticle synthesis processes.

Effects of Antimony on the Thermoelectric Properties of the Cubic Pb9.6SbyTe10−xSexMaterials

ABSTRACTThe thermoelectric properties of Pb9.6SbyTe10−xSexwere investigated in the intermediate temperature range of 300 – 700 K. The effect of the variation of Sb content (y) on the electronic properties of the materials is remarkable. Samples with compositions Pb9.6Sb0.2Te10−xSex(y = 0.2) show the best combination of low thermal conductivity with moderate electrical conductivity and thermopower. For Pb9.6Sb0.2Te8Se2(x = 2) a maximum figure of merit of ZT ∼ 1.1 was obtained around 700 K. This value is nearly 1.4 times higher than that of PbTe at 700 K. This enhancement of the figure of merit of Pb9.6Sb0.2Te8Se2derives from its extremely low thermal conductivity (∼0.7 at W/m.K at 700 K). High resolution transmission electron microscopy of Pb9.6Sb0.2Te10−xSexsamples shows broadly distributed Sb-rich nanocrystals, which may be the key feature responsible for the suppression of the thermal conductivity.

Reliability of Laser Flash Thermal Diffusivity Measurements of the Thermal Barrier Coatings

The thermal diffusivity of free standing thermal barrier coatings (TBCs) was measured by the laser flash technique. The combination of low thermal conductivity (1 to 2 W/m K) and small TBC thickness (300 to 600 µm thick) can cause errors in the measurements. Back surface (opposite the laser) temperatures of free standing plasma-sprayed TBCs were measured as a function of time and laser power. The front surface temperatures were calculated using thermal transport equations. In the high power region, thermal diffusivity decreased significantly with increasing laser power. In the moderate power region, thermal diffusivity remained constant. In the low power region, measurement became unreliable because of noise. The detector nonlinearity was believed to be a possible cause of deviation in the high power region. Measurements at different laser power levels should be conducted in order to obtain reliable thermal diffusivity values for TBCs.

Fire behaviour of composite laminates

The thermal response of laminated glass fibre reinforced panels to severe fire conditions has been investigated by furnace fire testing and thermal modelling. Excellent fire resistance has been demonstrated for several matrix materials and the materials response has been modelled to a high degree of accuracy. The thermal resistance properties are due to a combination of low thermal conductivity, good structural integrity and significantly, the endothermic decomposition of the matrix, which slows down the heat transmission through the laminate.

Appliance Rigid Foams Blown with Cyclopentane and Cyclopentane/Isopentane Blends

Since CFC 11 was eliminated from use in rigid foams in the mid 1990s, the blowing agent of choice in North America has primarily been HCFC 141b. In the USA, we are now faced with the phase-out of HCFC 141b by the year 2003. The blowing agent that will replace HCFC 141b must be non-ozone depleting and should also have low thermal conductivity and a low global warming potential. Hydrocarbons are some of the few blowing agents that meet these criteria and are approved by the United States Environmental Protection Agency to replace HCFC 141b in rigid polyurethane foams for appliance insulation [1]. Of the hydrocarbons being considered, cyclopentane is preferred, either by itself or in blends with other hydrocarbons. This is because of cyclopentane's favorable combination of low thermal conductivity, good solubility in polyols and reasonable boiling point, compared to other hydrocarbons. Not only is the thermal conductivity of cyclopentane lower than that of most other hydrocarbons, it is also lower than that of HFC 134a, another blowing agent which is approved for use after 2002. This paper will examine the effect of two important factors, both theoretically and experimentally, on cyclopentane blown foams. These factors are (1) the amount of blowing obtained from carbon dioxide that is formed from the reaction between water and isocyanate, and (2) the ratio of cyclopentane to isopentane, which is used as the hydrocarbon blowing agent.