Temperature Thermal Insulation Research Articles

Improvement in the high temperature thermal insulation performance of Y2O3 opacified silica aerogels

To improve the high-temperature thermal insulation characteristics of silica aerogels, it is important to reduce the radiative heat transfer. This can be achieved by Y2O3 doping in the silica sol to opacify the infrared radiation of silica aerogels. The purpose of the present work was to study the effect of Y2O3 incorporation on the structural and physicochemical properties of opacified silica aerogels prepared by a simple ambient pressure drying method. The influence of Y2O3 addition on specific extinction coefficient and high temperature thermal insulation of prepared aerogels were investigated. The synthesized aerogels were lightweight and crack-free, with a granular, nanoporous morphology. The specific surface area, pore diameter, and bulk density of the prepared samples were 917.5–937.6 m2/g, 5.64–6.58 nm, and 0.047–0.076 g/cm3, respectively. The thermal conductivity of opacified silica aerogel at 1000 K was 0.080 W/(m.K), which was lower than the unopacified silica aerogel and it was around 0.104 W/(m.K) at same temperature.

Carbon-fiber felt reinforced carbon/alumina aerogel composite fabricated with high strength and low thermal conductivity

The objective of this research was to develop a high-strength carbon-fiber felt reinforced carbon/alumina aerogel (CF-C/Al2O3) with low thermal conductivity. The carbon-fiber felt reinforced resorcinol formaldehyde/alumina aerogel (CF-RF/Al2O3) was initially prepared using sol­gel technique followed by ambient pressure drying technique. The CF-C/Al2O3 was obtained after the thermal treatment at 1600 °C under flowing argon atmosphere. Both CF-RF/Al2O3 and CF-C/Al2O3 displayed extremely little linear shrinkages due to the high structural strength of carbon fiber felt. The compression strength of CF-C/Al2O3 were 0.50, 0.78, 1.30 and 2.35 MPa, the corresponding compressive stress at the strains of Ɛ 20%, Ɛ 30%, Ɛ 40% and Ɛ 50%, respectively. The thermal conductivity of CF-C/Al2O3 is as low as 0.284 W/(m·K) at a ultrahigh temperature 1600 °C under argon, which is only about one half of the value of carbon fiber felt (0.538 W/(m·K)). The as-synthesized CF-C/Al2O3 possessed greatly controllable and stable structure, which would be fabricated and machined to a large-scale composites and be used as high temperature thermal insulators.

A Review of Parameters Influencing the Fire Performance of Light Gauge Steel Frame Walls

Load-bearing Light Gauge Steel Frame (LSF) walls are made of cold-formed steel frames and lined with fire-resistant gypsum plasterboards, and include cavity insulation based on energy performance requirements. Despite the importance of their fire performance, the current fire design methods rely mostly on a prescriptive approach, which relies on fire resistance ratings (FRR) provided by the wall manufacturers due to the lack of knowledge of the various parameters affecting the fire performance of LSF walls. This paper identifies the parameters affecting the fire performance of LSF walls, and presents a detailed review of the fire performance of LSF walls as a function of these parameters, based on recent research studies. It discusses the effects of elevated temperature thermal properties, plasterboard fall-off, cavity insulation, wall configuration, and stud section shapes on the thermal performance of LSF walls, and presents a detailed study of the temperature distribution patterns across the stud cross-sections. Negative effects of plasterboard joints are also demonstrated using fire test results. Elevated temperature mechanical properties of steel, time–temperature profiles of walls, stud section shapes and their sizes were found to be affecting the structural performance of LSF walls in fire, and their effects are discussed. The importance of using accurate elevated temperature mechanical properties in determining the FRR of LSF walls is also demonstrated. A good understanding of the identified parameters on the fire performance of LSF walls can lead to the use of a performance-based fire design method, and development of LSF walls with enhanced fire performance.

Effect of Carbon Black on Characteristics of a Selected EPDM-Based Thermal Insulator Composite

Ethylene-propylene-diene terpolymer (EPDM) based thermal insulator represent efficient class of the high temperature thermal insulators due to their excellent thermal, mechanical and ablative properties. This classed insulator is often reinforced by Kevlar pulp and fumed silica. Ammonium Sulphate and antimony trioxide mixture as a flame retardant was added. Carbon black (CB) was added to enhance the elastomer ablative properties. different concentrations of CB were investigated. In this work, the effect of carbon black content on the characteristics of a selected EPDM based insulating formulation has been investigated. The tensile strength, hardness and elongation have been slightly improved. The thermal stability via thermal gravimetric analysis (TGA-DTG) and differential scanning calorimetry (DSC) also has been slightly improved. Whereas the ablation resistance has been considerably improved. the weight loss from sample containing 10 phr decreased by 28% relative to the neat specimen.

Deposition of Tungsten-Doped V2O5 Thin Films on Non-Alkali Glass Substrate by RF Magnetron Sputtering for Thermal Insulation

The thermal insulation function of tungsten-doped vanadium dioxide (V2O5) thin films deposited on non-alkali glass substrate using RF magnetron sputtering was analyzed in this study. Grinded hot-dry V2O5 and tungsten powders, mixed in weight ratio of 98.1:1.9 or 97:3, were pressed at 800 psi for 10 min. These compounds were sintered at 550 or 600 °C for 8 hours, in oxygen gas environments (10 sccm and 0 sccm), to several W-doped V2O5 targets. The surface morphologies of these targets were analyzed by a SEM, and the crystal structure was characterized by a XRD. The experimental studies with reference to the thermal insulation property of V2O5 were conducted under various duration of deposition, substrate temperatures, rf powers and duration of annealing time. The influence of these factors was investigated using the Taguchi method, an orthogonal array L8. The results show that the targets contain a more homogenous structure and a larger grain size with higher oxygen gas flow rate. With a deposition-parameter combination of 60 min (duration of deposition), 300°C (substrate temperature), 150 W (rf power) and 60 min (duration of annealing time), the optimal thermal insulation temperature, 19.3°C, was observed.

High temperature oxidation and insulation behavior of plasma-sprayed nanostructured thermal barrier coatings

Abstract Thermal barrier coatings (TBCs) for more efficient gas turbine engines require higher operating temperature. Adopting pyrochlore structure materials such as rare-earth zirconate and double-ceramic-layer (DCL) structure can well satisfy service demands of TBCs at elevated temperature. The nanostructured single-ceramic-layer (SCL) 8YSZ, DCL Sm 2 Zr 2 O 7 (SZ)/8YSZ and doping 8 wt% CeO 2 nanoparticles in SZ (8CSZ)/8YSZ TBCs were prepared by atmospheric plasma spraying (APS) in this work. The high temperature oxidation behavior and thermal insulation property of the three kinds of as-sprayed TBCs were investigated systematically at 1000 °C and 1200 °C. The results indicate that the nanostructured DCL 8CSZ/8YSZ TBCs have the best oxidation resistance and thermal insulation property at different temperatures among others. The influence of microstructure on thermal insulation property of the coating has been discussed in detail.

Highly flexible, erosion resistant and nitrogen doped hollow SiC fibrous mats for high temperature thermal insulators

Novel free-standing, flexible, acid/alkali-resistant and nitrogen doped hollow SiC fibrous mats bearing ultralow thermal conductivity are reported.

Notes on the Calculation of the PMV Index by Means of Apps

In this work, all existing smartphone and web applications for the calculation of the PMV index running on different platforms for mobile devices have been compared with a reference software compliant with ISO 7730 Standard. Unfortunately, only four apps take into account all the six variables responsible for the thermal sensation (air temperature, mean radiant temperature, air velocity, relative humidity, metabolic rate and clothing thermal insulation) and in some cases data input appeared very hard (e.g. impossibility to consider decimal values). In addition, only one app seemed to take into account the correction of basic clothing insulation values due to pumping effect. Based on results from this preliminary test, we can conclude that web apps and smartphone tools for the evaluation of thermal comfort conditions by means of PMV presently on the market have to be used with special care (or, better, should not be used) especially by not skilled users could use them even for professional reasons.

Penguin activity modify the thermal regime of active layer in Antarctica: A case study from Hope Bay

Monitoring permafrost and active layer is essential for prognostic research of future climate scenarios. Although biotic factors have a prominent role in active layer behavior, apart from vegetation effects, their influence remains little investigated. It is generally assumed that frozen ground exerts influence on nesting fauna, whereas, on the other hand, land colonization by birds, such as penguins can also interfere with the soil thermal regime. However, to our knowledge, no studies report on permafrost and active layer regime changes under penguin activity. We present a comparative study of two adjacent sites located in Hope Bay, one representing an active penguin rookery-S1, and another, an adjacent abandoned site currently vegetated-S2. Soil temperature and water content, and air temperature were monitored hourly from 2009 to 2011. Current penguin activity and the resulting deposition of guano during spring and summer in S1 is an important factor for explaining the higher number of thaw degree days due to direct physical impact and chemical reactions caused by rapid guano decomposition. In the vegetated S2 site, the snow pack lasted longer, showing the highest mean minimum temperature and larger thermal insulation, as well as larger FDD than those found in the bare soil of the active rookery (S1), and lower n-F, due to greater thermal insulation. Penguins played a significant role in changing the active layer depth and thermal regime, and represent a neglected actor on the ground thermal regime in Antarctic terrestrial environments.

Effect of Moisture Absorption on High Temperature Thermal Insulation Performance of Fiber Insulation Materials

In this paper, effect of moisture absorption on high temperature thermal insulation performance of fiber insulation material is studied. Fiber material is stored in 40°C through a long period of time, then spontaneous moisture absorption of fiber material is measured. According this test result, 20% and 40% moisture absorption content gradient are determined. Pore distribution and high temperature thermal insulation property of samples of 20% and 40% moisture absorption and the dry samples are all tested in this study. The internal pore size distribution of fiber insulation material after moisture absorbing has a certain coarsening. Fiber material after moisture absorbing will make its pore size become large. And oxygen acetylene test results showed that: the larger moisture absorption, the more slowly the initial temperature rises. Fiber material after moisture absorbing will make its low temperature thermal insulation performance reduce, but it has little impact of its high temperature thermal insulation performance.

Effective blocking of radiative thermal conductivity in La2Zr2O7/LaPO4 composites for high temperature thermal insulation applications

Ceramics with low thermal conductivity have been widely used for high temperature thermal insulation applications, but their performance deteriorates rapidly at temperatures above 600–800°C due to the significantly enhanced radiation heat transfer effect. We found that by addition of secondary phase of LaPO4 into a matrix of La2Zr2O7, the radiative thermal conductivity could be remarkably suppressed due to lower photon mean free path as a result of photon scattering or absorption by the secondary phase. By adding more than 20wt.% of LaPO4, the radiative thermal conductivity of La2Zr2O7 can almost be completely blocked, as the LaPO4 phase has formed percolating interconnected network inside the matrix, resulting in remarkable reduction of infrared radiation transmittance. Furthermore, the mechanical properties of some composites were also optimized, including improved toughness and lowered elastic modulus, making the composites of particular interest for engineering applications, such as thermal barrier coatings for gas turbine.

Very low thermal conductivity in lanthanum phosphate–zirconia ceramic nanocomposites processed using a precipitation–peptization synthetic approach

A wet chemical synthetic approach involving precipitation–peptization mechanisms was successfully adopted for the development of LaPO4–ZrO2 nanocomposites with the ZrO2 content varying in the 5–20 wt% range. Stoichiometric lanthanum phosphate, formed as nanofibrils during the precipitation reaction with orthophosphoric acid, was subsequently transformed into nanorods of ∼10 nm width and 98% TD for the LaPO4–10 wt% ZrO2 composition upon sintering at 1600 °C. The addition of ZrO2 to LaPO4 impeded densification and grain growth inhibition of up to 50% was obtained for LaPO4–20 wt% ZrO2 nanocomposites. Furthermore, the nanocomposites indicated very low thermal conductivity values (1 W m−1 K−1) compared to single phase LaPO4. The non-reactivity of LaPO4 and ZrO2 at high temperatures and the low thermal conductivity values of LaPO4–ZrO2 render them effective for high temperature thermal insulation applications.

Particle in-flight behavior and its influence on the microstructure and properties of supersonic-atmospheric-plasma-sprayed nanostructured thermal barrier coatings

In order to accurately control the microstructure and property of yttria-partially-stabilized-zirconia (YPSZ) coatings, this paper elaborated the particle in-flight behavior during supersonic atmospheric plasma spraying (SAPS) and its significance in determining the microstructure and thermal insulation properties of SAPS-YPSZ coatings. The results suggested that, for a Laval nozzle with an internal injection port, the influence of spray power on the particles velocity is more significant than the temperature. When the spray power and argon flow rate were kept unchanged, the increase of hydrogen percentage in the mixed plasma gas can increase the temperature while decrease the velocity. Compared with other spray parameters, the standoff distance had the most significant influence on both the temperature and velocity of in-flight particles. The particles temperature and velocity determined the melting state of particles and the content of unmelted nanoparticles of as-sprayed coatings, which decreased from 12% to 6% when the particles temperature and velocity increased by 10% and 14%, respectively. For a 300-μm-thick YPSZ coating, thermal insulation temperature increased from 69°C to 90°C (increased by 30%) when the unmelted nanoparticle content increased from 6% to 12%.

Preparation and characterization of carbon foam derived from pitch and phenolic resin using a soft templating method

Carbon foam has been developed by soft templating method with phenolic resin and coal tar pitch as matrix precursor and polyurethane (PU) foam as an organic sacrificial template. Micron sized powdered pitch was mixed well in a diluted resin and the PU foam was soaked in it. The impregnated foam was dried at 70°C for 6h. This impregnation–stabilization (IS) process was repeated for four and six times. A slow curing cycle with hold steps at 80, 110, 140 and 2000°C was used for the sample being pressed isostatically in a steel mold. Carbonization at 800°C in a reduced and inert environment resulted in a geometric density of 0.48 and 0.60g/cm3 for IS cycles of four and six respectively. The carbon foam was characterized by porosity calculations, thermogravimetric analysis, compressive strength and scanning electron microscopy. The developed carbon foam showed an open porosity of 63–67%, compressive strength of 13.3–19.5MPa and pore size distribution of 10–500μm. An experimental setup was designed for the testing of carbon foam as a high temperature thermal insulation. The carbon foam specimen was heated to 975°C on the one face and the other face was monitored. The experiment was repeated with a carbon-felt specimen and the insulation indices compared. The carbon foam showed a higher insulation index but lower insulation density index due to higher density than that of the carbon felt.

Preparation of flexible ultra-fine Al2O3 fiber mats via the solution blowing method

Alumina fiber and fiber mat are important ceramic materials, which can be used in many fields including high temperature filtration, thermal insulation and catalyst support. This article demonstrates that alumina fiber mats can be obtained by the solution blowing process combined with the sol-gel method and a subsequent thermal treatment process. During the solution blowing process, the as-spun composite fibers were formed after the solvent evaporation and drawing of the gas flow. These fibers successfully converted into γ-Al2O3 and α-Al2O3 under the thermal treatment in air at 900°C and 1100°C. Diameter of as-spun fiber with the range from 2μm to 5μm was influenced obviously by adjusting the spinning parameters, such as gel viscosity and highly pressure gas flow. In addition, the resulting alumina fiber mat performed an excellent flexibility, which was significant to the high temperature filtration and other applications.

Thermal diffusivity of kaolinite–mullite ceramic matrix composite with silicon nitride nanoparticle filler

Thermal diffusivity of mullite fused ceramics was determined from room temperature up to 500°C at intervals of 100°C. The ceramic material was formed by combining kaolinite as the matrix and silicon nitride as the filler powder. The ceramics were sintered at 1000°C, 1100°C, and 1200°C to study the structural differences. Thermal diffusivity data showed that the addition of silicon nitride affects the composites heat flow and this behavior is noticeable at 1100°C and 1200°C samples. The thermal diffusivity data was plotted on 1000/T graph to demonstrate a linear relation for filler effects of silicon nitride on the mullite matrix. Additions of 25% and 30% silicon nitride sintered at 1200°C exhibited thermal diffusivity values lower than pure mullite crystal at 0.55–0.45mm2/s. These values suggest that heat conduction across the system was interrupted by silicon nitride nanoparticle additions via micropore formations. These samples have potential applications for high temperature thermal insulation.

Behavior and mechanism of the stress buffer effect of the inside ceramic layer to the top ceramic layer in a double-ceramic-layer thermal barrier coating

The Double-Ceramic-Layer Thermal Barrier Coating (DCL-TBC) consists of a top ceramic layer (TC1), an inside ceramic layer (TC2), bond coat (BC) and alloy substrate. The top ceramic layer is made by new ceramic materials which has the lower thermal conductivity, such as LZ, LZ7C3, LMA etc. Although these materials have good high temperature performance and thermal insulation properties, their thermal expansion coefficients are very low which cause higher degree of mismatch with material properties of alloy substrate.The behavior of the stress buffer effect of the layer of TC2 is investigated aiming at the DCL-TBC with the two ceramic layers as LZ7C3(TC1) and 8YSZ(TC2). A FE model of the DCL-TBC is established and the stress of the TC1 during the thermal cycles is calculated. Results show that the layer of TC2 has the behaviors of stress buffer effect on reducing the stress of TC1 and reducing the influence of the TGO growth upon the stress of TC1. The inner mechanism of the stress buffer effect is investigated based on the influence of the thermal expansion coefficient ρ of the TC2 layer. Results show that the TC1 layer varies with the TC2 layer, having different thermal expansion coefficients; and the stress buffer effect is dominant, affected by the thermal expansion coefficients of the TC2 layer. The thermal expansion coefficient should be neither too large nor too low; if its values are too low or too large, the stress levels of TC1 are all high. The reasonable range of the thermal expansion coefficient is investigated. Aiming at the DCL-TBC with the two ceramic layer thicknesses as 150μm, the reasonable range of the thermal expansion coefficient of TC2 is from 10.35°C−1 (×10−6) to 10.887°C−1 (×10−6).

Fabrication and characterization of boron nitride bulk foam from borazine

Boron nitride (BN) ceramic bulk foams larger than 10×10×1cm are fabricated by the pyrolysis of polyborazine foams that are prepared by fast curing of borazine. The principle of the self-foaming phenomenon is the emitting of H2 in condensation polymerization during the curing of borazine. The as-fabricated BN ceramic foams exhibit closed cells with sizes ranging from 50 to 100μm and controlled bulk densities varying from 0.08 to 0.25gcm−3 depending on the different cured pressures. Studies on the microstructure and properties of the porous material show that the BN ceramic foams exhibit high specific strength, low dielectric constants and loss tangents, and good thermal conductivity. These characteristics make them useful for high temperature radomes and thermal insulation.

La and Co substituted M-type barium ferrites processed by sol–gel combustion synthesis

The flake-like Ba1−xLaxCoxFe12−xO19 (x=0.0–0.4) were synthesized using sol–gel combustion synthesis process followed by the thermal insulation process and heating treatment. The synthesis process was investigated and the structure details, morphology, and magnetic properties were evaluated. TG/DTA was used to investigate the formation mechanism and to identify the thermal insulation temperature at 400°C followed by the heat treatment temperature at 1200°C. XRD patterns demonstrated that the unit cell volume and particle size decreased with the increase of the substitution content. The typical particles size was in the range of 1–2μm in the planar dimension whilst the thickness was in the range of 200–500nm. It was found from the VSM graphs that the saturation magnetizations Ms reached a maximum of 68.15emug−1 at x=0.3 and then decreases to 64.72emug−1 at x=0.3 whilst the coercivity Hc sustained decreases from 2190.70 to 1181.07Oeg−1 with substitution content increased from 0 to 0.4.

Optical and radiative properties of infrared opacifier particles loaded in silica aerogels for high temperature thermal insulation

An inverse model based on the shooting method, Mie theory and the improved Kramers–Kronig (KK) relation was combined with FTIR and Abbe refractometer measurements to calculate the complex refractive indices of various infrared opacifiers. The effects of opacifier sizes, types and shapes were then analyzed based on the Rosseland mean extinction coefficient using Mie theory and anomalous diffraction theory (ADT). This model provides theoretical guidelines for designing materials with optimized parameters, such as size, type and shape of opacifiers, to improve the aerogel thermal insulation at high temperatures. The results show that the optimum diameter of SiC particles to minimize the radiation is 4 μm for T < 400 K and 3 μm for T > 400 K. Carbon black is the optimum opacifier for T < 600 K while SiC is the optimum opacifier to minimize the radiative heat transfer for T > 600 K among the investigated opacifiers of SiC, TiO2, ZrO2, amorphous SiO2 and carbon black. The infrared extinction ability for various shapes is largest for oblate spheroids and decreases for spheres, cubes, cylinders with small length-to-diameter ratios, and then long, thin cylinders.