Porous silica matrices infiltrated with PCM for thermal protection purposes

In this work, numerical and experimental studies are proposed to predict and investigate the thermal absorption characteristics of porous silica infiltrated with phase change materials (PCM) for thermal protection applications. Several types of different solid–liquid phase change composites were introduced into a cylindrical enclosure while it was heated on the left of the enclosure. The numerical simulation was performed using a volume-averaging technique, and a finite volume modelling (FVM) was used to discretise the heat diffusion equation. The phase change process was modelled using the enthalpy–porosity method. The results are portrayed in terms of the temperature distribution and liquid fraction, and the numerical and experimental results showed good agreement. The results indicated that a higher latent heat storage capacity meant a more stable thermal performance of the phase change composite.     

Carbon foams for thermal management

A unique process for the fabrication of high-thermal-conductivity carbon foam was developed at Oak Ridge National Laboratory (ORNL). This process does not require the traditional blowing and stabilization steps and therefore is less costly. The resulting foam can have density values of between 0.2 and 0.6 g/cc and can develop a bulk thermal conductivity of between 40 and 180 W/m K. Because of its low density, its high thermal conductivity, its relatively high surface area, and its open-celled structure, the ORNL carbon foam is an ideal material for thermal management applications. Initial studies have shown the overall heat transfer coefficients of carbon foam-based heat sinks to be up to two orders of magnitude greater than those of conventional heat sinks.     

Carbon foams prepared from polyimide using urethane foam template

Polyimide and carbon foams were successfully prepared using polyurethane foams as a template. Impregnation of polyimide precursor, poly(amide acid), followed by imidization at 200 °C gave polyurethane/polyimide (PU/PI) composite foams, which resulted in PI foams by heating above 400 °C and then carbon foams above 800 °C. Foams carbonized at 1000 °C were graphitized by the heat treatment at 3000 °C, keeping foam characteristics. Two applications of these carbon foams, i.e., an adsorbent of ambient water vapor and a substrate of photocatalyst anatase TiO₂, were experimentally confirmed. For the former application, the present foam could be characterized by prompt adsorption of ambient water vapor. Some of carbon foams prepared were floating on water, even after loading photocatalyst anatase, which might be advantageous for photodecomposition of pollutants in water in respect to the UV rays efficiency.     

Modification of macroporous stainless steel supports with silica nanoparticles for size selective carbon membranes with improved flux

Silica nanoparticles were slip cast into porous stainless steel supports, which were then coated with polyfurfuryl alcohol and pyrolyzed to make nanoporous carbon membranes. The single gas permeances of the membranes formed on modified stainless steel supports were found to be between two and three orders of magnitude larger than the permeances of nanoporous carbon membranes (<10⁻¹¹ mol m⁻² s⁻¹ Pa⁻¹) synthesized on unmodified supports. Importantly, these high permeances (10⁻⁸-10⁻⁹ mol m⁻² s⁻¹ Pa⁻¹) were achieved within the same range of O₂/N₂ selectivities (3-5) that we have observed for single gases permeating at much lower fluxes through the nanoporous carbon membranes on unmodified supports. The nanoporous carbon membranes also were formed by combining the silica nanoparticles with polyfurfuryl alcohol resin and applying the mixture directly onto an unmodified support. This simpler process was as effective in producing selective-high permeance membranes. In both cases the significant increase in permeance without loss of selectivity is attributed to the silica nanoparticles filling the macropores of the stainless steel supports, thereby leading to the formation of very thin but selective carbon layers.     

Carbon nanoparticle-filled polymer flow in the fabrication of novel fiber composites

During the fabrication process of advanced composites using nanoparticle-filled matrices, many problems potentially could arise. One such problem is the clogging of the channels of the microfiber matrix used due to strong interactions between the nanoparticle additives and the matrix walls. In the present paper, a two-dimensional simulation model based on an Eulerian multiphase flow approach is introduced to investigate and predict the flow characteristics of carbon nanoparticle-filled fluid around carbon microfiber matrix. The interactions between the microfiber matrix walls and the nanoparticle additives have been studied, and an energy “imbalance” technique has been applied between the fluid and the microfiber walls to prevent any potential sticking of the nanoparticle additives on the microfiber matrix walls during the flow process. The concept of phasic volume fractions is utilized, and the effects of external body forces, lift forces, and virtual mass forces are introduced into the momentum equations. The phase coupled SIMPLE algorithm is employed to solve the model.     

Carbon foam derived from various precursors

A series of carbon foams was developed by using low-cost precursors, such as coal, coal tar pitch and petroleum pitch. The properties of the resultant carbon foams cover a wide range, e.g., bulk density, 0.32-0.67 g/cm³, compressive strength, 2.5-18.7 MPa, isotropic and anisotropic microstructure, etc. The investigation of foaming mechanism and the relationship between properties and structure indicate that the fluidity and dilatation of the foaming precursors significantly affect the foaming performance and foam structure. Raw coal samples were foamed directly without pretreatment in this work. However, for the pitch based foaming precursor, a thermal pretreatment is necessary to adjust its thermoplastic properties to meet the foaming requirement. The mechanical strength of carbon foam is found to be related to not only the foam cell structure, but also the fluidity and anisotropic domain size of the foaming precursors. In addition, the micro and mesopore structure in carbon foam matrix was investigated by gas adsorption and it was found that it also affects the strength of carbon foam and is related to the fluidity of foaming precursor.     

Synthesis and characterization of polyurethane foam matrices for the support of granular adsorbent materials

Polyurethane foams with different formulations were synthesized and characterized for use as supporting matrices of granular solid adsorbents. The open cell content, specific gravity, thermal stability, and hydrophobicity were determined and related to the formulation composition. The synthesized foams had open cell contents of 88.1–98.5% and specific gravity values of 120–28 kg m^?3. The thermal stability of the prepared foams was influenced mainly by the water content and the type of isocyanate used. The hydrophobicity was assessed by an analysis of the water adsorption isotherms determined on selected foams, and a correlation between these results and the formulation of the foams was attempted. Two types of activated carbons were supported in a polyurethane matrix. The adsorption characteristics evaluated before and after the supporting procedure, by nitrogen adsorption, revealed that there was only a moderate surface area reduction of 15–20%. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 2045–2053, 2004     

Thermal and hygric properties of Portland cement mortar after high-temperature exposure combined with compressive stress

Thermal conductivity λ, water vapor permeability δ, and liquid moisture diffusivity κ of cement mortar are measured on specimens subjected to four types of pretreatment, namely, unloaded, mechanically loaded to 90% of compressive strength, thermally loaded by subjecting to a temperature of 800 °C for 2 h, and loaded both mechanically and thermally. The values of λ and κ are found to depend very significantly on the loading mode. The maximum differences observed compared to the unloaded samples are almost one order of magnitude for λ and as much as three orders of magnitude for κ. In contrast, the values of δ are found to increase by only about 40% compared to the basic unloaded material. It is proposed that the observed large differences in λ and κ are due to the formation of cracks and the increase of total pore volume, which were shown by visual analysis and mercury porosimetry.     

The effective thermal conductivity of three-dimensional reticulated foam materials

In this paper, the effective thermal conductivity k _eff of three-dimensional (3D) reticulated SiC foams were investigated through experimental and numerical methods. The results showed that the k _eff of SiC foams increases as the volume fraction f increases from 30% to 50%. However, there are no systematic changes detected in k _eff when the cell size of the foam varies at a fixed volume fraction. The k _eff of SiC foams as a function of f was obtained. Compared the experimental results with the calculated ones, it indicated that the outcome can be widely applied in estimating the effective thermal conductivity of other foam materials.     

Porous amorphous carbon models from periodic Gaussian chains of amorphous polymers

An algorithm has been developed to create structural models for amorphous carbons using Monte Carlo simulations in the canonical ensemble. The simulation method used follows the experimental preparation of nanoporous carbons (NPC) by pyrolysis from polyfurfuryl alcohol as a guideline. The resulting structure exhibits properties that compare favorably to those observed experimentally for real NPCs. These atomistic NPC models are approaching a realistic representation of NPCs used for gas separations and as such, are being used to study the diffusion of small gas molecules in these materials. Limitations of the method and possible improvements are discussed.     

Microstructure-induced thermal stresses in pyrolytic carbon matrices at temperatures up to 2900 °C

Carbon/carbon composites produced by chemical vapor infiltration consist of carbon fibers embedded in a pyrolytic carbon matrix with a cylindrically layered structure at the microscale. Each coating layer has a different texture and different mechanical properties that depend on temperature. Stress distributions in such carbon matrices subjected to thermal loading and their possible failure scenarios are analyzed. A two-scale numerical model is developed. At the nanoscale, material properties of each layer are determined using a methodology based on the Eshelby's theory for continuously distributed inclusions. The resulting material parameters for each layer are then used in the finite element modeling at the microscale. Calculations are conducted for composites with different matrix structures for several cases of thermal loading. Calculated stress distributions show zones of maximal stress concentration and provide information on possible failure regions which correspond well with experimentally identified failure regions.     

Thermomechanical behavior of a graphite foam

The thermomechanical behavior of a graphite foam derived from pitch for use in thermal management was studied in air up to 150 °C. The damping capacity or loss tangent under flexure was 0.17 at 30 °C for the graphite foam, compared to 0.02 for conventional graphite (not a foam), 0.15 for flexible graphite and 0.22 for PTFE. The loss tangent of the graphite foam decreased with increasing temperature, whereas that of conventional graphite did not. The compressive strain of the graphite foam strongly depended on time, compressive stress and temperature. Due to creep at 30 °C, it reached 3% at 200 kPa, in contrast to 0.7% for conventional graphite. Thermal softening increased the compressive strain in the graphite foam upon heating and subsequent cooling, such that the thermal expansion phenomenon was overshadowed. In contrast, thermal softening was less in conventional graphite. The storage modulus of the graphite foam under flexure was lower than that of conventional graphite. Its fractional decrease with increasing temperature was more than that of conventional graphite.     

Fabrication of porous forsterite-spinel-periclase ceramics by transient liquid phase diffusion process for high-temperature thermal isolation

Porous forsterite-spinel-periclase ceramics with low thermal conductivity were synthesized via a transient liquid phase diffusion process by using pre-synthesized pellets and fused magnesia powder. The effects of sintering temperature on the pore formation, phase composition, sintering behavior, and properties of the resulting porous ceramics were investigated. The pre-synthesized pellets had a porous structure and contained a large amount of cordierite and enstatite. During the sintering progress, the pellets were converted into a transient liquid phase, which diffused into the solid MgO matrix. The liquid phase diffusion reaction promoted forsterite and spinel formation, which resulted in the in-situ formation of large pores. At elevated temperatures, the liquid phase disappeared and a large number of well-developed grains were simultaneously precipitated from the liquid phase. Porous ceramics with thermal conductivities of 0.42–0.48 W/(m·K) and refractoriness under load values of 1588 °C and 1624 °C were obtained after sintering at 1600 °C for 3 h.     

Synthesis of mesoporous carbon supports via liquid impregnation of polystyrene onto a MCM-48 silica template

Mesoporous MCM-48 was synthesised and used as a template to synthesise mesoporous carbon materials. Polystyrene, the carbon source, together with sulfuric acid and toluene were added to the template (160 °C for 6 h) and this procedure generated a low surface area carbon supported/MCM-48 material. A repeat addition and carbonisation step was needed to form the precursor carbon/MCM-48 material that was pyrolysed at 900 °C to generate graphitic mesoporous carbon materials, characterised by XRD, HR-TEM, Raman spectroscopy and surface area analysis. The effect of the amount of polystyrene as well as the role of the pyrolysis temperature on the final product was investigated. This synthesis methodology can readily be controlled to produce partially ordered graphitic mesoporous carbon supports with predictable pore width and surface area.     

A multi-scale model for predicting the thermal shock resistance of porous ceramics with temperature-dependent material properties

This paper develops a novel multi-scale thermal/mechanical analysis model which not only can efficiently measure the thermal shock response but also highly reflects the effects of diversiform micro-structures of porous ceramics. Knowledge of the temperature distribution and time-varied thermal stress intensity factors (SIF) is derived by finite element/finite difference method and the weight function method in the macro continuum model. The finite element analysis employs a micro-mechanical model in conjunction with the macro model for the purpose of relating the SIF to the thermal stress in the struts of the porous ceramics. The micro model around the crack tip was established by using Voronoi lattices to accurately explore the micro-architectural features of porous ceramics. Hot shock induced center crack and cold shock induced edge crack are both considered. Effects of relative density and pore size on the thermal shock resistance are investigated and the results are well coincident with the experimental tests. The influence of cell regularity and cross section shape of the cell struts is discussed and the corresponding explanations are provided. The importance of incorporating temperature-dependent material properties on the thermal shock resistance prediction is quantitatively represented. These multi-faceted models and results provide a significant guide to the design and selection of porous ceramics against the thermal shock fracture failure for the future thermal protection system of space shuttle.     

Porous anorthite ceramics with ultra-low thermal conductivity

Porous anorthite ceramics with an ultra-low thermal conductivity of 0.018 W/m K have been fabricated by hydrous foam-gelcasting process and pressureless sintering method using γ-alumina, calcium carbonate and silica powders as raw materials. Microstructure and phase composition were analyzed by SEM and XRD respectively. Properties such as porosity, pore size distribution and thermal conductivity were measured. High porosity (69–91%) and low thermal conductivity (0.018–0.13 W/m K) were obtained after sintering samples with different catalyst additions at 1300–1450 °C. Porosity, pore size, pore structure and grain size had obvious effect on heat conduction, resulting in the low thermal conductivity. The experimental thermal conductivity data of porous anorthite ceramics were found to be fit well with the computed values derived from a universal model.     

Carbon black dispersions as thermal pastes that surpass solder in providing high thermal contact conductance

Carbon black dispersions based on polyethylene glycol (PEG) or di(ethylene glycol) butyl ether, along with dissolved ethyl cellulose, provide thermal pastes that are superior to solder as thermal interface materials. The thermal contact conductance of the interface between copper disks reaches 3×10⁵ W/m² °C, compared to 2×10⁵ W/m² °C for a tin-lead-antimony solder. The pastes based on PEG are superior to those based on butyl ether in their thermal stability above 100 °C. Carbon black is superior to materials that are more conductive thermally (graphite, diamond and nickel particles, and carbon filaments) in providing thermal pastes of high performance. The performance of thermal pastes and solder as thermal interface materials is mainly governed by their conformability and spreadability rather than their thermal conductivity.     

High density carbon fiber/boron nitride matrix composites: Fabrication of composites with exceptional wear resistance

This paper describes the fabrication of high density (ρ ∼ 1.75 g/cc) composites containing a hybrid (carbon and boron nitride), or complete boron nitride matrix. The composites were reinforced with either chopped or 3D needled carbon fibers. The boron nitride was introduced via liquid infiltration of a borazine oligomer that can exhibit liquid crystallinity. The processing scheme was developed for the chopped carbon fiber/boron nitride matrix composites (C/BN) and later applied to the 3D carbon fiber reinforced/boron nitride matrix composites (3D C/BN). The hybrid matrix composites were produced by infiltrating the borazine oligomer into a low density 3D needled C/C composite to yield 3D C/C-BN. In addition to achieving high densities, the processing scheme yielded d₀₀₂ spacings of 3.35 Å, which afforded boron nitride with excellent hydrolytic stability. The friction and wear properties of the composites were explored over the entire energy spectrum for aircraft braking using an inertial brake dynamometer. The C/BN composites outperformed both the previously reported C/C-BN and chopped fiber reinforced C/C. The high density 3D C/BN performed as well as both the 3D C/C and the C/BN. The 3D C/C-BN provided outstanding wear resistance, incurring nearly zero wear across the entire testing spectrum. The coefficient of friction was relatively stable with respect to energy level, varying from 0.2 to 0.3.