Two types of natural graphite host matrix for composite activated carbon adsorbents

Two types of host for activated carbon (AC) adsorbents intended for use in compressed systems are studied: expanded natural graphite (ENG) and expanded natural graphite treated with acid (ENG-TA). Results show that compressed ENG-TA has much higher thermal conductivity than the compressed ENG. For a density of 830 kg/m³ the thermal conductivity of compressed ENG-TA is 336 W/(mK), and it is of the order of one hundred times higher compared with compressed ENG having similar density. The permeability of compressed ENG-TA is much more critical than the compressed ENG. For example for similar density of 300 kg/m³, the permeability of compressed ENG-TA is 2.01 × 10^?15 m² while the permeability of compressed ENG is 1.07 × 10^?13 m². Compressed composite adsorbents of AC with ENG as host were produced with a high density in the range 700–720 kg/m³. Considering that the permeability will be too low using composite AC with ENG-TA as host in high density, the density range was restricted to less than 500 kg/m³. The thermal conductivity of AC/ENG-TA composite is much higher than the thermal conductivity of AC/ENG composite, and it is about 7 times higher than the optimal value of AC/ENG composite.     

Performance predictions for a new zeolite 13X/CaCl2 composite adsorbent for adsorption cooling systems

Composite adsorbents synthesized from zeolite 13X and CaCl₂ were investigated for applications in solar adsorption cooling systems. The effects of Ca ion exchange on the adsorption properties of zeolite 13X were studied. Ca ion exchange was found to decrease the specific surface area of the zeolite while increasing the total pore volume. Soaking zeolite 13X in 46 wt.% CaCl₂ solution for 24 h gave the optimum Ca ion exchange. The increase in the total pore volume facilitated further impregnating the zeolite with CaCl₂. In all, 41.5 mol% of CaCl₂ was impregnated in the Ca-ion-exchanged zeolite from a 40 wt.% CaCl₂ solution to form the zeolite 13X/CaCl₂ composite adsorbent. A 0.4 g/g difference in equilibrium water uptake between 25 and 75 °C at 870 Pa was recorded for the composite adsorbent. This was 420% of that of zeolite 13X under the same conditions. Numerical simulation predicts that an adsorption cooling system using the composite adsorbent could be powered by a low grade thermal energy source using the temperature range 75–100 °C. Greatly improved efficiency is predicted compared to a system using pure zeolite 13X or impregnated silica gel.     

Optical probing of anisotropic heat transport in the quantum spin ladder Ca9La5Cu24O41

A transient thermal imaging technique is used to monitor heat diffusion at the surface of the antiferromagnetic spin ladder material Ca₉La₅Cu₂₄O₄₁. This material shows highly anisotropic thermal conductivity due to a large uni-directional magnetic heat transport along the ladders. The thermal conductivity is measured using optical heating as well as electrical heating, yielding 37 ± 3 W m^?1 K^?1 for the fast (ladder) direction and 2.5 ± 0.5 W m^?1 K^?1 for the slow direction, respectively. The fast direction result is in agreement with the thermal conductivity measured using other dynamic methods, but about 60% lower than the thermal conductivity measured using steady state methods.     

Sintered porous heat sink for cooling of high-powered microprocessors for server applications

This paper experimentally investigates the sintered porous heat sink for the cooling of the high-powered compact microprocessors for server applications. Heat sink cold plate consisted of rectangular channel with sintered porous copper insert of 40% porosity and 1.44 × 10^?11 m² permeability. Forced convection heat transfer and pressure drop through the porous structure were studied at Re ? 408 with water as the coolant medium. In the study, heat fluxes of up to 2.9 MW/m² were successfully removed at the source with the coolant pressure drop of 34 kPa across the porous sample while maintaining the heater junction temperature below the permissible limit of 100 ± 5 °C for chipsets. The minimum value of 0.48 °C/W for cold plate thermal resistance (R_cp) was achieved at maximum flow rate of 4.2 cm³/s in the experiment. For the designed heat sink, different components of the cold plate thermal resistance (R_cp) from the thermal footprint of source to the coolant were identified and it was found that contact resistance at the interface of source and cold plate makes up 44% of R_cp and proved to be the main component. Convection resistance from heated channel wall with porous insert to coolant accounts for 37% of the R_cp. With forced convection of water at Re = 408 through porous copper media, maximum values of 20 kW/m² K for heat transfer coefficient and 126 for Nusselt number were recorded. The measured effective thermal conductivity of the water saturated porous copper was as high as 32 W/m K that supported the superior heat augmentation characteristics of the copper–water based sintered porous heat sink. The present investigation helps to classify the sintered porous heat sink as a potential thermal management device for high-end microprocessors.     

Study of thermal conductivity,permeability, and adsorption performance of consolidated composite activated carbon adsorbent for refrigeration

Composite adsorbents, comprising activated carbon and expanded natural graphite, have been developed, and their thermal conductivity, permeability and adsorption performance were tested. The thermal conductivity varied with the ratio of activated carbon to expanded natural graphite. Thermal conductivity increased as the ratio of expanded graphite increased. Considering that the density of activated carbon for the composite adsorbent should not be lower than 200 kg/m³, otherwise the volumetric cooling capacity would be unacceptably low, the highest thermal conductivity obtained from experiments was 2.47 W m^?1 K^?1. The permeability was also measured, and the best result obtained was 4.378 × 10^?12 m². In order to evaluate the influence of heat and mass transfer on adsorption performance, the adsorption rate was tested using a Rubotherm magnetic suspension balance, and results showed that for the freezing conditions lower than ?10 °C the performance of granular activated carbon was better than that of solidified adsorbent because of the reduced mass transfer of ammonia at low saturated pressure. The adsorption performance of consolidated adsorbents increased rapidly when the evaporating temperature was higher than ?10 °C. When the evaporating temperature was 8 °C, the adsorption rate of consolidated adsorbent was improved by 29% if compared with that of granular adsorbent.     

PVDF-g-PSSA and Al2O3 composite proton exchange membranes

Poly(vinylidene fluoride) grafted polystyrene sulfonated acid (PVDF-g-PSSA) membranes doped with different amount of Al₂O₃ (PVDF/Al₂O₃-g-PSSA) were prepared based on the solution-grafting technique. The microstructure of the membranes was characterized by IR-spectra and scanning electron microscope (SEM). The thermal stability was measured by thermal gravity analysis (TGA). The degree of grafting, water-uptake, proton conductivity and methanol permeability were measured. The results show that the PVDF-g-PSSA membrane doped with 10% Al₂O₃ has a lower methanol permeability of 6.6 × 10⁻⁸ cm² s⁻¹, which is almost one-fortieth of that of Nafion-117, and this membrane has moderate proton conductivity of 4.5 × 10⁻² S cm⁻¹. Tests on cells show that a DMFC with the PVDF/10%Al₂O₃-g-PSSA has a better performance than Nafion-117. Although Al₂O₃ has some influence on the stability of the membrane, it can still be used in direct methanol fuel cells in the moderate temperature.     

Assessing the effects of soil grading on the moisture content-dependent thermal conductivity of stabilised rammed earth materials

New data for both the dry-state and the moisture content-dependent thermal conductivity of cement-stabilised rammed earth (SRE) materials is presented. For highly compacted SRE materials, no correlation was found between thermal conductivity and dry density or void ratio. The thermal conductivity of SRE materials increases linearly with the saturation ratio, S_r of the material and can be expressed as λ¹, the moisture content-dependent thermal conductivity. The sensitivity of λ¹ to an increase in the saturation ratio of SRE materials varies according to soil grading. The influence of grading parameters on λ¹ can cause material variations of approximately 0.8 m² K/W. The experimental data has been applied to standard SRE wall design configurations and the effect of wall moisture content on the total thermal resistance has been shown. The R-value of an SRE wall irrespective of cavity insulation can vary by as much as 0.13 m² K/W.     

The adsorption refrigeration characteristics of alkaline-earth metal chlorides and its composite adsorbents

A study was conducted on the adsorption characteristics of the adsorption refrigeration working pairs using alkaline-earth metal chlorides as adsorbents and ammonia as refrigerant. The adsorption isotherms between alkaline-earth metal chlorides and nitrogen were studied. The study shows that the adsorbents of CaCl₂, SrCl₂ provide better adsorption capability associated with ammonia when compared to that of MgCl₂, BaCl₂. CaSO₄ was added into the CaCl₂, SrCl₂ with a mass ratio 20%, respectively, in order to solve the swelling and smashing problems encountered with CaCl₂, SrCl₂ adsorbent particles. The adsorption refrigeration experiments of composite adsorbents were investigated. The results show that the refrigeration capacity of the unit adsorbent of CaCl₂/CaSO₄ is 1.26 times higher than that of CaCl₂, and SrCl₂/CaSO₄ is 1.6 times higher than that of SrCl₂ at 100 °C. The mechanism of NH₃ adsorption and the gelatification of CaSO₄ were also discussed. BET (Brunauer–Emmett–Teller) specific surface area and pore structure of adsorbents were examined. Results show that the BET specific surface area and pore structure of composite adsorbents are retained well. This study indicates that the refrigeration capacity could be enhanced by compositing the adsorbents which indicates that composite adsorbents can perform better in adsorption refrigeration, and can be employed in adsorption refrigeration system using low-grade heat source.     

Preparation and characterization of activated carbon from rubber-seed shell by physical activation with steam

The use of rubber-seed shell as a raw material for the production of activated carbon with physical activation was investigated. The produced activated carbons were characterized by Nitrogen adsorption isotherms, Scanning electron microscope, Thermo-gravimetric and Differential scanning calorimetric in order to understand the rubber-seed shell activated carbon. The results showed that rubber-seed shell is a good precursor for activated carbon. The optimal activation condition is: temperature 880 °C, steam flow 6 kg h^?1, residence time 60 min. Characteristics of activated carbon with a high yield (30.5%) are: specific surface area (S_BET) 948 m² g^?1, total volume 0.988 m³ kg^?1, iodine number of adsorbent (q_iodine) 1.326 g g^?1, amount of methylene blue adsorption of adsorbent (q_mb) 265 mg g^?1, hardness 94.7%. It is demonstrated that rubber-seed shell is an attractive source of raw material for producing high capacity activated carbon by physical activation with steam.     

Development research on composite adsorbents applied in adsorption heat pump

A novel adsorbent design technique base on the concept of Kelvin equation was proposed to develop hydrophilic adsorbent applicable to water vapor adsorption heat pump (AHP) for high performance. In the process, the composite adsorbent was prepared after silica gel was synthesized in the pores of activated carbons by impregnating activated carbons in sodium silicate solution. Two kinds of activated carbons were tested to produce composite adsorbent and to investigate the performance by measuring the adsorption isotherms of water vapor and pore structure characteristics. All adsorption isotherms of the silica impregnated activated carbons prepared shifted to a lower region of water vapor pressure compared to those of the raw activated carbons. The volume-based amount of adsorption in the AHP operation range (φ = 0.1–0.4) for the adsorbents prepared at sodium silicate solution concentration of 10 wt.% and impregnating time of 48 h are 5.88 and 2.62 times that of the raw activated carbons (AC1 and AC2), respectively. Based on the Kelvin equation, it is clarified that the contact angle and the volume of pore radius greater than 1.2 nm decrease with the increase of sodium silicate solution concentration for the novel composite adsorbents, which contributes the isothermals shift to lower relative pressure range.     

Ionic liquid–polymer gel electrolytes based on morpholinium salt and PVdF(HFP) copolymer

New ionic liquid–polymer gel electrolytes (IPGEs) are prepared from N-ethyl-N-methylmorpholinium bis(trifluoromethanesulfonyl)imide (Mor_1,2TFSI) and poly(vinylidene fluoride)-hexafluoropropylene copolymer (PVdF(HFP)). To investigate the effect of propylene carbonate (PC) on the ionic conductivity of the IPGEs, the preparation methods are roughly divided into two groups according to the presence or absence of PC. The ionic conductivity for each IPGE is measured with increasing temperature and changing weight ratio of Mor_1,2TFSI. The results show that the ionic conductivity increases as the temperature and weight ratio of the Mor_1,2TFSI increase, and that the added PC improves the ionic conductivity of the IPGEs. In addition, thermogravimetric analysis and the data from infrared spectroscopy demonstrate the thermal stability of each IPGE and the presence of PC in the polymer network. Although the IPGEs that contain PC display high conductivity (∼1.1 × 10⁻² S cm⁻¹) at 60 °C, they are thermally unstable.     

Influence of composition and acid treatment on proton conduction of composite polybenzimidazole membranes

Inorganic/organic composite membranes formed by polybenzimidazole, silicotungstic acid and silica with different ratio between them have been prepared and characterized before and after treatment in phosphoric acid in order to evaluate the influence of composition and acid treatment on some main characteristics of the membranes. In particular the proton conductivity, the mechanical stability and the structural characteristics of the membranes were evaluated. Silica behaved as a support on which the heteropolyacid remained blocked in finely dispersed state and as an adsorbent for water, thus determining a beneficial effect on proton conduction. The membrane with 50 wt.% of SiWA–SiO₂/PBI, mechanically stable, gave proton conductivity of 1.2×10⁻³ S cm⁻¹ at 160°C and 100% relative humidity. After treatment with phosphoric acid the proton conductivity of membranes increased to 2.23×10⁻³ S cm⁻¹ under the same test conditions. All the materials prepared had amorphous structure.     

Vacuum insulation properties of phenolic foam

Characteristic properties of phenolic foam as the interstitial material of a vacuum insulation panel are investigated experimentally. For the measurement of effective thermal conductivity, a vacuum guarded hot plate (VGHP) apparatus is used and the conductivity is measured at various vacuum levels. Radiative properties are found using a Fourier transform infrared spectroscopy (FT-IR) device. Solid conductivity is estimated using the porosity of the foam. Effective thermal conductivity at high level of vacuum is measured to be 5 mW/m K which is sum of solid conductivity (2.56 mW/m K) and radiative conductivity (2.44 mW/m K) with 5% of measurement uncertainty. The pore size of the foam is estimated to be 260 μm using rarefied gas conduction theory. This ensures insulation performance of phenolic foam up to about 10^?3 atm. Other practical characteristics of phenolic foam as the VIP core material are also discussed.     

Thermal conductivities study on silica aerogel and its composite insulation materials

This paper presents a theoretical and experimental study on thermal conductivities of silica aerogel, xonotlite-type calcium silicate and xonotlite–aerogel composite insulation material. The transmittance spectra of silica aerogel and xonotlite-type calcium silicate samples are obtained through FTIR measurements. The corresponding extinction coefficient spectra of the three materials are then obtained by applying Beer’s law. The thermal conductivities of aerogel, xonotlite-type calcium silicate, and xonotlite–aerogel composite insulation material are measured from 300 to 970 K and from 0.045 Pa to atmospheric pressure with the transient hot-strip (THS) method. The thermal conductivity models developed for coupled heat transfer of gas and solid based on the unit cell method are compared with the experimental measurement results. It is shown that the effective thermal conductivity models matches well with the experimental data. The specific spectral extinction coefficients of xonotlite-type calcium are larger than 10 m² kg^?1, and the specific spectral extinction coefficients of aerogel are larger than 7 m² kg^?1 over the whole measured spectra. The density of xonotlite-type calcium silicate is the key factor affecting the effective thermal conductivity of xonotlite–aerogel composite insulation material, and the density of aerogel has little influence. The effective thermal conductivity can be lowered greatly by composite of the two materials at an elevated temperature.     

Simulation of turbulent natural convection in a porous cylindrical annulus using a macroscopic two-equation model

This work presents numerical computations for laminar and turbulent natural convection within a horizontal cylindrical annulus filled with a fluid saturated porous medium. Computations covered the range 25 < Ra_m < 500 and 3.2 × 10⁻⁴ > Da > 3.2 × 10⁻⁶ and made use of the finite volume method. The inner and outer walls are maintained at constant but different temperatures. The macroscopic k–ε turbulence model with wall function is used to handle turbulent flows in porous media. First, the turbulence model is switched off and the laminar branch of the solution is found when increasing the Rayleigh number, Ra_m. Subsequently, the turbulence model is included and calculations start at high Ra_m, merging to the laminar branch for a reducing Ra_m. This convergence of results as Ra_m decreases can be seen as an estimate of the so-called laminarization phenomenon. Here, a critical Rayleigh number was not identified and results indicated that when the porosity, Prandtl number, conductivity ratio between the fluid and the solid matrix and Ra_m are kept fixed, the lower the Darcy number, the higher is the difference of the average Nusselt number given by the laminar and turbulent models.     

Effect of organic acids and nano-sized ceramic doping on PEO-based solid polymer electrolytes

Composite solid polymer electrolytes (CSPEs) consisting of polyethyleneoxide (PEO), LiClO₄, organic acids (malonic, maleic, and carboxylic acids), and/or Al₂O₃ were prepared in acetonitrile. CSPEs were characterized by Brewster Angle Microscopy (BAM), thermal analysis, ac impedance, cyclic voltammetry, and tested for charge–discharge capacity with the Li or LiNi_0.5Co_0.5O₂ electrodes coated on stainless steel (SS). The morphologies of the CSPE films were homogeneous and porous. The differential scanning calorimetric (DSC) results suggested that performance of the CSPE film was highly enhanced by the acid and inorganic additives. The composite membrane doped with organic acids and ceramic showed good conductivity and thermal stability. The ac impedance data, processed by non-linear least square (NLLS) fitting, showed good conducting properties of the composite films. The ionic conductivity of the film consisting of (PEO)₈LiClO₄:citric acid (99.95:0.05, w/w%) was 3.25 × 10⁻⁴ S cm⁻¹ and 1.81 × 10⁻⁴ S cm⁻¹ at 30 °C. The conductivity has further improved to 3.81 × 10⁻⁴ S cm⁻¹ at 20 °C by adding 20 w/w% Al₂O₃ filler to the (PEO)₈LiClO₄ + 0.05% carboxylic acid composite. The experimental data for the full cell showed an upper limit voltage window of 4.7 V versus Li/Li⁺ for CSPE at room temperature.     

The effect of cycle boundary conditions and adsorbent grain size on the water sorption dynamics in adsorption chillers

The aim of this work was an experimental study of the temporal evolution of isobaric adsorption uptake (release) for simplest configuration of an adsorbent-heat exchanger unit, namely, a monolayer of loose adsorbent grains located on a metal plate. The study was performed by a large temperature jump method at four various boundary conditions of an adsorptive heat transformation cycle typical for air-conditioning application driven by low temperature heat: T_e = 5 and 10 °C, T_c = 30 and 35 °C and T_HS = 80 °C. The size of the Fuji silica grains was varied from 0.2 to 1.8 mm to investigate its effect on water sorption dynamics. For each boundary set and grain size the experimental kinetic curve could be described by an exponential function up to 80–90% of the equilibrium conversion. Desorption runs are found to be faster than appropriate adsorption runs by a factor of 2.2–3.5, hence, for optimal durations of the isobaric ad- and desorption phases of the chilling cycle should be selected accordingly. The size R of the adsorbent grains was found to be a powerful tool to manage the dynamics of isobaric water ad-/desorption. For large grains the characteristic time was strongly dependent on the grain size and proportional to R². Much less important appeared to be an impact of the boundary conditions which variation just weakly affected the dimensionless kinetic curves for the four tested cycles. The maximal specific cooling/heating power was proportional to the maximal temperature difference ΔT and the contact area S between the layer and the metal plate, and can exceed 10 kW/kg. The heat transfer coefficient α estimated from this power was as large as 100–250 W/(m² K) that much exceeds the value commonly used to describe the cycle dynamics.     

Preparation and characterization of sulfonated PEEK-WC membranes for fuel cell applications: A comparison between polymeric and composite membranes

Sulfonated poly(etheretherketone) with a cardo group (SPEEK-WC) exhibiting a wide range of degree of sulfonation (DS) was used to prepare polymeric membranes and composite membranes obtained by incorporation of an amorphous zirconium phosphate sulfophenylenphosphonate (Zr(HPO₄)(O₃PC₆H₄SO₃H), hereafter Zr(SPP)) in a SPEEK-WC matrix. The nominal composition of the composite membranes was fixed at 20 wt% of Zr(SPP). Both types of membrane were characterized for their proton conductivity, methanol permeability, water and/or methanol uptake, morphology by SEM and mechanical properties. For comparison, a commercial Nafion 117 membrane was characterized under the same operative conditions. The composite membranes exhibited a reduced water uptake in comparison with the polymeric membranes especially at high DS values and temperature higher than 50 °C. As a result, the water uptake into composite membranes remained about constant in the range 20–70 °C. The methanol permeability (P) of both polymeric and composite membranes was always lower than that of a commercial Nafion 117 membrane. At 22 °C and 100% relative humidity (RH), the proton conductivities (σ) of the polymeric membranes increased from 6 × 10⁻⁴ to 1 × 10⁻² S cm⁻¹ with the increase of DS from 0.1 to 1.04. The higher conductivity value was comparable with that of Nafion 117 membrane (3 × 10⁻² S cm⁻¹) measured under the same operative conditions. The conductivities of the composite membranes are close to that of the corresponding polymeric membranes, but they are affected to a lesser extent by the polymer DS. The maximum value of the σ/P ratio (about 7 × 10⁴ at 25 °C) was found for the composite membrane with DS = 0.2 and was 2.5 times higher than the corresponding value of the Nafion membrane.     

Experimental investigation and model development for thermal conductivity of α-Al2O3-glycerol nanofluids

In order to investigate the effect of nanoparticle volume fraction, nanoparticle size and temperature on the thermal conductivity of glycerol based alumina (α-Al₂O₃) nanofluids, a set of experiments were carried out for temperature ranging from 20 °C to 45 °C. The nanofluids contained α-Al₂O₃ nanoparticles of three different sizes (31 nm, 55 nm and 134 nm) were prepared by two-step method at volume fractions ranging from 0.5% to 4%. The experimental results show that α-Al₂O₃-glycerol nanofluids have substantially higher thermal conductivity than the base fluid and the maximum enhancement of the relative thermal conductivity was 19.5% for the case of 31 nm at 4% volume fraction. The data analyses indicated that the volume fraction and size of the nanoparticles have significant effects on the thermal conductivity ratio (TCR) of Al₂O₃-glycerol nanofluids, while the temperature has almost no significant effect on the data for range of this study. At room temperature, the effective thermal conductivity remains almost constant for 50 h at 4% volume fractions. The comparison of the obtained experimental data and predictions from some existing theoretical and empirical models reveals that the thermal conductivity ratio and its trend could not be accurately explained by the models in open literature. Consequently, a new empirical correlation based on the experimental data has been developed in this study.     

Heat exchange performance of stainless steel and carbon foams modified with carbon nano fibers

Carbon nanofibers (CNF), with fishbone and parallel wall structures, were grown by catalytic chemical vapor deposition on the surface of carbon foam and stainless steel foam, in order to improve their heat exchange performance. Enhancement in heat transfer efficiency between 30% and 75% was achieved with CNF-modified stainless steel foam as measured in a filled copper channel for varied lengths of heat exchanger (between 0.05 and 0.01 m). The heat transfer coefficient of carbon foam decreased when modified with CNF by ～40% in average. The increase in heat transfer efficiency of the CNF-modified stainless steel foam is explained by an increase in surface area provided by the carbon filaments grown on stainless steel of one order of magnitude, and by the carbon fibers’ specific parallel wall structure, providing excellent thermal conductivity in the axial direction (h = 1130 W/K^?1m^?2). However, nanofibers grown on carbon foam have fishbone type structure, exhibiting lesser conductivity in the axial direction resulting in lower thermal conductivity of the fibers from the wall to the air (h = 428 W/K^?1m^?2). The higher crystallinity of carbon nanotubes of modified stainless steel material in contrast to chaotic mal-alignment and relatively high concentration of structural defects of carbon nanofibers grown on carbon foam can also contribute to the big difference in heat exchange properties.