Low Heats Research Articles

Self-heating in a small mesa of BSCCO intrinsic Josephson junctions at very low temperatures

Temperature rise due to self-heating at very low temperatures near 0.4 K has been numerically estimated for a 2 μm on a side and 3 nm-thick mesa of Bi2Sr2CaCu2O8+δ intrinsic Josephson junctions. In spite of very low thermal conductivities and specific heats of the materials constructing the mesa at these temperatures, the temperature rise is much less than 1 K at a usual current injection level. It is also found that the temperature rise shows a tendency to increase with the mesa size, which was also observed at higher temperatures. The temperature rise in the presence of the contact resistance is also estimated to be small when the contact resistivity is of the order of 1 × 10−10 Ωm2. When the contact resistance is larger, the temperature rise becomes comparable with 1 K. The temperature rise in the presence of a voltage state junction is much less than 1 K at a critical current level. This implies that the saturation of the escape temperature of the recent switching probability experiments is totally unlikely to be due to simple heating characterized by the thermal equilibrium in phonons.

Synthesis of some novel hydrazono acyclic nucleoside analogues

The syntheses of novel hydrazono acyclic nucleosides similar to miconazole scaffolds are described. In this series of acyclic nucleosides, pyrimidine as well as purine and other azole derivatives replaced the imidazole function in miconazole and the ether group was replaced with a hydrazone moiety using phenylhydrazine. To interpret the dominant formation of (E)-hydrazone derivatives rather than (Z)-isomers, PM3 semiempirical quantum mechanic calculations were carried out which indicated that the (E)-isomers had the lower heats of formation.

Development of a model for compensating the influence of temperature gradients within the sample on DSC-results on phase change materials

The use of phase change materials (PCMs) in thermal storage is not a new concept, but engineers are continually finding new ways to utilize them in a wide range of applications. A PCM takes advantage of high latent heat in the phase change process to store large amounts of heat while undergoing only a small change in temperature. This property makes PCMs suitable for thermal storage purposes in a wide range of engineering applications. Due to the nature of these applications, it is vital to have a precise knowledge of the thermal characteristics of any PCM. Unfortunately, due to the low thermal conductivities and high latent heats found in PCMs, current measuring tools such as differential scanning calorimetry, provide inconsistent results. This paper conjectures that these errors come from the effects of low thermal diffusivity samples as well as improper data analysis methods.

CO 2 adsorption, selectivity and water tolerance of pillared-layer metal organic frameworks

The potential for two water tolerant three-dimensional pillared-layer metal organic frameworks (MOFs) M 2(BDC) 2(Dabco) (M = Zn, Ni) to selectively adsorb and separate CO 2 is considered. Isotherms for CO 2, CH 4 and N 2 were measured from 0 to 15 bar and for O 2 from 0 to 2.5 bar at temperatures between 25 and 105 °C. The isotherms give CO 2 adsorption capacities of 13.7 and 12.5 mol/kg for Zn 2(BDC) 2(Dabco) and Ni 2(BDC) 2(Dabco) at 25 °C and 15 bar, respectively. Both materials have higher CO 2/N 2 selectivity than CO 2/CH 4 selectivity and low heats of adsorption. The structures of the two MOFs are stable after O 2 and 30% relative humidity water vapor sorption at 25 °C, but collapsed after 60% relative humidity water vapor sorption at 25 °C.

Molecular Simulation Study on the Separation of Xylene Isomers in MIL-47 Metal−Organic Frameworks

The separation performance of the metal−organic framework MIL-47 for xylene isomers was studied by Monte Carlo simulations in the grand-canonical ensemble. The experimental adsorption isotherms of xylene isomers in MIL-47 between 343 and 423 K were reproduced by using force fields available in the literature. Mixture isotherms were computed and compared with the mixture isotherms predicted by using pure component adsorption data and the ideal adsorption solution theory. This theory accurately predicts mixture isotherms of xylene isomers in MIL-47. the ideal adsorption sorption theory was further employed to calculate the separation factors of xylene isomers in MIL-47. The order of preferential adsorption was found to be ortho > para > meta. The adsorption selectivity was found to increase with pressure, and the results showed a good agreement with the experimental data available. Henry coefficients and low coverage heats of adsorption and adsorption entropies were computed at 543 K showing an excellent agreement with experiments. It was found that the reason for adsorption selectivity is the interaction of CH3 groups of neighboring molecules at high loadings, mainly for molecules adsorbed on the same wall of the MIL-47 channels.

Thermodynamics and regeneration studies of CO2 adsorption on multiwalled carbon nanotubes

Multiwalled carbon nanotubes (CNTs) were fabricated and modified by 3-aminopropyl-triethoxysilane (APTS) solutions to study thermodynamics and regeneration of CO2 adsorption from gas streams. The CO2 adsorption capacities of CNTs and CNT(APTS) decreased with temperature indicating the exothermic nature of adsorption process while the thermodynamic analysis gave low isosteric heats of adsorption, which are typical for physical adsorption. The cyclic CO2 adsorption on CNT(APTS) showed that the adsorbed CO2 could be effectively desorbed via thermal treatment at 120 °C for 25 min while the adsorbed CO2 due to physical interaction could be effectively desorbed via vacuum suction at 0.145 atm for 30 min. If a combination of thermal and vacuum desorption was conducted at 120 °C and 0.145 atm, the time for effectively desorbing CO2 could be further shortened to 5 min. The adsorption capacities and the physicochemical properties of CNT(APTS) were preserved during 20 cycles of adsorption and regeneration. These results suggest that the CNT(APTS) can be stably employed in prolonged cyclic operation and they are thus possibly cost-effective sorbents for CO2 capture from flue gases.

Thermodynamics and regeneration of CO 2 adsorption on mesoporous spherical-silica particles

Mesoporous spherical-silica particles (MSPs) were modified by N-[3-(trimethoxysilyl) propyl]ethylenediamine (EDA) solution and were employed as sorbents to study thermodynamics and regeneration of CO 2 adsorption from gas streams. The thermodynamic analysis gave low isosteric heats of adsorption, which are typical for physical adsorption. The cyclic CO 2 adsorption on MSP(EDA) showed that the adsorbed CO 2 could be effectively desorbed via thermal treatment at 120 °C for 25 min while the adsorbed CO 2 due to physical interaction could be effectively desorbed via vacuum suction at 0.145 bar for 30 min. If a combination of thermal treatment and vacuum suction was conducted at 120 °C and 0.145 bar, the time for effectively desorbing CO 2 could be shortened to 7.5 min and thus reduces a significant amount of energy penalty. The adsorption capacities and the physicochemical properties of MSP(EDA) were preserved after 15 cycles of adsorption and regeneration. These results suggest that the MSP(EDA) can be stably employed in the prolonged cyclic adsorption and that they are thus possibly cost-effective sorbents for CO 2 capture from flue gases.

On the source of entropic elastomeric force in polypeptides and proteins: Backbone configurational vs. side-chain solvational entropy

At physiological temperature In water, the relaxed state of the protein, elastin, and model elastomeric sequential polypeptides derived from this biological elastomer is the result of an inverse temperature transition. At lower temperatures, say at 20°C. the hydrophobic side chams of the elastomers are hydrated with a low-entropy net of water. On raising the temperature, the more ordered waters of hydrophobic hydration are converted to less ordered bulk water as the polypeptide chains fold increasing intra- and intermolecular hydrophobic contacts. Following Flory and colleagues, thermoelasticity studies suggests that these polypeptide elastomers are dominantly entropic elastomers above the temperature of the inverse temperature transition. A central question becomes the source of the entropic elastomeric force. On stretching, hydrophobic side chains become exposed to water, resulting in an exothermic reaction of hydrophobic hydration. The issue addressed by the present report is whether this decrease in solvent entropy on stretching might make a major contribution to the entropic elastomeric restoring force. It has previously been argued that the free energy of solvation can be made very small by a 30% ethylene glycol (EG):70% water solvent mixture. This is demonstrated here using the repeating pentapeptide sequence of elastin (Val '-Pro2-Gly'-Va14-Gly '). or poly( VPGVG) and its 7-irradiation cross-linked elastomeric matrix. Differential scanning calorimetry of the inverse temperature transition of poly( VPGVG) shows the endothermic heat of the transition to become very small in EG/HZO when compared with H20 alone, which also indicates a very small entropy change for the transition on exposure of the hydrophobic side chains to the EG/H20 solvent mixture. A similar result is found for the crosslinked elastomeric matrix. Significantly, however, in spite of the lower heats for hydrophobic solvation. the elastic modulus and the entropic elastomeric forces generated are greater in EG/H2O. Thus, even though the heat of the transition and the entropy change on solvation are markedly reduced, the entropic elastomeric force is increased. Accordingly, it is argued that the entropy change due to hydrophobic sidechain solvation on extension is considered not to be a primary source of the entropic elastomeric force.

CO2Adsorption-Based Separation by Metal Organic Framework (Cu-BTC) versus Zeolite (13X)

The potential for the metal organic framework (MOF) Cu-BTC to selectively adsorb and separate CO2 is considered. Isotherms for CO2, CH4, and N2 were measured from 0 to 15 bar and at temperatures between 25 and 105 °C. The isotherms suggest a much higher working capacity (×4) for CO2 adsorption on Cu-BTC relative to the benchmark zeolite 13X over the same pressure range. Higher CO2/N2 and CO2/CH4 selectivities in the higher pressure range (1−15 bar) and with lower heats of adsorption were also demonstrated. Cu-BTC was observed to be stable in O2 at 25 °C, but its crystallinity was reduced in humid environments. The CO2 adsorption capacity was progressively reduced upon cyclic exposure to water vapor at low relative humidity (<30%), but leveled out at 75% of its original value after several water adsorption/desorption cycles.

Development of novel tertiary amine absorbents for CO2 capture

Abstract In the framework of the COCS (Cost Saving CO 2 Capture System) project we continue to develop high performance CO 2 absorbents to reduce the regeneration energy cost to almost half the current cost. To achieve this target we investigated twenty five tertiary amine based CO 2 absorbents with different chemical structures. Solvent selection procedures were carried out based on their CO 2 absorption rate, loading capacity and heat of reaction measurements. We found several high performance absorbents with advantages of high absorption rate and low heats of reaction compared to MDEA.

N-Paraffins adsorption with 5A zeolites: The effect of binder on adsorption equilibria

The effect of binder on adsorption equilibria of n-paraffins on 5A zeolite was investigated. Some differences in adsorption properties were found between binderless 5A zeolite and 5A zeolite with binder. Compared with binderless zeolite, 5A zeolite with binder showed lower adsorption capacities, larger coalescing factors of characteristic curves, and lower heats of adsorption for n-paraffins. The adsorption amounts comparison and scanning electron microscopes (SEM) analysis indicated that the influence of binder on adsorption capacities of n-paraffins on 5A zeolite adsorbent could be ascribed to the dilution and blocking effects which were brought to zeolite crystals by binder. The binder may change the polar interactions between adsorbates and adsorbent therefore affects the characteristic curves. A decrease in heats of adsorption is mainly due to the adsorption of some adsorbate molecules on the surface of the binder.

Radiation and postirradiation crosslinking and structure of two unsaturated polyester resins

AbstractRadiation and postirradiation crosslinking of two unsaturated polyester (UP) resins were monitored, and substantial differences in the reaction course and extents were observed. DSC thermograms of one of the resins showed double peaks and significantly lower residual reaction heats. Extraction revealed that gelation dose of the resin with double peak was twice the gelation dose of the other resin that had single peak in DSC thermograms. Although other components of the polyesters were identical, NMR spectra of the resin with a single peak revealed isophthalic units while in the polyester of the resin having double DSC peaks orthophthalic units were detected. Orthophthalate reduced the compatibility of polyester and styrene and caused the reaction‐induced phase separation, influencing gel structure that was visible in scanning electron microscope micrographs. Previously, the double peaks in crosslinking thermograms of UP resins were usually attributed to initiator effects, but here no initiator was used, and, in the literature, we found that the double peaks are almost exclusively present in the thermograms of UP resins containing orthophthalates, whereas in resins with isophthalates double peaks almost never appear. Crosslinking extents were significantly higher in the resin‐containing isophthalate and in both cases enhanced by postirradiation reaction that is often neglected. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers

Effects of intumescent formulation for acrylic-based coating on flame-retardancy of painted red lauan (Parashorea spp.) thin plywood

Acrylic emulsion based painted red lauan plywood (Parashorea spp.) is most commonly used for indoor furnishings. This study investigated the enhancement of the fire retardance of painted plywood by interaction among four major components of intumescent formulation: (1) acrylic emulsion resin as binder resin (BR), (2) pentaerythritol as carbonizing substance (CS), (3) melamine as foam producing substance (FPS) and (4) ammonium polyphosphate as dehydrating agent (DA). Effects of changing BR/CS ratios (designated as FRA series) and FPS/DA ratios (designated as FRM series) on flame-retardance of painted plywood were investigated using a cone calorimeter. The intumescent formulation significantly enhanced fire retardancy of painted plywood by exhibiting lower peak release rates and longer times to reach peak release rates, compared with uncoated plywood (UP) panel and plywood panel solely coated with acrylic emulsion resin. Lower BR content in the FRA series and lower FPS content in the FRM series were shown to enhance flame retardancy of painted plywood. The positive correlation between total heat release values under increasing combustion duration and incremental changes of BR and FPS contents in two series further verified the above findings. Consistent with the observed flame retardancy enhancement of painted plywood, lower heats of combustion and weight losses for paints in the FRM series were also identified by oxygen bomb calorimeter measurements and thermogravimetrical analysis. Infrared analysis of the chars indicated the formation of phosphate ester linkages with the lowest BR content in the FRA series and the lowest FPS content in the FRM series showing superior enhancements of flame retardancy for painted red lauan plywood.

Hyperpolarized 129Xe Nuclear Magnetic Resonance Studies of Isoreticular Metal-Organic Frameworks

The pore environments of a series of isoreticular metal-organic frameworks (IRMOF) have been studied using hyperpolarized (HP) 129Xe nuclear magnetic resonance (NMR) spectroscopy. Xenon gas behaved as an efficient probe molecule for interrogating the variability of adsorption sites in functionalized IRMOF materials through variations in the NMR chemical shift of the adsorbed xenon. The xenon adsorption enthalpies extracted from variable-temperature HP 129Xe NMR were found to be lower than published values for the physisorption of xenon. The low heats of adsorption were corroborated by xenon adsorption measurements that revealed two atoms per pore under one atmosphere of pressure at 19 °C. Average pore diameters estimated from the empirical chemical shift and pore size correlations based on a geometrical model were compared with X-ray crystallography data. The exchange processes of xenon in IRMOFs also were explored using 2D exchange spectroscopy (EXSY) 129Xe NMR. It was found the exchange of xenon from adsorption sites within the IRMOF to the free gas space is much slower than that between the adsorption sites within the lattice. Cross-polarization experiments showed that the preferred adsorption sites were spatially removed from the phenylene rings of the network. This agrees with previous spectroscopic, structural, and computational studies of gas adsorption (H2, N2, Ar) in IRMOFs that indicate the preferred binding sites reside near the carboxylate groups of the inorganic clusters.

THERMODYNAMICS OF MECHANICAL OSCILLATIONS IN CRYSTALLINE SUPERLATTICES

Phonon spectra as well as thermodynamic characteristic of superlattice are analyzed using the method of two-time dependent Green's functions. Free and internal energy of the system as well as specific heat and entropy are found. The temperature behavior of superlattice specific heat is compared with specific heats of bulk structures and thin films. It was shown that at extremely low temperature, superlattice and film specific heats are practically equal and considerably lower than specific heat of bulk sample. The consequences of this fact to the thermal, conducting and superconducting properties of materials was discussed.

Linear Adsorption of Nonionic Organic Compounds from Water onto Hydrophilic Minerals: Silica and Alumina

To characterize the linear adsorption phenomena in aqueous nonionic organic solute-mineral systems, the adsorption isotherms of some low-molecular-weight nonpolar nonionic solutes (1,2,3-trichlorobenzene, lindane, phenanthrene, and pyrene) and polar nonionic solutes (1,3-dinitrobenzene and 2,4-dinitrotoluene) from single- and binary-solute solutions on hydrophilic silica and alumina were established. Toward this objective, the influences of temperature, ionic strength, and pH on adsorption were also determined. It is found that linear adsorption exhibits low exothermic heats and practically no adsorptive competition. The solute-solid configuration and the adsorptive force consistent with these effects were hypothesized. For nonpolar solutes, the adsorption occurs presumably by London (dispersion) forces onto a water film above the mineral surface. For polar solutes, the adsorption is also assisted by polar-group interactions. The reduced adsorptive forces of solutes with hydrophilic minerals due to physical separation by the water film and the low fractions of the water-film surface covered by solutes offer a theoretical basis for linear solute adsorption, low exothermic heats, and no adsorptive competition. The postulated adsorptive forces are supported by observations that ionic strength or pH poses no effect on the adsorption of nonpolar solutes while it exhibits a significant effect on the uptake of polar solutes.

Physical properties of poly( n-alkyl acrylate) copolymers. Part 2. Crystalline/non-crystalline combinations

The physical properties of n-alkyl acrylate copolymers containing one crystallizeable monomer and one non-crystallizeable or slightly crystallizeable monomer, including thermal characteristics, structure as determined by small angle X-ray scattering, and gas permeability as a function of temperature, were examined in detail and compared to the corresponding homopolymers and copolymer systems containing two crystallizeable comonomers. The current copolymers exhibit melting point depression and, for a given average side-chain length, have lower heats of fusion than the corresponding homopolymers and crystalline/crystalline copolymers. Limited SAXS experiments revealed an increase in the d-spacings, above and below the melting point, with side-chain length consistent with expectations. The crystallites predominantly exhibit end-to-end packing similar to other poly( n-alkyl acrylates) previously studied. Poly( n-alkyl acrylates) exhibit a ‘jump’ in their gas permeability at the T m of the side-chain lengths that is mainly caused by a switch in the side-chain morphology from crystalline to amorphous upon melting. The reduced crystallinity of the crystalline/non-crystalline copolymers results in a smaller permeation jump, which in some cases were extremely broad. All jump breadths correlate with the melting endotherms for these copolymers as determined by DSC similar to that for homopolymers and crystalline/crystalline copolymers. The magnitude of the jump correlates with the heat of fusion, irrespective of the comonomer type, in all cases.

Interaction of Alcohols and Ethers with a-CFx Films

The surfaces of the magnetic data storage hard disks used in computers are coated with a thin film of amorphous carbon and a layer of perfluoropolyalkyl ether (PFPE) lubricant. Both protect the surface of the magnetic layer from contact with the read-write head flying over the disk surface. Although the most commonly used carbon films are amorphous hydrogenated carbon, a-CH(x), it has been suggested that the thermal properties of amorphous fluorinated carbon films, a-CF(x), might be superior. This work has probed the interaction of small fluorinated ethers and alcohols with the surfaces of a-CF(x) films to understand the effects of carbon film fluorination on the interaction of the lubricant with its surface. Temperature-programmed desorption was used to measure the desorption energies of small fluorocarbons from the a-CF(x) surface and to compare their desorption energies with those from the surfaces of a-CH(x) films. These measurements reveal that, similarly to a-CH(x) films, a-CF(x) films expose a heterogeneous surface on which fluorocarbons adsorb at sites with a range of binding energies. The fluorocarbon ethers all have lower heats of adsorption than their hydrocarbon counterparts, suggesting that the ethers adsorb by donation of electron density from the oxygen lone-pair electrons to sites on the surface. Fluorinated alcohols have roughly the same heats of adsorption as their hydrocarbon counterparts. There is little significant difference between the interactions of fluorinated ethers (or alcohols) with the surfaces of either a-CF(x) or a-CH(x) films.

Physical properties of poly( n-alkyl acrylate) copolymers. Part 1. Crystalline/crystalline combinations

The physical properties of n-alkyl acrylate copolymers containing two crystallizeable monomers, including thermal characteristics, structure as determined by small angle X-ray scattering, and gas permeability as a function of temperature, were examined in detail and compared to the corresponding homopolymers. The copolymers exhibit co-crystallization and, thus, for a given average side-chain length have comparable melting temperatures as the corresponding homopolymers. For a given side-chain length, the copolymers have somewhat lower heats of fusion than the corresponding homopolymers because of a reduction in crystallite size as revealed by SAXS. This depression in crystallinity is reflected in the permeability data for the copolymers. Poly( n-alkyl acrylates) exhibit a ‘jump’ in their gas permeability at the T m of the side-chain lengths that is mainly caused by a switch in the side-chain morphology from crystalline to amorphous upon melting. The depression in crystallinity for the copolymers results in a smaller permeation jump. The jump breadth correlates with the melting endotherms for these polymers as determined by DSC. Ultimately, the melting endotherms for these copolymer systems provide an excellent tool for predicting permeability changes across the melting region.

Effect of the Precise Branching of Polyethylene at Each 21st CH2 Group on Its Phase Transitions, Crystal Structure, and Morphology

Three linear polyethylenes with branches at every 21st backbone atom have been analyzed by differential scanning calorimetry (DSC) and quasi-isothermal, temperature-modulated DSC. The branches were methyl (PE1M), dimethyl (PE2M), and ethyl groups (PE1E). Linear polyethylene (HDPE) and atactic poly(octadecyl acrylate) (PODA) were also analyzed. All were compared to a random poly(ethylene-co-octene-1) of similar branch concentration (LLDPE) and poly(4,4'-phthaloimidobenzoyldoeicosyleneoxycarbonyl) (PEIM-22). The HDPE has the highest melting temperature and crystallinity with relatively large contributions of reversing melting when grown as folded-chain crystals. The precisely branched polyethylenes and copolymers have lower melting temperatures and heats of fusion. Of the branched samples, PE1M crystallizes more readily, followed by PE1E and PE2M, with PE2M showing cold crystallization. In contrast to paraffins of equal length which melt fully reversibly, the precisely designed, branched polymers melt largely irreversibly with small amounts of reversing melting, which is least for the best-grown crystals. The PE1M forms monoclinic, PE1E, pseudohexagonal, or triclinic crystals, and PE2M has a multitude of crystal structures.