Short-term Thermal Stability Research Articles

Improving the thermal properties of NaNO3-KNO3 for concentrating solar power by adding additives

Concentrating Solar Power (CSP) system with storage can provide consistent power and thus are attractive relative to intermittent power sources, e.g., solar photovoltaics and wind. Mixed molten salt is a promising medium for both heat transfer and energy storage in solar thermal power because of its many advantages such as large heat capacity, low vapor pressure, low cost, wide range of temperature in application, etc. However, there are the disadvantages of higher melting point and poor thermal stability with the commonly mixed molten salts, such as Solar salt (60wt%NaNO3+40wt%KNO3, melting point @220°C, limit use of temperature @565°C). In order to obtain molten salt with lower melting point and better short-term thermal stability, 14 kinds of mixed molten salts are obtained by adjusting the component ratio and adding carbonates or nitrates based on Solar salt. Through the Differential Scanning Calorimetry (DSC) and Thermogravimetric (TG) methods, their melting point, crystallization point, thermal decomposition temperature and short-term thermal stability are measured. 4 kinds of mixed molten salts are selected preliminarily at last. Their melting points and decomposition temperatures are about 100°C and 600°C respectively. They show good short-term thermal stability in the 4-days repeated heating-cooling experiments. The specific heat and sensible thermal storage cost of preferred molten salts were also obtained.

Detection of volatile impurities in turbine oils by the heat-pulse testing method

The research is aimed at development and implementation of methods and devices to control critical sections of the oil system of the power equipment that operates in the real time mode. The task was to develop a method for rapid detection of volatile impurities in turbine oils. The approach to the study is based on quantitative assessment of the short-term thermal stability of the substance that is formally associated with the content of the volatile impurity. The approach was selected on the basis of the results of search experiments taking into consideration the formulation of requirements for the method and the device, viz., (1) the method should reliably determine the moisture content in the range of 10–150 g of the impurity per ton of oil and (2) the device is to be applicable “in situ.” For this purpose, a variant of the method of the controlled pulse heating of a wire probe, a resistance thermometer, has been developed. The advantages of the method are its speed, sensitivity to small contents of volatile impurities regardless of the nature of the impurity, and smallness of methodologically contributed perturbation. The heating conditions of the probe most sensitive to the appearance of moisture— including its trace amounts—in the system, has been defined. The duration of the measurement is on the order of milliseconds; the heat flux density through the surface of the probe reaches 1 MW/m2. The essence of the method consists in measuring, in the characteristic time interval, the temperature of the thermal instability onset associated with the content of the volatile impurity. The approach proposed by the authors is aimed at increasing the lifetime of the oil and preventing unpredictable failures of the operating equipment.

Effect of dual functional ionic liquids on the thermal degradation of poly(vinyl chloride)

The short-term thermal stability of PVC on addition of functional ionic liquids (ILs) based on phosphonium and pyridinium cations, and a docusate anion, was studied using FTIR, TGA-MS and TGA-FTIR. The thermal stability of PVC plasticized with these ILs is reduced and the activation energy for thermal degradation for the first step of PVC degradation lowered, relative to neat PVC. This feature is not dependent on the thermal stability of the IL itself, as their addition to neat PVC did not result in increased thermal stability of the resin. Analysis of the gases evolved during thermal degradation showed that there is no variation in the mechanism of PVC degradation in the presence of ILs, the only difference observed is that HCl is evolved at lower temperatures for PVC-ILs samples. The ILs had no effect on the second step of the thermal degradation of PVC. The decrease in the thermal stability of PVC-ILs is associated with two different effects: firstly, the anion weakens the CH bond while the cation forms complexes and weakens the CCl bond; second, the IL interaction with the polymer chains partly substitutes the interaction between the polymer chain and heat stabilizers (e.g. stearates) added to the PVC compound. In this way, the latter tends to be expelled from the bulk polymer, thus not being available for PVC stabilization at high temperature. Heat stabilizer migration is related to the solvating power of the ILs: the higher the solvating power, the higher the migration of heat stabilizer, the lower the thermal stability of the polymer.

Organic Based Heat Stabilizers for PVC: A Safer and more Environmentally Friendly Alternatives

Organic based stabilizers have been considered as a new technology providing environmentally friendly heat stabilizer for PVC pipe production to substitute conventional lead stabilizer as well as calcium zinc stabilizer. In this research, PVC samples stabilized with 5 types of heat stabilizers i.e. 1) commercial lead stabilizer, 2) commercial calcium zinc stabilizer, 3) commercial organic based stabilizer (OBS), 4) 1,3-dimetyl-6-aminouracil (DAU) and 5) eugenol, were investigated. From dynamic mechanical analysis, storage modulus at room temperature of PVC stabilized with DAU was found to provide the highest value among those stabilizers. Glass transition temperature of the PVC stabilized with all types of heat stabilizers was determined to be approximately 99°C except the value of about 89°C in eugenol stabilized PVC. Furthermore, PVC stabilized with commercial lead, calcium zinc stabilizer and commercial OBS could be reprocessed up to at least 5 cycles. Whereas, PVC stabilized with DAU was found to be able to withstand the processing cycle up to 4 cycles. Additionally, PVC stabilized with DAU showed the most outstanding short term thermal stability and can maintain its original color for at least up to 4 processing cycles. Finally, repeated processing of PVC stabilized with each type of heat stabilizers showed negligible effect on mechanical properties for at least up to 3 processing cycles. From the above results, it is evident that DAU showed high potential use as a safe and effective organic based heat stabilizer for PVC to substitute traditional lead or calcium zinc compounds.

Investigation of not fully stable fluids by the method of controlled pulse heating. 2. Short-term thermal stability of polymethyl metacrylate

The short-time thermal stability of glassy PMMA (polymethyl metacrylate) under conditions of high-power local heating has been studied experimentally. For this purpose, a peculiar procedure in the framework of the method of controlled pulse heating of implantable probe has been developed. The procedure is based on combining the thermal shock and monitoring heating functions with characteristic pulse lengths of the order of 1μs and 1ms, respectively. The value of the characteristic probe temperature T*pk was determined, a reproduction of which in a series of measurements under identical parameters of heating function resulted in irreversible changes in the thermal resistance of a polymeric sample. The depth of penetration into the region of thermal instability of polymer (T*pk−Td), where Td is the onset temperature of thermal decomposition in quasi-static process, attains a few hundreds of degrees.

Preparation of Magnesium-zinc Alloy by Mechanical Alloying

Mechanical alloying is one of the ways how to prepare nanostructured and amorphous metallic materials. In this paper we used this method to prepare Mg-Zn alloy containing 50 wt.% of zinc. Powders produced by milling in a planetary ball mill were consequently compacted by the SPS method, a very fast method which prevents grain coarsening. The prepared samples were subjected to a closer examination - microstructure, phase composition, hardness and short-term thermal stability were studied. We found out that the prepared powder consisted of Mg and Mg7Zn3 phases, which were very fine and homogeneously distributed. After the SPS compacting, the metastable Mg7Zn3 phase decomposed and new phases (Mg, MgZn, MgZn2, Mg2Zn11) formed. The compacted sample possessed relatively inhomogeneous microstructure and hardness about 260 HV5. Investigation of the short-term thermal stability was accomplished by annealing at temperatures up to 300 °C. We observed changes in hardness - it regularly decreased up to 200 °C and then it slightly increased.

Preparation of cation-exchange hybrid membranes with multi-functional groups and their performance on alkali recovery

Cation-exchange hybrid membranes have been prepared via the blending of sulfonated poly(2,6-dimethyl-1,4-phenylene oxide) (SPPO) with poly(vinyl alcohol) (PVA), which is cross-linked with multisilicon copolymer of poly(acrylic acid-co-γ-methacryloxypropyl trimethoxy silane) (poly(AA-co-γ-MPS)). The SPPO phase, PVA phase, and multisilicon copolymer can provide the membranes with functional groups of –SO3Na, –OH and –COOH, respectively. Hence, multi-functional groups are formed within the hybrid membranes. The hybrid membranes have a short-term thermal stability of 234–260°C, tensile strength (TS) of 12–13 MPa, and elongation at break of 27–49%. Membrane stability in alkaline solution is much affected by the content of SPPO. The membranes are potentially applied in diffusion dialysis (DD) for alkali recovery (NaOH/Na2WO4). The dialysis coefficients of NaOH (UOH) are in the range of 0.0075–0.032 m/h, which are higher than those of commercial membranes and reported membranes. The separation factor can reach up to 21.2 at 25°C for the membrane containing 10 wt% SPPO. Meanwhile, the membrane shows relative stability during long-term DD process. Hence, the multi-functional groups are effective for the diffusion dialysis process.

Novel low melting point quaternary eutectic system for solar thermal energy storage

A new low melting point quaternary eutectic mixture consisting of alkali nitrates and sodium nitrite was modeled by using thermodynamic principles. The eutectic temperature of the quaternary salt mixture was predicted to be 371.75K. The quaternary eutectic mixture was synthesized based on the predicted eutectic composition and the eutectic temperature was determined using the Differential Scanning Calorimeter (DSC). The experimental melting point was 372.17K which was in excellent agreement with that of the predicted value. The heat capacity of the eutectic mixture was determined by the DSC as function of temperature in the liquid state and heat capacity increased with increase in temperature. Density of the molten quaternary eutectic mixture decreases linearly with increase in temperature. Thermal stability of the eutectic mixture was determined by both short-term and long-term thermogravimetric analyses. Short-term thermal stability test showed negligible weight loss. However, 7.4wt.% weight loss was observed in the long–term thermal stability test at 773.15K which corresponds to thermal decomposition of the salt mixture. Upper temperature limit for long-term thermal stability of the eutectic mixture was determined to be 703.15K. Thermal energy storage capacity of the novel eutectic mixture was found to be larger than that of conventional solar salt and HITEC® salt.

High temperature phase stability and chemical analysis of the highly doped yttria stabilized zirconia with alumina

Short and long term thermal stability of YSZ was studied considering the equilibrium and non-equilibrium phase transformation of YSZ. To improve the long term thermal stability of YSZ, electrical treatment was introduced. Electrical treatment consists of applying electric field into YSZ at high temperatures under a reducing atmosphere. By optimizing the conditions of treatment, electrical resistivity reduced around 40% and thermal stability improved dramatically. These improvements are due to the generation of tetragonal metastable phases. Arguments are put forth to show that electrical treatment induces a relaxation in the lattice. This relaxation is associated with a reduction in lattice tetragonality and replaces the tetragonal grains of YSZ with domain of metastable phases. High resistance of metastable phase to creep deformation and lower electrical resistivity of domains of metastable phases in comparison with tetragonal grains brings about these improvements.

Thermal stability of the eutectic composition in LiNO3–NaNO3–KNO3 ternary system used for thermal energy storage

The new eutectic composition in the LiNO3–NaNO3–KNO3 ternary salt system has a very low melting point (118°C) and is a potential candidate for use in parabolic trough solar power generation. The short and long-term thermal stabilities and reliability of the eutectic composition in this ternary system were determined using the Thermogravimetric Analyzer (TGA) and Differential Scanning Calorimetry (DSC), respectively. The system demonstrates excellent short-term thermal stability and reliability during thermal cycling. However, the system showed 8% weight loss during long-term thermal stability test. Long-term isothermal stepwise study reveals that below 435°C the weight change is minimal and this temperature is taken as the upper limit of thermal stability of the system. The eutectic composition in the ternary system was characterized by XRD and SEM techniques before and after the thermal stability experiments to identify the morphology and compositions of the phases. From the present study it can be concluded that the major contributor to the weight loss is the dissociation of lithium nitrate to lithium oxides.

PVA/SiO2 anion exchange hybrid membranes from multisilicon copolymers with two types of molecular weights

Multisilicon copolymer has a long main chain and many branched chains, with plenty of ion-exchange (N+(CH3)3Cl−) and Si(OCH3)3 groups. Multisilicon copolymer with the low molecular weight (Mr of 795) is difficult to gel, while that with the high molecular weight (Mr of 4152) has two gelation times. One is the “pseudo gelation” within 0.03–2.5 days, and the other is within 14–48 days. The two types of multisilicon copolymers are crosslinked with polyvinyl alcohol (PVA) to prepare anion exchange hybrid membranes.Membranes from the low molecular weight copolymer have lower water uptakes (WR, 62–87%), ion exchange capacities (IECs, 0.6–0.9mmol/g), thermal stability (223–239°C) and mechanical strength (7–12MPa, 42–97%). Membranes from the high molecular weight copolymer are homogeneous, transparent and colourless. They have higher WR of 195–464%, IECs of 1.0–1.5mmol/g, short-term thermal stability of 244–247°C, mechanical strength (15–23MPa, 122–208%) and organic solvent resistance. All the membranes can be potentially applied in diffusion dialysis (DD) process for recovery of CH3COOH and HCl. The dialysis coefficients of CH3COOH (UCH3COOH) can reach up to 0.009m/h, and the dialysis coefficients of HCl (UH) are in the range of 0.01–0.043m/h with the separation factors of 22–39 (0.01–0.029m/h and 28–39 for the low molecular weight, and 0.024–0.043m/h and 22–26 for the high molecular weight), superior to those of commercial DF-120 membrane (0.009m/h, 18.5).

Physicochemical properties determined by ΔpKa for protic ionic liquids based on an organic super-strong base with various Brønsted acids

Neutralization of an organic super-strong base, 1,8-diazabicyclo-[5,4,0]-undec-7-ene (DBU), with different Brønsted acids affords a novel series of protic ionic liquids (PILs) with wide variations in the ΔpK(a) of the constituent amine and acids. The physicochemical properties of these PILs, such as thermal properties, density, conductivity, viscosity, self-diffusion coefficient, vibrational stretching frequency, and (1)H-chemical shifts of the N-H bond, have been studied in detail. The generated PILs have melting temperatures below 100 °C, and six are liquids at ambient temperatures. Thermogravimetric analyses (TGA) conducted under isothermal and programmed heating conditions have shown that PILs with ΔpK(a)≥ 15 exhibit good thermal stability similar to aprotic ionic liquids. For instance, PILs with ΔpK(a) > 20 show remarkably high short-term thermal stability up to ca. 450 °C under a nitrogen atmosphere. The viscosity, ionic conductivity, and molar conductivity of the PILs fit well with the Vogel-Fulcher-Tamman equation for their dependencies on temperature. The relative cationic and anionic self-diffusion coefficients of the PILs estimated by the pulsed-field gradient spin-echo (PGSE) NMR method appear to be dependent on the structure and strength of the Brønsted acids. Evaluation of the ionicity based on both the Walden plot and PGSE-NMR revealed that it increases until ΔpK(a) becomes 15 for the PILs.

The Influence of Various Mucoadhesive Polymers on In Vitro Performance of the Resulting Artificial Saliva Pump Spray Formulations

The aim of this study was to investigate the rheological behavior and mucoadhesive nature of saliva substitutes, by incorporating various mucoadhesive polymers into an artificial saliva pump spray formulation. For this purpose various mucoadhesive polymers including cellulosic polymers in the range of 0.1-1.0% and Carbomers such as C974p, C971, C934p and C971 in the range of 0.01-0.1% were added to a formulated aqueous-based artificial saliva pump spray formulation, containing fixed amounts of some essential electrolytes. The pH of the formulations was between 6.3-7.4. The formulations were examined in terms of appearance, taste, odor, spray-ability, short-term thermal and mechanical stability, pH, viscosity and rheological behavior, particle size distribution, as well as invitro mucoadhesive strength (MS). The mucoadhesivity ratio (MR) was also calculated as follows: MR=MS test /MS control , using natural saliva as the control. Natural saliva showed a pseudoplastic rheological behavior, with a viscosity in the range of 12.85-28.15cP. Hence, artificial saliva samples having viscosities within this range were selected. The rheological behavior and viscosity of the test samples as well as the natural saliva were subsequently determined. Similar to that found for the natural saliva, all the prepared formulations showed a pseudoplastic rheological behavior. Among the polymers, C974p had the highest viscosity (25.97±0.11 cp) and mucoadhesive strength (34.84±0.21 mN/cm 2 ) followed by hydroxypropylmethylcellulose which had a viscosity of 25.48±0.11 cp and a mucoadhesive strength of 34.03±0.24 mN/cm 2 . Furthermore, the mucoadhesivity of C974p containing artificial saliva was 1.186 times greater than natural saliva and 1.387 times more than water. In conclusion, it seems that the presence of mucoadhesive polymers within the artificial saliva pump spray formulations could help to improve the adhesive nature of the formulation to mucosal surfaces, making it even more effective than the natural saliva.

M Stars as Targets for Terrestrial Exoplanet Searches And Biosignature Detection

The changing view of planets orbiting low mass stars, M stars, as potentially hospitable worlds for life and its remote detection was motivated by several factors, including the demonstration of viable atmospheres and oceans on tidally locked planets, normal incidence of dust disks, including debris disks, detection of planets with masses in the 5-20 M() range, and predictions of unusually strong spectral biosignatures. We present a critical discussion of M star properties that are relevant for the long- and short-term thermal, dynamical, geological, and environmental stability of conventional liquid water habitable zone (HZ) M star planets, and the advantages and disadvantages of M stars as targets in searches for terrestrial HZ planets using various detection techniques. Biological viability seems supported by unmatched very long-term stability conferred by tidal locking, small HZ size, an apparent short-fall of gas giant planet perturbers, immunity to large astrosphere compressions, and several other factors, assuming incidence and evolutionary rate of life benefit from lack of variability. Tectonic regulation of climate and dynamo generation of a protective magnetic field, especially for a planet in synchronous rotation, are important unresolved questions that must await improved geodynamic models, though they both probably impose constraints on the planet mass. M star HZ terrestrial planets must survive a number of early trials in order to enjoy their many Gyr of stability. Their formation may be jeopardized by an insufficient initial disk supply of solids, resulting in the formation of objects too small and/or dry for habitability. The small empirical gas giant fraction for M stars reduces the risk of formation suppression or orbit disruption from either migrating or nonmigrating giant planets, but effects of perturbations from lower mass planets in these systems are uncertain. During the first approximately 1 Gyr, atmospheric retention is at peril because of intense and frequent stellar flares and sporadic energetic particle events, and impact erosion, both enhanced, the former dramatically, for M star HZ semimajor axes. Loss of atmosphere by interactions with energetic particles is likely unless the planetary magnetic moment is sufficiently large. For the smallest stellar masses a period of high planetary surface temperature, while the parent star approaches the main sequence, must be endured. The formation and retention of a thick atmosphere and a strong magnetic field as buffers for a sufficiently massive planet emerge as prerequisites for an M star planet to enter a long period of stability with its habitability intact. However, the star will then be subjected to short-term fluctuations with consequences including frequent unpredictable variation in atmospheric chemistry and surficial radiation field. After a review of evidence concerning disks and planets associated with M stars, we evaluate M stars as targets for future HZ planet search programs. Strong advantages of M stars for most approaches to HZ detection are offset by their faintness, leading to severe constraints due to accessible sample size, stellar crowding (transits), or angular size of the HZ (direct imaging). Gravitational lensing is unlikely to detect HZ M star planets because the HZ size decreases with mass faster than the Einstein ring size to which the method is sensitive. M star Earth-twin planets are predicted to exhibit surprisingly strong bands of nitrous oxide, methyl chloride, and methane, and work on signatures for other climate categories is summarized. The rest of the paper is devoted to an examination of evidence and implications of the unusual radiation and particle environments for atmospheric chemistry and surface radiation doses, and is summarized in the Synopsis. We conclude that attempts at remote sensing of biosignatures and nonbiological markers from M star planets are important, not as tests of any quantitative theories or rational arguments, but instead because they offer an inspection of the residues from a Gyr-long biochemistry experiment in the presence of extreme environmental fluctuations. A detection or repeated nondetections could provide a unique opportunity to partially answer a fundamental and recurrent question about the relation between stability and complexity, one that is not addressed by remote detection from a planet orbiting a solar-like star, and can only be studied on Earth using restricted microbial systems in serial evolution experiments or in artificial life simulations. This proposal requires a planet that has retained its atmosphere and a water supply. The discussion given here suggests that observations of M star exoplanets can decide this latter question with only slight modifications to plans already in place for direct imaging terrestrial exoplanet missions.

M Stars as Targets for Terrestrial Exoplanet Searches And Biosignature Detection

The changing view of planets orbiting low mass stars, M stars, as potentially hospitable worlds for life and its remote detection was motivated by several factors, including the demonstration of viable atmospheres and oceans on tidally locked planets, normal incidence of dust disks, including debris disks, detection of planets with masses in the 5–20 M⊕ range, and predictions of unusually strong spectral biosignatures. We present a critical discussion of M star properties that are relevant for the longand short-term thermal, dynamical, geological, and environmental stability of conventional liquid water habitable zone (HZ) M star planets, and the advantages and disadvantages of M stars as targets in searches for terrestrial HZ planets using various detection techniques. Biological viability seems supported by unmatched very long-term stability conferred by tidal locking, small HZ size, an apparent short-fall of gas giant planet perturbers, immunity to large astrosphere compressions, and several other factors, assuming incidence and evolutionary rate of life benefit from lack of variability. Tectonic regulation of climate and dynamo generation of a protective magnetic field, especially for a planet in synchronous rotation, are important unresolved questions that must await improved geodynamic models, though they both probably impose constraints on the planet mass. M star HZ terrestrial planets must survive a number of early trials in order to enjoy their many Gyr of stability. Their formation may be jeopardized by an insufficient initial disk supply of solids, resulting in the formation of objects too small and/or dry for habitability. The small empirical gas giant fraction for M stars reduces the risk of formation suppression or orbit disruption from either migrating or nonmigrating giant planets, but effects of perturbations from lower mass planets in these systems are uncertain. During the first ∼1 Gyr, atmospheric retention is at peril because of intense and frequent stellar flares and sporadic energetic particle events, and impact erosion, both enhanced, the former dramatically, for M star HZ semimajor axes. Loss of atmosphere by interactions with energetic particles is likely unless the planetary magnetic moment is sufficiently large. For the smallest stellar masses a period of high planetary surface temperature, while the parent star approaches the main sequence, must be endured. The formation and retention of a thick atmosphere and a strong magnetic field as buffers for a sufficiently massive planet emerge as prerequisites for an M star planet to enter a long period of stability with its habitability intact. However, the star will then be subjected to short-term fluctuations with consequences including frequent unpredictable variation in atmospheric chemistry and surficial radiation field. After a review of evidence concerning disks and planets associated with M stars, we evaluate M stars as targets for future HZ planet search programs. Strong advantages of M stars for most approaches to HZ detection are offset by their faintness, leading to severe constraints due to accessible sample size, stellar crowding (transits), or angular size of the HZ (direct imaging). Gravitational lensing is unlikely to detect HZ M star planets because the HZ size decreases with mass faster than the Einstein ring size to which the method is sensitive. M star Earth-twin planets are predicted to exhibit surprisingly strong bands of nitrous oxide, methyl chloride, and methane, and work on signatures for other climate categories is summarized. The rest of the paper is devoted to an examination of evidence and implications of the unusual radiation and particle environments for atmospheric chemistry and surface radiation doses, and is summarized in the Synopsis. We conclude that attempts at remote sensing of biosignatures and nonbiological markers from M star planets are important, not as tests of any quantitative theories or rational arguments, but instead because they offer an inspection of the residues from a Gyr-long biochemistry experiment in the presence of extreme environmental fluctuations. A detection or repeated nondetections could provide a unique opportunity to partially answer a fundamental and recurrent question about the relation between stability and complexity, one that is not addressed by remote detection from a planet orbiting a solar-like star, and can only be studied on Earth using restricted microbial systems in serial evolution experiments or in artificial life simulations. This proposal requires a planet that has retained its atmosphere and a water supply. The discussion given here suggests that observations of M star exoplanets can decide this latter question with only slight modifications to plans already in place for direct imaging terrestrial exoplanet missions. Key Words: M star planets—Habitable planets — Life and stellar activity — Spectral biosignatures — Terrestrial planet formation — Exoplanet properties. Astrobiology 7(1), 85 – 166.

Effect of oxygen impurity on long-term thermal stability of Zr-based metallic glasses below glass transition temperature

Long-term thermal stability of a series of Zr-based metallic glasses with different oxygen contents below their glass transition temperatures was compared based on their deductive continuous-heating-transformation diagrams created by using the corollary of Kissinger analysis method. It is found that the influence of oxygen on the long-term thermal stability of Zr-based metallic glasses exhibits at lower temperature is different from that on their short-term thermal stability presented at higher temperature. For each kind of the Zr-based metallic glasses, there is a critical heating rate, ø c , which corresponds to a critical temperature, T c . As heating rate is smaller than ø c , and onset devitrification temperature is below T c , the glass with higher oxygen content will have longer incubation period for onset devitrification. The values of ø c and T c are related with the glasses' reduced glass transition temperature T rg. The improving effect of oxygen impurity on the long-term thermal stability of Zr-based metallic glasses was discovered.

An experimental investigation of heat transfer in thermally unstable polymer systems

An experimental study is made into the process of heat transfer from a pulse-superheated probe to a solidified polymer system at probe temperatures above the temperature Td∞ of the beginning of thermal destruction of matter in a quasi-static process. Use is made of the procedures of thermal stabilization of the pulse-superheated probe (with the characteristic time of constancy of superheated probe temperature of 1 ms) and of shock heating (with the characteristic time of increasing the probe temperature of 1 µs and that of monitoring its cooling-down of 1 ms). The effect of short-term thermal stability of polymers in the region of T > Td∞ is revealed. A procedure is developed for identifying the signs of thermal destruction of polymers in the pulsed process. The maximal values are estimated of the density of heat flux through samples without their thermal destruction.

Thermal cycle stability of a novel glass–mica composite seal for solid oxide fuel cells: Effect of glass volume fraction and stresses

A novel glass–mica composite seal was developed based on a previously reported concept of “infiltrated” mica seals for solid oxide fuel cells. Ba–Al–Ca silicate sealing glass powder and Phlogopite mica flakes were mixed at glass volume fractions of 10–50 vol.% to make the glass–mica composite seals. The seals were leak tested for short-term thermal cycle stability as a function of glass volume fraction. Composite seals with 10 and 20 vol.% glass were also leak tested under compressive stresses from 3 to 100 psi and helium pressures of 0.2 or 2 psi. Post-mortem microstructure analyses were used to characterize the fracture (leak path) of the glass–mica composite seals and were related to the high temperature leakages. Open circuit voltage tests on dense 8YSZ electrolyte with the glass–mica composite seal showed very good thermal cycle stability over 250 cycles with minute (<1%) voltage drop.

Effects of ambient-temperature curing agents on the thermal stability of poly(methylphenylsiloxane)

Two kinds of curing agents, triethanolamine and polysilazane (KH-CL), were used to make polymethylphenylsiloxne (PMPS) cured at room temperature. Their effects on the short-term and long-term thermal stability of PMPS are reported and discussed. The influences of the curing agents on the thermal degradation mechanisms of PMPS were studied by thermogravimetric analysis (TG), differential thermogravimetry (DTG) and Fourier-transform infrared spectroscopy (FTIR). The results indicate that both kinds of PMPS had higher short-term thermal stability than traditional heat-cured PMPS. For long-term thermal stability, the PMPS cured with KH-CL was much higher than the one cured with triethanolamine. The “unzip degradation” of PMPS was eliminated through the reaction between KH-CL and silanol groups, and then the thermal stability of PMPS was improved. However, it was initiated at a low temperature (about 300 °C) with catalysis by triethanolamine, and the thermal stability was decreased.

Semi-Aromatic Polyimides with Low Dielectric Constant and Low CTE

Semi-aromatic polyimides (PIs) possessing simultaneously a low dielectric constant ( K) and low linear coefficient of thermal expansion ( CTE) were molecularly-designed. Two PI systems were focused on, i.e. the s-BPDA/CHDA polyimide derived from 3,3′,4,4′-biphenyltetracarboxylic dianhydride with trans-1,4-cyclohexanediamine and CBDA/TFMB derived from 1,2,3,4-cyclobutanetetracarboxylic dianhydride and bis(2,2′-trifluoromethyl)benzidine. In the s-BPDA/CHDA system, poly(amic) acid (PAA) polymerization using the common procedure was prevented by strong salt formation at the initial stage. However, heating the salt-forming reaction mixture at a selected temperature for a short period allowed the system to polymerize and resulted in a high-molecular-weight PAA (reduced viscosity ηred &gt; 2 dl g−1). On the other hand, in the CBDA/TFMB system, a high-molecular-weight PAA ( ηred &gt; 3 dl g−1) was readily obtained by choosing the solid content without any salt formation. The chosen PIs almost achieved the current goal; CTE &lt; 20 ppm K−1, K &lt; 3.0, and Tg &gt; 300°C concurrently with adequate film flexibility. In particular, CBDA/TFMB accomplished a very low K (∼2.6). Although these PIs have a much lower thermo-oxidative stability than wholly aromatic PIs, the results suggested that there is no serious problem for the short-term thermal stability required for soldering processes.