Long-term Thermal Stability Research Articles

Long-term impact of temperature on the hydraulic permeability of bentonite

Abstract In a Swedish repository for the disposal of heat-emitting waste, the long-term thermal stability of the bentonite engineered barrier forms a key component of the safety case. Central to such consideration is the evolution of hydraulic permeability and a potential degradation of hydraulic properties, in response to prolonged thermal exposure of the clay. To address this issue, a detailed programme of laboratory-based experiments has been undertaken at both the British Geological Survey and Studiecentrum voor Kernenergie/Centre d'Etude de L'Energie Nucleaire, in order to examine the hydraulic behaviour of bentonite that had previously been exposed to elevated temperatures. Hydraulic properties were calculated from both steady-state pressure gradients and from analysis of the pressure transients. Inspection of the data found no significant difference in hydraulic behaviour between the virgin material and clay samples taken from the Canister Retrieval Test. Based on these observations, the authors find no evidence for an adverse increase in hydraulic conductivity of bentonite as a result of prolonged thermal exposure to temperatures of 80 °C.

ON THE THERMAL STABILITY OF RADIATION-DOMINATED ACCRETION DISKS

We study the long-term thermal stability of radiation dominated disks in which the vertical structure is determined self-consistently by the balance of heating due to dissipation of MHD turbulence driven by the magneto-rotational instability (MRI), and cooling due to radiation emitted at the photosphere. The calculations adopt the local shearing box approximation, and utilize the recently developed radiation transfer module in the Athena MHD code based on a variable Eddington tensor rather than an assumed local closure. After saturation of the MRI, in many cases the disk maintains a steady vertical structure for many thermal times. However, in every case in which the box size in the horizontal directions is at least one pressure scale height, fluctuations associated with MRI turbulence and dynamo action in the disk eventually trigger a thermal runaway which causes the disk to either expand or contract until the calculation must be terminated. During runaway, the dependence of the heating and cooling rates on total pressure satisfy the simplest criterion for classical thermal instability. We identify several physical reasons why the thermal runaway observed in our simulations differ from the standard alpha disk model, for example the advection of radiation contributes a non-negligible fraction to the vertical energy flux at the largest radiation pressure, most of the dissipation does not happen in the disk mid-plane, and the change of dissipation scale height with mid-plane pressure is slower than the change of density scale height. We discuss how and why our results differ from those published previously. Such thermal runaway behavior might have important implications for interpreting temporal variability in observed systems, but fully global simulations are required to study the saturated state before detailed predictions can be made.

Investigation of the Stability of the Ruthenium-Based Dye (N719) Utilizing the Polarization Properties of Dispersive Raman Modes and/or of the Fluorescent Emission

Dye-sensitized solar cells (DSCs) offer intriguing new possibilities with the integration of these into, e.g., power generating windows or facade applications. For the DSCs to constitute a viable investment, the thermal appliance with respect to the working conditions typically encountered must be considered. One of the aspects governing the long-term thermal stability of the DSC is the dye stability and hence whether or not the dye is degraded upon heating or illumination. This emphasizes the need for a sensitive and nondestructive measuring technique with which it is possible to distinguish between a dye and the degradation products possibly formed in a DSC under working conditions. We have carried out the preliminary steps in this direction by showing that a distinction between the commonly used dye N719 and the main degradation product [Ru(LH)2(NCS)(4-tert-butylpyridine)][N(Bu)4] (N719-TBP for short) is possible by exploiting a combination of the polarization properties of the dispersive Raman modes with the small spectral change in the visible absorption spectrum and/or the difference in the polarization of the fluorescence related to the difference in molecular configuration. By measuring the polarized resonance Raman spectra in the region 650–1900 cm–1 it is demonstrated that the polarization dispersion is sufficient for four (1021, 1060, 1313, and 1606 cm–1) out of ten Raman modes to discriminate between N719 and N719-TBP. It is also demonstrated that the difference in molecular configuration between N719 and N719-TBP gives rise to an even larger change in the polarization of the fluorescence in the same spectral region. It therefore follows that a discrimination between the dye and degradation product can be based on the changes observed in the polarization properties of both the Raman as well as the fluorescence spectra.

The Study of an Inorganic Gel for Profile Modification in High-Temperature and Low-Permeability Sandstone Reservoirs

Chemical water control technique has been successfully used to improve oil recovery of mature waterflooding reservoirs. However, for high-temperature and low-permeability sandstone reservoirs, not many options are available. An inorganic gel composed of aluminum chloride hexahydrate as gelling agent and urea as an activator was developed. Based on the proper gelation time and degree of syneresis, the optimum mass ratio of aluminum chloride and urea was 1:0.625. According to the gel strength and economical cost, the reasonable concentration of aluminum chloride should range from 3.0% to 6.0%. Gel compatibility with various formation brines showed that the gel could tolerate sodium chloride concentrations up to 80 g·L −1 and calcium chloride concentrations up to 40 g·L −1. Long-term thermal stability and sandpack flow results showed that after aging for 60 days at 110°C, the gel showed approximately 82% permeability reduction when about 16% syneresis was observed in bulk samples.

The effect of permafrost changes on embankment stability along the Qinghai–Xizang Railway

After construction of the Qinghai–Xizang Railway, the resultant heat exchange between soil and atmosphere caused changes in the thermal and mechanical stability of permafrost beneath the railway embankment. Monitoring from 2005 to 2010 indicated 12 sections of embankment that experienced more than 5 cm of settlement, with three showing deformations of more than 10 cm and signs of continuing settlement. Embankment stability is closely related to permafrost changes beneath the embankment. Large-scale deformations have contributed to permafrost thaw and artificial permafrost table deepening, and this deformation has not stabilized over the short term. In contrast, small-scale deformations have contributed to a warming of the permafrost that has gradually stabilized as soil temperature decreases. Only three sections of the Qinghai–Xizang Railway have exhibited settlement deformation that exceeds 10 cm, through a deepening of the artificial permafrost table and a gradual increase in permafrost thawing result in embankment settlement deformation. However, with climate warming trends and the long-term operation of the railway, the long-term thermal and mechanical stability of the embankment needs to be carefully monitored to ensure the safe operation of the Qinghai–Xizang Railway.

System-level simulation of a solar power tower plant with thermocline thermal energy storage

A thermocline tank is a low-cost thermal energy storage subsystem for concentrating solar power plants that typically utilizes molten salt and quartzite rock as storage media. Long-term thermal stability of the storage concept remains a design concern. A new model is developed to provide comprehensive simulation of thermocline tank operation at low computational cost, addressing deficiencies with previous models in the literature. The proposed model is then incorporated into a system-level model of a 100MWe power tower plant to investigate storage performance during long-term operation. Solar irradiance data, taken from measurements for the year 1977 near Barstow, CA, are used as inputs to the simulation. The heliostat field and solar receiver are designed with DELSOL, while the transient receiver performance is simulated with SOLERGY. A meteorological year of plant simulation with a 6-h capacity for the thermocline tank storage yields an annual plant capacity factor of 0.531. The effectiveness of the thermocline tank at storing and delivering heat is sustained above 99% throughout the year, indicating that thermal stratification inside the tank is successfully maintained under realistic operating conditions. Despite its good thermal performance, structural stability of the thermocline tank remains a concern due to the large thermal expansion of the internal quartzite rock at elevated molten-salt temperatures, and requires further investigation.

기능성 엘라스토머를 이용한 선형 폴리페닐렌 설파이드의 개질 및 그 특성

장기내열성과 파단신율이 우수한 블렌드 수지를 개발하고자 polyphenylene sulfide(PPS)에 7종류의 elastomer를 용융혼합하여 그 가능성을 조사해 보았다. PPS/elastomer(90/10, 80/20, 70/30) 블렌드를 제조한 후 시험편을 압축성형 또는 sheet를 제조하여 capillary rheometer, UTM, impact tester, SEM 등으로 유변학적 거동, 기계적 특성 및 모폴로지를 측정하였고 오븐에서 일주일간 열을 가한 후 다시 기계적 특성을 측정하였다. 열을 가하기 전의 인장강도는 elastomer 종류에 관계없이 비슷하였고 파단신율은 m-EVA의 경우가 가장 큰 값을 보였다. Elastomer 첨가량을 증가시킴에 따라 신율이 증가하였는데 30 wt%정도 첨가 시 박리가 발생하였다. 오븐테스트 이후에는 인장 강도는 더 증가한 반면에 신율은 급격히 감소하였다. 상기의 elastomer들이 PPS와 부분적인 상용성이 있는 것으로 보이는 바 소량의 elastomer 첨가 만으로도 PPS 가공과 관련된 여러 문제 등을 해결할 수 있어 다양한 그레이드의 개발에 적극 활용될 수 있을 것으로 기대된다. In order to develop the blends with good long-term thermal stability and tensile elongation, the blends of polyphenylene sulfide (PPS) and 7 kinds of elastomer were tested. PPS/elastomer (90/10, 80/20, 70/30) blend samples were prepared by compression molding after twin screw extrusion or punching after sheet extrusion. Rheological, mechanical property and morphology of the blends were analyzed by capillary rheometer, UTM, impact tester, and SEM. For long-term thermal stability tests, the mechanical properties were measured again after the samples were stored in a convection oven for a week. The tensile strengths were almost same regardless of kinds of elastomer and the tensile elongation was the maximum for the PPS/m-EVA blend. As the content of elastomer increased, the elongation increased but delamination occurred at 30 wt% of elastomer content. The tensile strength increased but the elongation decreased seriously after thermal aging. Many problems related with PPS processing could be solved by adding a small amount of the elastomers partially compatibile with PPS and it would be applicable to develop various PPS grades.

Long-Term Thermal Stability of Liquid Dye Solar Cells

Laboratory-size dye solar cells (DSCs), based on industrially feasible materials and processes employing liquid electrolytes, have been developed. Cells based on two electrolyte solvents with different physical properties were subjected to thermal stress test at 80 °C for 2000 h in the dark to monitor their long-term thermal stability. The DSCs incorporating a methoxypropionitrile (MPN)-based electrolyte presented a severe efficiency loss at 1 sun AM 1.5G of more than 70% upon thermal aging, while the solar cells using tetraglyme (TG) as a high boiling point solvent attained a promising stability with only 20% loss of performance. To better understand the above behavior, systematic experiments, including optical microscopy, linear sweep voltammetry, UV–vis absorption, electrochemical impedance, and Raman spectroscopies were conducted. Virtually no dye degradation/desorption, electrolyte decomposition, semiconductor passivation, or loss of cathode activity could be identified. For the MPN-based cells, a sh...

Evaluation of the Gel Profile Control Agent Polyacrylamide in Enhanced Oil Recovery

Based on the system of anionic delayed cross-link polymer gel type deep profile control agent, experiments are conducted to evaluate the performance of long-term thermal stability, shearing resistance, and compatibility with natural debris of profile control agent and compatibility with ASP compound system. Besides, experiments are completed to simulate core displacement profile controlling and evaluate effect of oil displacement. Formula of anionic delayed cross-link polymer gel profile control agent with ideal long-term stability is confirmed. Profile control effect referred to core with diverse permeability and influence law of different injection time and injection rate on effect of oil displacement. Following conclusions are made according to the research. Long-term thermal stability, shearing resistance, and compatibility with natural debris of profile control agent and compatibility of ASP compound system of anionic delayed cross-link polymer gel profile control agent all have good performance, as well as core plugging effect and resistance to erosion. Displacing effect varies with the profile control agent injecting time, which shows, injection before polymer displacing proves the best, injection during polymer displacing the second, and injection after polymer displacing the worst. At the same time, the value of enhanced oil recovery increases with the injection rate of profile control agent increasing, however, when the rate comes to the point of 0.1 pv, the increasing extent begins to slow down. Key Words : Profile control agent; Anionic; Delayed cross-link

Inter-crosslinking through both donor and acceptor with unsaturated bonds for highly efficient and stable organic solar cells

A novel approach for thermally stable bulk heterojunction (BHJ) solar cells based on the inter-crosslinking of a vinyl-functionalized low band-gap copolymer donor poly{4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b′]dithiophene-alt-4,6-thieno[3,4-b]thiophen-2-yl-2-ethylhexan-1-one-alt-4,8-bis(oct-7-en-1-yloxy)benzo[1,2-b:4,5-b′]dithiophene} (PBDTTT-V), and vinyl-functionalized fullerene derivative acceptor [6,6]-phenyl C61 butyric acid dec-9-en-1-yl ester (PCBD), is developed. Compared with non-crosslinked corresponding heterojunctions and mono-crosslinked corresponding heterojunctions, inter-crosslinked heterojunctions with and without the addition of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) showed well-maintained and stable morphology after thermal annealing at 150 °C for 8 hours, in which the large-scale phase separation caused by the aggregation of fullerenes was dramatically suppressed. Although the mono-crosslinked PBDTTT-V:PCBM blend efficiently improved the stability of device with respect to a non-crosslinked corresponding heterojunction device, the power conversion efficiency (PCE) still decreased to an half of its initial efficiency value after annealing at 150 °C for 8 h. On the contrary, a remarkably enhanced stability was obtained in inter-crosslinked devices with performance maintained at 60% for PBDTTT-V:PCBM:PCBD and at 79% for PBDTTT-V:PCBD. It is worthy to note that the device based on PBDTTT-V:PCBM:PCBD showed comparable performance to the non-crosslinked one, indicating that the inter-crosslinking of both donor and acceptor with unsaturated bonds is a promising approach to ensure a device with long-term thermal stability, and maintain the good performance at the same time.

Ionic liquids of superior thermal stability

In a head-to-head study, ILs based upon tetraarylphosphonium cations exhibit superior long-term thermal stability compared to familiar salts based upon imidazolium, quaternary ammonium, or tetraalkylphosphonium cations.

Melilite glass–ceramic sealants for solid oxide fuel cells: effects of ZrO2 additions assessed by microscopy, diffraction and solid-state NMR

The influence of adding 0–5 mol% zirconia (ZrO2) to a series of melt-quenched alkaline-earth aluminosilicate glasses designed in the gehlenite (Ca2Al2SiO7)–akermanite (Ca2MgSi2O7) system has been investigated for their potential application as sealants for solid oxide fuel cells (SOFCs). The work was implemented with a dual aim of improving the sintering ability of the glass system under consideration and gaining insight into the structural changes induced by ZrO2 additions in the glasses consequentially leading to their enhanced long-term thermal stability. That the degree of condensation of SiO4 tetrahedra increased with increasing amounts of zirconia was confirmed by 29Si magic-angle (MAS) NMR. 1D 27Al, 11B MAS as well as two-dimensional (2D) 11B MQMAS/STMAS NMR experiments gave structural insight into the number and nature of aluminum and boron sites found in the glass and glass–ceramic (GC) samples. Irrespective of the heat treatment time, increasing the zirconia content in glasses suppressed their tendency towards devitrification, while the glasses exhibited good sintering behavior resulting in mechanically strong GCs with higher amounts of residual glassy phase making them suitable for self-healing during SOFC operation. All the GCs exhibited low total electrical conductivity; appropriate coefficients of thermal expansion (CTE), good joining and minimal reactivity with SOFC metallic components at the fuel cell operating temperature, thus, qualifying them for further appraisal in SOFC stacks.

Synergism of pentaerythritol-zinc with β-diketone and calcium stearate in poly(vinyl chloride) thermal stability

Pentaerythritol-zinc (called Penzinc), which is a novel thermal stabilizer for PVC, was synthesized, and its performance was characterized during the thermal processing of PVC. The stabilization effects of this new stabilizer combined with calcium stearate (CaSt2) and stearoyl benzoyl methane (β-diketone) were investigated using conductivity tests, thermal aging tests, and torque rheometry tests. The results revealed that the addition of Penzinc can improve the color and thermal stability of PVC in both static and dynamic tests. There are no obvious synergistic effects between Penzinc and CaSt2. However, the synergistic action between Penzinc and β-diketone is excellent. The thermal stability mechanisms of Penzinc and β-diketone are also discussed with the help of quantum chemical calculations. Pentaerythritol-zinc (Penzinc) has been synthesized and its thermal stability, especially its synergism with β-diketone and calcium stearate, was characterized. Experimental results show that PVC stabilized with complex of Penzinc and β-diketone has an excellent initial color and long-term thermal stability. Moreover, the thermal stability mechanism of Penzinc and β-diketone are also discussed with the help of quantum chemical calculations. Results indicate that both α-H atom of β-diketone and zinc atom of Penzinc, having low electron density, can easily react with labile chloride atom of PVC, which has high electron density.

Areal density limitation in bit-patterned, heat-assisted magnetic recording using FePtX media

The future evolution of magnetic recording data storage toward its ultimate limit is expected to involve a combination of energy-assisted recording on bit-patterned media, according to recent publications. In this work, we assess the effectiveness of single magnetic grain reversal under heat-assisted recording conditions by analyzing macrospin magnetization dynamics with the Landau-Lifshitz-Bloch equation. The simulations reported pertain to FePtX recording media and recording system parameters constrained by expected practical limitations. The approach adopted is assessment of the patterned media writing error rate as a function of applied bias field and areal density (AD), taking account of the relevant physics of the heat-assisted recording process. Additionally, we require that long-term thermal stability of recorded information be maintained, and that sufficient thermal and effective writing field gradients to support AD targets are available. For the long-time analysis, an Arrhenius-Nèel model of single grain switching probability is helpful. In this context, an investigation of achievable areal density with respect to tradeoffs in writing error rate at practical applied fields and thermal conditions is provided.

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.

Cobalt-free composite cathode for SOFCs: Brownmillerite-type calcium ferrite and gadolinium-doped ceria

Abstract A cobalt-free composite Ca2Fe2O5 (CFO) – Ce0.9Gd0.1O1.95 (GDC) is investigated as a new cathode material for intermediate-temperature solid oxide fuel cells (IT-SOFCs) based on a Gd0.1Ce0.9O1.95 (GDC) electrolyte. The cathodes had brownmillerite structure with x wt.% Gd0.1Ce0.9O1.95 (GDC) – (100−x) wt.% Ca2Fe2O5 (CFO), where x = 0, 10, 20, 30 and 40. The effect of GDC incorporation on the thermal expansion coefficient (TEC), electrochemical properties and thermal stability of the CFO–GDC composites is investigated. The composite cathode of 30 wt.% GDC – 70 wt.% CFO (CG30) coated on Gd0.1Ce0.9O1.95 electrolyte showed the lowest area specific resistance (ASR), 0.294 Ω cm2 at 700 °C and 0.122 Ω cm2 at 750 °C. The TEC of the CG30 cathode was 13.1 × 10−6 °C−1 up to 900 °C, which is a lower value than for CFO alone (13.8 × 10−6 °C−1). Long-term thermal stability and thermal cycle testing of CG30 cathodes were performed. Stable ARS values were observed during both tests without delamination at the cathode–electrolyte interface. An electrolyte-supported single cell with a 300-μm-thick GDC electrolyte and an anode-supported single cell with ∼10-μm-thick yttria-stabilized zirconia (YSZ) with a GDC buffer layer attained maximum power densities of 395 mW cm−2 at 750 °C and 842 mW cm−2 at 800 °C, respectively. The unique composite composition of CG30 demonstrates enhanced electrochemical performance and good thermal stability for IT-SOFCs.

High temperature structure stability of GH4169 superalloy

The long-term thermal stability of GH4169 superalloy has been studied as a function of heat treatment and thermal exposure. The GH4169 superalloy was subjected to long-term unstressed heat treatment at 650°C, from 200 to 1000h. The microstructure, tension and stress rupture properties of the samples before and after long-term thermal exposure were characterized. The coarsening of strengthing phase γ′ and γ″ was observed after exposing at 650°C. Coarsening of two different precipitates proceeds by Oswald ripening controlled by volume diffusion of solute atoms in the alloy. The tension strength at room-temperature and 650°C decreases slowly and the stress rapture life decreases remarkably with the increase of thermal exposure time. The decreasing of volume fraction and increasing of mean distance for the precipitates appears to be the main factor in the degradation of the mechanical properties.

Zn-Al-La hydrotalcite-like compounds as heating stabilizer in PVC resin

ZnAlLa-CO3 layered double hydroxides (LDHs) with different Zn/Al/La molar ratios were prepared by the constant pH coprecipitation method. The synthetic materials were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), Brunauer-Emmett-Teller method (BET) and particle analysis. The results showed that the hydrotalcite-like materials had a layered structure. The ZnAlLa-CO3-LDHs as stabilizers were mixed with PVC resin. The tests of thermal aging, mass loss test and Congo red for the PVC composites were carried out at 180±1 °C, respectively. The results showed that ZnAlLa-CO3- LDHs used as single thermal stabilizers could enhance both the long-term thermal stability and initial thermal stability of PVC, and 2.4 phr (parts per hundred PVC resin) ZnAlLa-CO3-LDHs with Zn/Al/La (molar ratio)=20/8/2 had a better stabilizing effect on PVC than others. Compared with single thermal stabilizers, the composite thermal stabilizers including 0.3 phr calcium stearate (CaSt2), 0.4 phr β-diketone and 2.4 phr ZnAlLa-CO3-LDHs significantly enhanced the thermal stability of PVC sample. And rare earth ion could optimize the conformation of PVC and refrain the initiation of the thermal stability and the thermal stable time was over 220 min. The main reason could be concluded to the special structure of rare earth ion, which had longer atom radius and more coordination numbers. So the rare earth ion (REδ+) could react with the labile chlorine atoms (Clr−) to form a stable coordinate complex.

A New Class of Ionic Liquids: Anion Amphiprotic Ionic Liquids

We here present a new class of protic ionic liquids, anion amphiprotic ionic liquids (AAILs). These materials are protonation equilibrium free protic ionic liquids and interesting in their own right by not following the classical Brønsted acid-base neutralization concept. Due to the very simple synthesis route applied and their stable basic chemistry, we believe in a potential use for manifold applications. This is supported by the combination of practical material properties, foremost, a general intrinsic stability versus reversal of the formation reaction toward neutral species, broad liquidus ranges, long-term thermal stabilities, high conductivities, protic characteristics, and a general stability versus water.

Microstructure and properties of extruded rapidly solidified AlCr4.7Fe1.1Si0.3 (at.%) alloys

The rapidly solidified AlCr4.7Fe1.1Si0.3 (at.%) alloy produced at large scale by direct extrusion of gas atomised powders was investigated. Hot-extrusion yielded sound compacts and did not create any major structural changes compared to the as-atomised state. Microstructural characterisation of the as-extruded material revealed submicron scale α-Al grains with an embedded Al7Cr crystalline phase and traces of a nano-scale quasicrystalline Al84.6Cr15.4 phase. The long-term thermal stability up to 300 °C of compacts was proven. The thermal stability of compacts was accompanied by high mechanical properties, especially high strength and Young's modulus, and creep performance up to 300 °C. The same material was prepared by laboratory scale extrusion of cryo-chopped melt-spun ribbons as a reference material. No major differences in the structure or properties of the industrially prepared material were observed compared to the reference material.