Thermal Soak Research Articles

MoOx back contact for CdS/CdTe thin film solar cells: Preparation, device characteristics, and stability

A novel MoOx back contact buffer has been successfully developed for thin-film CdS/CdTe solar cells. To achieve a good ohmic contact, water rinse step is necessary in order to remove surface residues from the CdTe surface prior to MoOx deposition by thermal evaporation. With MoOx as the high work function buffer, various metals can be used as the electrode to realize an ohmic back contact to p–CdTe. Thermal and light soaking stability of CdTe cells is also improved with the MoOx buffer acting as a diffusion barrier to minimize the interaction between the metal electrode and CdTe. The best performance is obtained with MoOx/Ni as the back contact for the CdS/CdTe solar cell.

Simultaneous Transfer and Doping of CVD-Grown Graphene by Fluoropolymer for Transparent Conductive Films on Plastic

Chemical doping can decrease sheet resistance of graphene while maintaining its high transparency. We report a new method to simultaneously transfer and dope chemical vapor deposition grown graphene onto a target substrate using a fluoropolymer as both the supporting and doping layer. Solvent was used to remove a significant fraction of the supporting fluoropolymer, but residual polymer remained that doped the graphene significantly. This contrasts with a more widely used supporting layer, polymethylmethacrylate, which does not induce significant doping during transfer. The fluoropolymer doping mechanism can be explained by the rearrangement of fluorine atoms on the graphene basal plane caused by either thermal annealing or soaking in solvent, which induces ordered dipole moments near the graphene surface. This simultaneous transfer and doping of the graphene with a fluoropolymer increases the carrier density significantly, and the resulting monolayer graphene film exhibits a sheet resistance of ∼320 Ω/sq. Finally, the method presented here was used to fabricate flexible and a transparent graphene electrode on a plastic substrate.

Investigation of Properties of Thixoprocessed LM4

Thermomechanical treatment on cast Al-Si-Cu alloy (LM4) was carried out. The aim is to determine effects of degree of cold work, slurry thermal treatment and soaking time on the tensile strength, ductility, hardness and microstructure of the alloy. LM4 was prepared and cast in sand mould into rods of Φ50 mm and 200 mm in length. The prepared rods were then thermally treated at 635°C which fall within the slurry region of the alloy. The heated samples were soaked at this temperature for various times of: 5, 10, 15 and 20 min. A none-treated sample was also kept as control specimen. The thermally treated rods were subjected to various degrees of cold work. 10 %, 15%, 20%, 30% degrees of cold work were used. Hardness, tensile strength and elongation of the samples were determined. The metallographic examination of the samples was also carried out. Results of mechanical property test show sensitivity to degree of cold work, thermal treatment soaking time. The microstructures of the alloy also show significant modifications as a result of thermal treatment.

Fan air flow analysis and heat transfer enhancement of vehicle underhood cooling system – Towards a new control approach for fuel consumption reduction

We report here experimental results focused on the optimization of vehicle underhood cooling module. These results constitute the basis for a new approach of controlling the cooling module positioning according to the engine energy requirements. Measurements are carried out on a simplified vehicle body designed based on the real vehicle front block. We report here velocity and temperature measurements by Particle Image Velocimetry (PIV), by Laser Doppler Velocimetry (LDV) and by thermocouples. The underhood of the simplified body is instrumented by 59 surface and fluid thermocouples. Measurements are carried out for conditions simulating both the slowdown and the thermal soak phases with the fan in operation. Different fan rotational speeds, radiator water flow and underhood geometries have been experimented. The ultimate aim is to apply the new control approach to a real vehicle so as to reduce the energy delivered to the pump and compressor and therefore to reduce the vehicle fuel consumption.

Durability test of PVP‐capped Pt nanoclusters counter electrode for highly efficiency dye‐sensitized solar cell

ABSTRACTIn this paper, the durability of PVP‐capped Pt nanoclusters counter electrode (PVP‐Pt CE) for dye‐sensitized solar cell (DSSC) has been extensively evaluated including electrochemical reaction durability, thermal stress durability and light soaking durability. It is revealed that PVP‐Pt CE exhibits both electrochemical and thermal durability by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) test. Moreover, DSSC containing PVP‐Pt CE shows over 9.37% conversion efficiency in highly volatile electrolyte system. As to device thermal durability, both low‐volatile and non‐volatile electrolyte systems were tested and the results show relative efficiency can maintain more than 85% after accelerated thermal test at 85°C for 1000 h, and 110% after 60°C for 1000 h. Finally, after continuous light soaking test under 60°C for 1000 h, the relative efficiency can still maintain at 94%. Copyright © 2011 John Wiley & Sons, Ltd.

Thermal and Stability Analysis of Formed Platelet in Platelet Transpiration Cooling

Platelet transpiration cooling is one of the key techniques for the developing reusable launch vehicle. The thinner of the platelet, the better effect plate transpiration will gain. Meanwhile, the platelet unit may get wrinkled if thermal stress is large enough. One-dimensional non-equilibrium model is adopted to analyze the heat transfer characteristics of platelet transpiration cooling. The assumption of rectangular plate with three-side simple supported and one-side free is taken and Galerkin method is applied for analyzing the stability of platelet unit. Analysis indicates that as the heat transfer intensity increases, the thermal soaking depth decreases, while more coolant is needed to keep the heated wall at design temperature. Also, the critical thickness for the platelet not to get wrinkled gets larger with the increasing thermal soaking depth.

Effects of solar spectrum and module temperature on outdoor performance of photovoltaic modules in round‐robin measurements in Japan

AbstractThe performance of six photovoltaic (PV) modules composed of polycrystalline silicon (pc‐Si), amorphous silicon (a‐Si), and hydrogenated amorphous silicon/crystalline silicon (a‐Si:H/c‐Si) modules was investigated at eight locations in Japan from August 2007 to December 2008. In addition, solar irradiance, solar spectrum, and module temperature were simultaneously measured in these round‐robin measurements. In this study, we evaluate quantitatively the effects of module temperature and solar spectrum on the performance of the PV modules as thermal factor (TF) and spectral factor (SF), respectively. Furthermore, we investigate the variation in module performance, which is converted into module performance under standard test conditions (STC) using the TF and SF. In the case of the pc‐Si modules, the variations in performance ratio under STC (PRSTC) for these modules range from 0.056 to 0.074 through the round‐robin measurements. The TF indicates that the contribution of module temperature to the variation in performance is large, between about 15 and 20%. However, the SF suggests that the contribution of solar spectrum is quite small, less than 3%. In the case of the a‐Si modules, the contribution of module temperature is about 8%. The performance is largely influenced by solar spectrum, more than 12% at its maximum. Consequently, the variations in the corrected PRSTC of the a‐Si modules are between 0.117 and 0.141. These large variations may result from the effects of thermal annealing and light soaking. The variation in PRSTC of the a‐Si:H/c‐Si module is similar to that of the pc‐Si modules. Copyright © 2010 John Wiley & Sons, Ltd.

Surface passivation of crystalline silicon by plasma-enhanced chemical vapor deposition double layers of silicon-rich silicon oxynitride and silicon nitride

Excellent surface passivation of crystalline silicon (c-Si) is desired for a number of c-Si based applications ranging from microelectronics to photovoltaics. A plasma-enhanced chemical vapor deposition double layer of amorphous silicon-rich oxynitride and amorphous silicon nitride (SiNx) can provide a nearly perfect passivation after subsequent rapid thermal process (RTP) and light soaking. The resulting effective minority carriers’ lifetime (τeff) is close to the modeled maximum on p-type as well as on n-type c-Si. Restrictions on the RTP of passivated surfaces, typical of other common passivation schemes (e.g., amorphous Si), are relieved by this double layer. Harsher thermal treatments can be adopted while still obtaining salient passivation. Furthermore, characterization of the same, such as, surface photovoltage, capacitance voltage, and electron paramagnetic resonance, enables the reproducibility and the understanding of the passivation scheme under test. It is shown that the strong quality of surface passivation is ensured by a mechanism that emits electrons from shallow donor states in the passivation layer system and therefore creates a positive field effect.

Electron-rich heteroaromatic conjugated polypyridine ruthenium sensitizers for dye-sensitized solar cells

Ru(II) heteroleptic complexes as photosensitizers for dye-sensitized solar cells (DSCs) are presented. The article outlines design strategies, synthetic routes, optical and photovoltaic properties of ruthenium dyes based on polypyridines as ancillary ligands containing π-conjugated electron-rich heteroaromatic groups. The integration of donor heteroaromatic substituents, typically thiophene-based moieties, strongly improves the optical properties of the sensitizers in terms of bathochromic and hyperchromic shift compared to prototypical dyes N3 and N719. These favorable properties in turn yield DSCs with superior light harvesting abilities, higher external quantum efficiencies, improved device photocurrents, and top-ranked power conversion efficiencies. In combination with excellent stabilities under thermal stress and light soaking, this class of DSC photosensitizer has great potential for practical applications.

Generalization of effect of oxygen exposure on formation and performance of carbon molecular sieve membranes

Previously, a pyrolysis method was developed to control separation performance of carbon molecular sieve (CMS) membranes by doping with trace oxygen. This method involved oxygen exposure during pyrolysis to tune the selective pore windows. During the development, it was observed that oxygen concentration in inert gas, rather than the total amount of oxygen exposed controls performance. In this study, we hypothesized that mass transfer of oxygen in CMS membranes during pyrolysis is governed by chemical reaction at critical pore opening. Effect of thermal soak time, inert flow rate, and precursor thickness were conducted to test this hypothesis with 6FDA/BPDA-DAM and Matrimid® polymer precursors. Results of oxygen consumption from pyrolysis process and CO 2/CH 4 separation performance showed that the process is likely governed by reaction kinetics. This observation implies simplicity and easy scale-up for the oxygen “doping” method on CMS formation by tuning the oxygen concentration in the pyrolysis atmosphere.

Vaseline-loaded expanded graphite as a new adsorbent for toluene

A new kind of vaseline-loaded expanded graphite (VEG) complex material was prepared by a thermal soaking method and characterized by Brunauer–Emmett–Teller surface area, contact angle meter, scanning electron microscopy and mercury porosimeter. The use of VEG as an adsorbent for the control of toluene vapors in air was investigated at different humidities and compared with commercial granular activated carbon (GAC). The results showed that the VEG was much more hydrophobic than the original expanded graphite and could adsorb more toluene. The amounts of loaded vaseline on the expanded graphite surface were optimized and the VEG-2 was the best one. For example, the sorption capacity of 1 g of the VEG-2 was 213 mg of toluene, more than 4.5 times that of the expanded graphite. The high adsorption capacity of the VEG could be attributed to the partitioning action of vaseline for toluene. It was also found that the VEG for toluene was independent on the humidity of the streams, while the sorption capacity of the GAC was strongly dependant on humidity. Therefore, it was expected to have a longer durability than GAC in wet weather.

Estimation of the maximum power temperature coefficients of PV modules at different time scales

The maximum power temperature coefficients of a-Si modules are negative in indoor measurements, whereas the performance increases with module temperature in outdoor measurements. In order to resolve the discrepancy, we investigate the temperature coefficients of a-Si modules by two different methods corresponding to two different time scales. The results are that the temperature coefficient for a shorter time scale of several hours is negative, although the temperature coefficient for a longer time scale of seasons is positive. These results suggest that the discrepancy in temperature coefficient is partially caused by the effects of thermal annealing and light soaking. Therefore, the history of module temperature and received solar irradiance would be required for the correction of the performance of a-Si modules. In addition, we reveal that the effect of solar spectrum is also a contributory factor in the discrepancy.

Evaluation of equivalent (by residual life) periods of thermal soaking during complex fatigue tests of samples of structural components

For the purpose of the substantiation of the parameters of forced fatigue tests, equivalent (by residual life) periods of thermal soaking during complex fatigue tests of smooth samples were determined by the calculation method. The variation of the lifetimes of samples with a hole and samples of riveting joints of D16AT and AK4-1AT1 aluminum alloys was experimentally investigated as a function of the previous stress history. The effect of aging processes on the fatigue strength of the samples that occur during soakings with different durations under natural conditions during the action of heightened temperature was evaluated. During the selection of the parameters of forced fatigue tests, it was shown to be necessary to take into account the different nature of the variation of the mechanical characteristics of structural components, which depended on the type of the structural component, the alloy, the soaking temperature, and modes of the fatigue tests. The results can be used during the design of experiments for the evaluation of effects of various factors on the fatigue life of structural components and during the forecasting of their operational life.

Stress analysis at the interface between Ni–P coating and SiCp/Al substrate of space mirror

This paper investigates the mechanical properties at the interface of the coating–substrate system, which comprises the electroless nickel–phosphorus (Ni–P) coating and the aluminum matrix composite substrate reinforced by the silicon carbide particles (SiCp/Al), and is used for the space mirror. To estimate the adhesion of Ni–P coating on SiCp/Al substrate, the scratch adhesion testing has been performed by drawing a spherically tipped diamond indenter with a radius of 200μm over the coated surface. The influence of the coating thickness on the interfacial stress induced by the inertial accelerations, temperature gradients and thermal soaks has been evaluated by simulation analysis based on the finite element method. The results of the scratch testing indicate that the adhesion strength of Ni–P coating to SiCp/Al composite is more than 3.0GPa. Compared the maximum value of the interfacial stress obtained by simulation analysis with results of the scratch testing, it is can be seen that the mirror has enough safety margin. Furthermore, the most significant conclusion that can be drawn from this work is that the coating thickness should not exceed 45μm in order to ensure the performance and reliability of Ni–P coating and SiCp/Al substrate system for space applications.

Nanoscale Alumina-Reinforced Aluminum Matrix Composites: Microstructure and Mechanical Properties

Studies were carried out on microstructure evolution and mechanical behavior of an Al matrix–nanoscale Al2O3 particulate-reinforced composite. The thermal stability of the composite, evaluated by heat treating specimens at temperatures from 300 to 600 °C for times varying from 1 to 100 hours, revealed that the nano-sized (30-100 nm) Al2O3 particles present in the as-received/ascast material coalesced into larger particles, but with sizes still in the 100 to 500 nm range. Despite the coarsening of the particles, high hardness was retained. The tensile properties of both the as-cast DSC material and those thermally soaked for 500 hours at a number of temperatures were evaluated. The results showed that the yield strength was quite high (283 MPa) at room temperature and decreased nearly linearly with temperature, though values as high as 110 MPa were obtained at 400oC. Thermal soaking did not have a detrimental effect on strength. Although the macroscopic ductility of both unsoaked and soaked materials remained quite low over the entire temperature range, SEM observations of the fracture surfaces provided substantial evidence for high localized plasticity as manifested by stretching, tearing and void formation in the Al matrix around the oxide particles. Possible strengthening mechanisms, including grain size reduction, Orowan bypass and forest hardening, were considered and modeled. Good agreement between the calculated and experimental strengths was obtained, and majority of the strengthening at room temperature was found to come from forest hardening (i.e, increase in dislocation density caused by the thermal expansion mismatch between Al and Al2O3), with secondary contributions from the Orowan mechanism. TEM observations provided confirmatory evidence for these mechanisms. The decrease in strength at higher temperatures was attributed to a diminishing contribution from forest hardening due to recovery processes.

High-performance dye-sensitized solar cells based on solvent-free electrolytes produced from eutectic melts

Low-cost excitonic solar cells based on organic optoelectronic materials are receiving an ever-increasing amount of attention as potential alternatives to traditional inorganic photovoltaic devices. In this rapidly developing field, the dye-sensitized solar cell (DSC) has achieved so far the highest validated efficiency of 11.1% (ref. 2) and remarkable stability. However, the cells with the best performance use volatile solvents in their electrolytes, which may be prohibitive for outdoor solar panels in view of the need for robust encapsulation. Solvent-free room-temperature ionic liquids have been pursued as an attractive solution to this dilemma, and device efficiencies of over 7% were achieved by using some low-viscosity formulations containing 1-ethyl-3-methylimidazolium thiocyanate, selenocyanate, tricyanomethide or tetracyanoborate. Unfortunately, apart from tetracyanoborate, all of these low-viscosity melts proved to be unstable under prolonged thermal stress and light soaking. Here, we introduce the concept of using eutectic melts to produce solvent-free liquid redox electrolytes. Using a ternary melt in conjunction with a nanocrystalline titania film and the amphiphilic heteroleptic ruthenium complex Z907Na (ref. 10) as a sensitizer, we reach excellent stability and an unprecedented efficiency of 8.2% under air-mass 1.5 global illumination. Our results are of importance to realize large-scale outdoor applications of mesoscopic DSCs.

Experimental and Numerical Investigations of Thermal Soak

Experimental and Numerical Investigations of Thermal Soak

Pseudo-transient computational fluid dynamics analysis of an underbonnet compartment during thermal soak

Underbonnet simulations are proving to be crucially important within a vehicle development programme, reducing test work and time-to-market. While computational fluid dynamics (CFD) simulations of steady forced flows have been demonstrated to be reliable, studies of transient convective flows in engine compartments are not yet carried out owing to high computing demands and lack of validated work. The present work assesses the practical feasibility of applying the CFD tool at the initial stage of a vehicle development programme for investigating the thermally driven flow in an engine bay under thermal soak. A computation procedure that enables pseudo time-marching CFD simulations to be performed with significantly reduced central processing unit (CPU) time usage is proposed. The methodology was initially tested on simple geometries and then implemented for investigating a simplified half-scale underbonnet compartment. The numerical results are compared with experimental data taken with thermocouples and with particle image velocimetry (PIV). The novel computation methodology is successful in efficiently providing detailed and time-accurate time-dependent thermal and flow predictions. Its application will extend the use of the CFD tool for transient investigations, enabling improvements to the component packaging of engine bays and the refinement of thermal management strategies with reduced need for in-territory testing.

Ionically Conducting Composite Membranes from the Li2O–Al2O3–TiO2–P2O5 Glass–Ceramic

This paper reports processing of lithium ion‐conducting, composite membranes comprised of 14Li2O·9Al2O3·38 TiO2·39P2O5 glass–ceramic and polyethylene. The processing involved tape casting of 14Li2O·9Al2O3·38TiO2·39P2O5 glass powder with organic additives into tapes, subjecting the green tape to binder burnout and thermal soaking in the temperature range of 950°–1100°C, and finally infiltrating the porous tape with polyethylene solution. The ionic conductivity and microstructure of 150–350 μm thick membranes were characterized and are discussed in this paper. The crystallites of the glass–ceramic show liquid‐like conductivity at ambient temperature, whereas the grain boundary conductivity is lower by a factor of five. The lower grain boundary conductivity is explained on the basis of crystallographic mismatch and the existence of AlPO4 at the grain boundary. The polyethylene infiltration in the porous membrane improved mechanical resilience with a minor adverse effect on conductivity.

Influence of carbonisation parameters on the transport properties of carbon membranes by statistical analysis

A statistical two-level eight-run factorial design of experiment was used to evaluate the influence of various carbonisation parameters on the transport properties of the carbon membranes prepared from Kapton ® 100HN polyimide. The effects of the carbonisation atmosphere, the final temperature, the heating rate and the thermal soak time at the final temperature on the permeation rates of He, CO 2, O 2 and N 2 were examined. Statistically significant carbonisation parameters, including the main factors and the two-factor interactions of the selected factors, were studied by analysing the probability values. The results indicate that the carbonisation temperature is the controlling parameter that will significantly affect the transport properties of the carbon membranes. The two-factor interactions also have some effects on the transport properties.