Poor Thermal Properties Research Articles

Forced convective heat transfer characteristics of solar salt-based SiO2 nanofluids in solar energy applications

Nowadays, molten salts are widely used in concentrated solar plants as working media to transfer and store solar energy. However, the relatively poor thermal properties of molten salts have restricted their further application. Nevertheless, the addition of nanoparticles to molten salts excellently improves these thermal properties. In present work, solar salt-based nanofluids were prepared using a lyophilizer. The specific heat of solar salt-based SiO2 nanofluids was experimentally measured based on the sapphire method, and its heat transfer performance was numerically investigated using the lattice Boltzmann method. Results indicated that an optimum nanoparticle mass fraction that could enhance heat transfer performance existed. This optimum nanoparticle mass fraction, which was found to be 1.0 wt%, improved the heat transfer coefficient and Nu number by 8.58 and 7.29%, respectively. The parameter analysis indicated that the considerably large change in the specific heat dominated the change in heat transfer performance. Meanwhile, the simulation results of the average Nu numbers exhibited good agreement with the Shah-London correlation prediction to within ±3.0%, indicating the feasibility of using this correlation in designing heat exchangers that utilize solar salt-based SiO2 nanofluids.

A comparison of non-isocyanate and HDI-based poly(ether urethane): Structure and properties

DMC-derived poly (ether urethanes) were prepared via isocyanate-free route. The transurethanization reaction of the non-isocyanate route needs higher temperature as compared with the traditional route. There may be some unknown side reactions under high temperature which lead to changes in chemical structure and subsequent thermal and mechanical properties of the novel DMC-derived non-isocyanate polyurethanes (NIPUs). Two 1,6-hexamethylene diisocyanate based polyurethanes (HDIPUs) were prepared as a control. HDIPUs possessed a higher melting point and better mechanical properties than NIPUs. In order to find out the causes of the difference in their thermal and mechanical properties, the structure, crystallization behavior, morphology and microphase separation behavior of NIPUs and HDIPUs were systematically studied and compared in this work. It was found that the structural distinction between NIPUs and HDIPUs was originated from the hard segments. The urea content in the hard segments of NIPUs was much higher than that of HDIPUs. The formation of urea groups was explained by the side reactions during the transurethanization process of NIPUs. Due to the random insertion of urea groups on the hard segments of NIPU, the strength of hydrogen bonding between hard segments and the crystallinity of the hard phase were lower than those of HDIPU. Therefore, the compatibility between the hard phase and the soft phase of NIPU was increased, resulting in poor thermal and mechanical properties.

Evaluation of energy renovation strategies for 12 historic building types using LCC optimization

The life cycle cost (LCC) optimization is a vital method when performing building energy renovation. The present paper provides an evaluation of cost-optimal energy renovation strategies for historic buildings using LCC optimization software OPERA-MILP. The evaluation is performed based on preset targets depending on LCC (LCC optimum) and energy use (decrease by 50%), where the environmental performance is also addressed. Twelve building types, which are typical of the historic building stock in Visby, Sweden, are used as the study object.The results show possible decreases of 12–38% in LCC when targeting LCC optimum. When targeting a 50% decrease in energy use, the LCC is decreased in 21 of 26 cases compared to before energy renovation. Cost-efficient EEMs on the building envelope are characterized by low renovation costs and additional insulation of building components with poor thermal properties. Furthermore, the environmental performance from the energy renovations is highly dependent on the chosen energy system boundary.

Preparation and characterization of gelatin-based nanocomposite containing chitosan nanofiber and ZnO nanoparticles

There is an increasing interest toward biodegradable active packaging because of consumer demand and environmental concerns. Despite this interest, poor thermal, mechanical, and water barrier properties of biodegradable polymers such as gelatin limit their application in food packaging. In this study, to prevail these limitations, the gelatin-based nanocomposite containing chitosan nanofiber (CHNF) and ZnO nanoparticles (ZnONPs) were fabricated and characterized by FTIR, SEM, and DSC analyses. The results showed the appropriate interactions between gelatin matrix, CHNF and ZnONPs due to their good compatibility. Additionally, the nanocomposite showed high mechanical and water barrier properties due to its high dense and less permeable structure. The incorporation of CHNF compensated the negative effect of ZnONPs on the color properties of gelatin film. In addition, the synergistic effect between CHNF and ZnONPs improved the antibacterial activity of nanocomposite. In conclusion, the fabricated bio nanocomposite indicated considerable potential for food packaging.

Prediction of Viscosity Values of Nanofluids at Different pH Values by Alternating Decision Tree and Multilayer Perceptron Methods

Due to the poor thermal properties of conventional thermal fluids such as water, oil and ethylene glycol, small solid particles are added to these fluids to enhance heat transfer. Since the viscosity change determines the rheological behavior of a liquid, it is very important to examine the parameters affecting the viscosity. Since the experimental viscosity measurement is expensive and time-consuming, it is more practical to estimate this parameter. In this study, CuO (copper oxide) nanoparticles were produced and then Scanning Electron Microscope (SEM) images analyses of the produced particles were made. Nanofluids were obtained by using pure water, ethanol and ethylene glycol materials together with the produced nanoparticles and the viscosity values were calculated by experimental setups at different density and temperatures. For the viscosity values of nanofluids, predictive models were created by using different computational intelligence methods. Mean square error (MSE), root mean square error (RMSE) and mean absolute percentage error (MAPE) error analyses were used to determine the accuracy of the predictive models. The multilayer perceptron method, which has the least error value in computational methods, was chosen as the best predicting method. The multilayer perceptron method, with an average accuracy of 51%, performed better than the alternating decision tree method. As a result, the viscosity increased with the increase in the pH of the nanofluids produced by adding CuO nanoparticles and decreased with the increase in the temperature of the nanofluids. The importance of this study is to create a predictive model using computational intelligence methods for viscosity values calculated with different pH values.

Effect of a fluoroalkyl-functional curing agent on the wettability, thermal and mechanical properties of hydrophobic biobased epoxy coatings

Over the past several decades, producing materials from fossil-based sources has become a serious problem because such action continues to contribute to global warming and contamination of the ecosystem. Thus, it is highly desirable to address this issue through producing functional materials via green chemistry such as the use of renewable compounds. In this paper, the preparation of hydrophobic coatings based on isosorbide, a derivative of polysaccharides, is reported. The inherently hydrophilic thermoset derived from isosorbide was made hydrophobic through the use of the hydrophobic curing agent F8815. The influence of the F8815 concentration on the decomposition temperature, glass transition temperature and storage modulus of the biobased samples was explored using TGA, DSC and DMA, respectively. The results showed that, despite of its poor mechanical and thermal properties, incorporation of F8815 served to improve the thermal and mechanical properties of the isosorbide-based polymers. Deconvolution analysis of FTIR spectra and SEM images provided insightful information on the participation of F8815 in the polymer network.

Thermal Simulation for Unconditioned Single Zone with Modified Roof

Roof in the Iraqi houses normally flattening by a concrete panel. This concrete panel has poor thermal properties. The usage of materials with low thermal conductivity and high specific heat gives a good improvements to the thermal properties of the concrete panel, thus, the indoor room temperature improves. A Mathcad program based on a mathematical model employing complex Fourier series built for a single room building. The model input data are the ambient temperature, solar radiation, and sol-air temperature, which have been treated as a periodic function of time. While, the room construction is constant due to their materials made of it, except the roof properties are taken as a variable generated practically from the improved mixing ratios.The result showed that using concrete panel with components (cement, sand, coarse aggregate, wood ash and Alabaster aggregates) with a ratio (1:1:2:1:1) and 3-plastic layer denoted by roof No.4, gives the best improvement of the thermal performance for the building. Where, the thermal conductivity is reduced by 42% and the specific heat increased by 41.2% compared to the traditional concrete panel mixing ratio denoted by roof No.1. Also, the mechanical properties are agreed with the Iraqi standards No. 1107 on 2002.

Sustainable Framework of Chitosan–Benzoxazine with Mutual Benefits: Low Curing Temperature and Improved Thermal and Mechanical Properties

Polybenzoxazines (PBzs) are emerging as a highly promising and superior class of thermoset polymers for a variety of applications. However, it remains a significant challenge to substantially lower the ring-opening polymerization (ROP) temperature with an ease in processability. On the other hand, biomacromolecule chitosan (CS) is explored extensively, but its practical applications have been precluded by poor thermal and mechanical properties. Here, we developed a fully biobased copolymer of vanillin benzoxazine (V-fa) monomer with CS, which is effective in providing mutual benefits, effective lowering in ROP temperature of benzoxazine (70 °C), and an enhanced thermal stability of CS (by 85 °C, and with char yield of ∼32%). To understand this unusual lowering in ROP temperature, we investigated the structural interaction mechanism between solvated CS and V-fa using in situ NMR studies. The analysis of fully intercalated co-structure demonstrated that there is a strong preference for ROP over Schiff base ...

Surface texturing of Tribological Interfaces –An Experimental Analysis

Abstract Surface texturing principle has already been reported to be successful in enhancing the tribological characteristics of various mechanical components including bearings, mechanical seal, piston rings, cylinder liner, etc. The current research work investigates the application of surface texturing principle on drill tools while machining titanium alloy. One of the major challenges pertaining to drilling of Ti-6Al-4V is the accumulation of heat at the machining zone due to the poor thermal conductivity property of the material. This factor deteriorates its applicability in various fields including aerospace, automobile, spacecraft, bio medical, etc. Accumulation of heat over the cutting regime will lead to rapid tool wear affecting the tool life and machined surface quality, which will eventually increase the total production cost. In drilling as the machining phenomenon occurs inside the hole, dissipation of accumulated heat is a real challenge. Moreover, the reachability of cutting fluids will get obstructed by the upward motion of chips along the helical groove. The above-mentioned challenge can be addressed by creating micro scale textures on drill tool surfaces, thereby controlling the heat generation by minimizing the sliding friction. Hence in the present work, micro scale textures in the form of circular dimples were created on the drill tool, and the same was coated for increasing the wear strength. Drilling experiments were performed under dry, wet and MQL conditions for evaluating the effectiveness of micro textured tools in cutting force reduction. From the experimental results, a net thrust force reduction of 13.83% in dry, 22.85% in wet and 21.65% in MQL condition were achieved while machining Ti-6Al-4V using coated flute and margin textured tool. Reduction in chip clogging phenomenon and lubrication enhancement provided by the coated micro scale textures are observed to be the underlying mechanism responsible for the lower cutting forces under all machining conditions.

Evaluation of dynamic properties of nano oil palm empty fruit bunch filler/epoxy composites

Cured epoxy resins pretense a constraint for variety of advanced applications due to its notably poor thermal and dynamic (viscoelastic) properties, hence required to minimize their properties prior to their usage. The aim of the present study is to evaluate the effect of nano oil palm empty fruit bunch (OPEFB) fibers at different loadings (1%, 3% and 5%) on the dynamic mechanical properties through dynamic mechanical analysis (DMA) in terms of storage modulus E′, loss modulus E″ and glass transition temperature (Tg) of epoxy composites. Results explored that dynamic properties of the epoxy composites get improved remarkably by the incorporation of nano OPEFB to epoxy composites, while 3% loading displays marked decrease in damping factor with relative to pure epoxy composites and the rest. Overall we perceived that the 3% loading of nano OPEFB filler is the best and optimal to enhance dynamic mechanical properties and to modify the damping factor of the epoxy composites resulting in most promising light weight and thermally stable composite structural materials.

Numerical evaluation of thermal discomfort in conditions of surface heating and asymmetric radiation

This paper presents the results of analytical analysis of thermal comfort and radiation asymmetry in case of wall heating depending on the room geometry and thermal properties of the external wall. The negative effects of radiation asymmetry on thermal comfort in case of summer conditions can be lowered using advanced personalized ventilation systems. In case of buildings with poor thermal properties of the envelope during the winter period low surface temperatures may occur. The aim of this research was to analyse the thermal asymmetry in the case of a room with one external wall and wall heating installed on the opposite wall. It was assumed that the radiation asymmetry will lead to discomfort and it was hypothesised that the discomfort might be reduced increasing the air velocity. The results have proven that thermal asymmetry in the middle of the room will not lead to thermal discomfort even for walls without any additional thermal insulation. However, the mean radiant temperature varies significantly depending on the position of the occupant in the room. In this case, the personalized control on the air velocity can help to improve the thermal comfort conditions.

Characterization and properties of biodegradable thermoplastic grafted starch films by different contents of methacrylic acid

Due to poor mechanical and thermal properties of native starch (NS) film; in the present work; NS was modified by graft copolymerization. Thermoplastic starch (TPS) grafted by methacrylic acid (MAA) with different percentage of grafting, i.e., 0%, 18.3%, 36.3%, 52.1% and 89.7% were prepared and tested. The result demonstrated that the intensity of IR peak of acrylic group increased with the increasing percentage of grafting. The higher graft copolymerization with MAA also significantly reduced degree of crystallinity. The strain at maximum load of TPS film grafted by MAA increased with the increasing percentage of grafting. However, water uptake of TPS film grafted by MAA reduced with high percentage of grafting (52.1% and 89.7%). In addition, different TPS films grafted by MAA were also examined for morphology, water vapor permeability, thermal property and biodegradable property by soil buried test.

A green physical approach to compatibilize a bio-based poly (lactic acid)/lignin blend for better mechanical, thermal and degradation properties

The poly (lactic acid) (PLA) and lignin (LG) are promising candidates to develop green plastic. However, the blending of lignin with PLA leads to incompatible blend with poor mechanical and thermal properties. Hence, in the present work, a green and simple approach was employed to make PLA/LG compatible blend with high lignin percentage (5 and 20%). The E-beam irradiated lignins having different absorbed dosages were blended with two different percentages 5 and 20 with PLA in the presence of 3 phr triallyl isocyanurate (TAIC). FTIR and DSC studies have confirmed the formation of PLA-TAIC-Lignin crosslinked structures which act as an interface between the dispersed lignin phase and PLA matrix and hence improved their compatibility in the resulting blend. The compatibility of resulting blends was further validated by the morphology study, Glass transition temperature (Tg) behavior of PLA/LG blends and by observing the significant improvement in the mechanical, thermal and hydrolytic degradation properties.

High refractive index dispersion of compositionally optimized Ge-Ga-Sb-S sulfide glass for use as molded lens in the long-wavelength infrared range

In an effort to endow Ge-Sb-S sulfide glass with capability to transmit the long-wavelength infrared (LWIR) photons as well as to possess highly dispersive refractive indexes over the LWIR spectral range, we have compositionally adjusted ternary Ge-Sb-S glass prior to tuning quaternary Ge-X-Sb-S (X = Ga, Bi, In or Sn) glass. Among the constituent atoms of ternary Ge-Sb-S glass, Ge content turned out to exert the strongest influence not only on thermal and mechanical properties but also on infrared transmission edge. Specifically, glass transition temperature and Vickers hardness tended to be directly proportional to Ge content, but infrared transmission edge was blue-shifted upon increase of Ge content, simultaneously producing an additional multiphonon absorption peak at ~ 9 µm. As such, Ge-Sb-S glass containing Ge atoms less than ~ 15 at% was considered to feature applicability in LWIR range, which on the other hand suffers from relatively poor thermal and mechanical properties. In our experimental attempts on quaternary sulfide glasses performed to mitigate such deteriorations, presence of relatively small amount of Ga appeared to be most effective among the elements employed in this study as the fourth constituent. Moreover, only Ga atoms improved thermal stability with significantly increasing crystallization temperature. Quaternary Ge-Ga-Sb-S glass compositionally optimized in this study was then verified to be much more dispersive than typical Se-based chalcogenide glasses in the LWIR region.

Ultrasound assisted acid hydrolyzed structure modification and loading of antioxidants on potato starch nanoparticles

Starch is second most abundant biomaterial available after cellulose but the intensity of research on starch is less compared to cellulose. It is a carbohydrate based polymer synthesized in plants for the storage of the energy. Major percentage of starch is being utilized by food industries as raw material for giving texture, flavor, gelling, fat replacement etc. and also has multiple applications in different area due to its biological origin and properties. Native starch possesses low shear stress, poor thermal properties and less digestion resistance and retro-gradation. Thus, it has to be modified using physical, chemical, enzymatic and/or genetic treatments. Physical and chemical modifications using ultrasound and acid hydrolysis is time-efficient and effective process. These economical treatments are predominant for production of digestion resistant starch with increased shelf-life and thermal properties. Ultrasound assisted acid hydrolyzed starch (potato) exfoliates the native starch and modifies the structural arrangement. On acid treatment the amorphous nature of starch converted to crystalline nature. The physical and structural properties of the native starch were enhanced. The digestibility and structure of the modified starch effects on the double helices structure of starch. The size of the starch particle was changed from 1596 nm (Conventional) to 80 and 42 nm on ultrasonication and acid hydrolyzed ultrasonication approaches respectively. The crystallite of the particles was evaluated from XRD analysis. From TEM analysis the starch nanoparticles were found to have spherical morphology.

Graphene-modified carbon/epoxy nanocomposites: Electrical, thermal and mechanical properties

A primary limitation of fiber-reinforced polymer composites in aircraft applications is susceptibility to lightning because of poor electrical, thermal and electromagnetic properties. The current methods to mitigate the lightning strike in aircraft have added weight and reduced the performance. Previous graphene-modified epon 828 epoxy matrix study showed that three-roll dispersion is effective, repeatable and potentially scalable to disperse graphene in to epoxy to increase the electrical conductivity. Percolation threshold of graphene was found to be about 1.0 wt.% that enhanced electrical conductivity of epon 828 epoxy matrix from 4.3 × 10−15 to 2.6 × 10−6 S/m, thermal conductivity doubled and fracture toughness increased by one-third. In the present study, the same graphene/epon 828 is reinforced by carbon fabric by hand lay-up followed by compression molding. The resulting composite laminate was tested for electrical, thermal and mechanical properties and results of this nanocomposite laminates were compared with base composite laminate.

Chemical and Hydrophobic Properties of PLA/HNTs-ZrO2 Bionanocomposites

The utilization of biopolymers in the applications that meet demands in society for sustainability and environmental safety has been limited. That is owing to the poor mechanical and thermal properties of biopolymers. The goal of this study is to improve the limitations of the biopolymers properties. polylactic acid (PLA) as one of the most important biopolymer was selected to investigate the effect of incorporating of natural halloysites nanotubes HNTs with different concentrations (0–8 wt%) on PLA chemical and hydrophobic properties. In the second series, Zirconium dioxide (ZrO2) nanoparticles were added to the PLA/HNTs film with the optimum mechanical properties in the first series. the chemical structure and surface wettability were investigated for both of PLA/HNTs and PLA/HNTs-ZrO2 bionanocomposites. The addition of ZrO2 resulted in appearance of OH groups in the FTIR graph. In addition, the increasing of ZrO2 loading led to increase the hydrophobicity of the PLA nanocomposites.

The Improvement of Thermal Insulating Concrete Panel

The Iraqi houses flattening the roof by a concrete panel, and because of the panels on the top directly exposed to the solar radiation become unbearably hot and cold during the summer and winter. The traditional concrete panel components are cement, sand, and aggregate, which have a poor thermal property. The usage of materials with low thermal conductivity with no negative reflects on its mechanical properties gives good improvements to the thermal properties of the concrete panel. The practical part of this work was built on a multi-stage mixing plan. In the first stage the mixing ratio based on the ratios of the sand to cement. The second stage mixing ratios based on replacing the coarse aggregate quantities with the Alabaster aggregates, and the third stage the mixing ratios based on the replacement of wood ash instead of the sand. While the fourth stage mixing ratios based on decreasing the thermal conductivity and increasing mechanical properties by adding a multilayer of a plastic net. The result shows that using a concrete panel with components (cement, sand, coarse aggregate, wood ash, and Alabaster aggregates) with a mass ratio of (1:1:2:1:1) and 3-plastic layers, gives the best improvement of the thermal properties. Where, the thermal conductivity is reduced by 42% and the specific heat increased by 41.2% as compared to the traditional concrete panel mixing ratio, with mechanical properties are agreed with the Iraqi standards.

(Invited) Process Monitoring of 100 GaN-on-Diamond Wafers

The introduction of chemical vapor deposition (CVD) diamond as a viable substrate for GaN HEMTs [1], [2] makes GaN-on-diamond an attractive means to run higher power densities through the GaN HEMT compared to traditional GaN on host substrates. While power densities of 20 and 40W/mm have been demonstrated for pulsed devices on silicon carbide (SiC) [3], continuous powers exceeding 10W/mm will reduce the lifetime of the GaN HEMTs due to self-heating. GaN-on-diamond holds a promise of reducing the problems associated with self-heating and allows for increases in the power density of HEMT devices. To date, GaN-on-diamond has been shown to allow for higher areal power densities than GaN-on-SiC [4], [5]. The process of making GaN-on-diamond was first demonstrated more than 10 years ago [1], [2] has evolved and improved over the years from broken samples 1cm on a side with a 25 micron (800 W/mK thermal conductivity) diamond substrate to a 4” process with a 100 micron (1500 W/mK) diamond substrate. In this paper we will look at wafer level electrical comparisons of GaN-on-diamond to GaN on host substrates and at process monitoring for the 100 GaN-on-diamond wafers. The process we use to make GaN-on-diamond is to transfer an epitaxial layer of GaN from a silicon substrate to diamond. In this process we flip the GaN grown on silicon onto a sacrificial substrate and remove the growth silicon. At this point the AlGaN transition layers (TLs) which are necessary to grow low defect density GaN on silicon are etched away. The TLs while critical to the growth of low defect GaN are themselves highly defective and have poor thermal properties [6]. It is therefore advantageous both electrically and thermally to remove these layers. Once the TLs are removed we protect the GaN back side with a nucleation/adhesion layer and we grow the CVD diamond. The standard metric for GaN-on-diamond is the thermal boundary resistance (TBR) which measures how efficiently the heat can be transferred from the GaN to the diamond. With collaborators we have been able to demonstrate a drop from 135 m2K/GW to as low as 10 m2K/GW by removing the TLs and maintaining a consistent adhesion layer thickness of silicon nitride between the GaN and the diamond [7]. Process monitoring shows that we can consistently maintain a 35 +/- 5nm SiN adhesion layer between the GaN and the diamond. In this paper we will present process monitoring data of this and other parameters for the last 100 GaN wafers. Less well established are the electrical metrics for comparing GaN-on-diamond to GaN on host substrates. One important metric of electrical performance for GaN-HEMTs is the flat capacitance at high reverse bias. It allows us to characterize the presence of traps and/or doping at the GaN to host interface [8]. In our case the GaN to diamond interface. We measure this flat capacitance by taking the capacitance voltage (C-V) profile. Results show that we can take GaN-on-silicon which has a flat band capacitance of 25pF and turn it into GaN-on-diamond with a flat band capacitance of ~ 2pF Figure[1a]. This low capacitance is indicative of negligible GaN buffer and epi interface charge/doping. The capacitance of the GaN-on-diamond is now comparable to GaN-on-silicon carbide Figure [1b]. Figure 1

Continuous-wave room-temperature diamond maser.

The maser-the microwave progenitor of the optical laser-has been confined to relative obscurity owing to its reliance on cryogenic refrigeration and high-vacuum systems. Despite this, it has found application in deep-space communications and radio astronomy owing to its unparalleled performance as a low-noise amplifier and oscillator. The recent demonstration of a room-temperature solid-state maser that utilizes polarized electron populations within the triplet states of photo-excited pentacene molecules in a p-terphenyl host paves the way for a new class of maser. However, p-terphenyl has poor thermal and mechanical properties, and the decay rates of the triplet sublevel of pentacene mean that only pulsed maser operation has been observed in this system. Alternative materials are therefore required to achieve continuous emission: inorganic materials that contain spin defects, such as diamond and silicon carbide, have been proposed. Here we report a continuous-wave room-temperature maser oscillator using optically pumped nitrogen-vacancy defect centres in diamond. This demonstration highlights the potential of room-temperature solid-state masers for use in a new generation of microwave devices that could find application in medicine, security, sensing and quantum technologies.