Shape stable composite phase change material with improved thermal conductivity for electrical-to-thermal energy conversion and storage

Limited thermal conductivity and leakage of phase change material (PCM) are among the most challenging obstacles that impede their effective applications in real-world scenarios. This study focussed on enhancing the thermal conductivity (TC), address leakage issues and incorporate thermoelectric conversion capabilities by using a single multifunctional scaffold. The shape stable PCM (ss-PCM) composite has been prepared using medium temperature range (~46 °C) commercial grade paraffin wax (PW) as organic PCM while expanded graphite (EG) as an encapsulating scaffold. The composite was prepared using vacuum impregnation method, incorporating various weight percentages (wt.%) of EG. In particular, the three wt.% of EG that has been used to encapsulate PCM are 5 % (ss-PCM1), 10 % (ss-PCM2) and 15 % (ss-PCM3). Then the composite was evaluated for its thermal stability, potential chemical interactions, leakage prevention, optical properties, thermal conductivity and thermo-electric conversion capability. Results revealed that the incorporation of 15 wt% EG in PCM (ss-PCM3) demonstrated no traces of leakage even after heating the composite at 60 °C. In addition, a significant increment of 447 % in thermal conductivity and 98 % in light absorbance has been observed. However, the composite showed a slight decrement of 13.83 % in latent heat related to base PCM. Finally, ss-PCM3 was put through to 500 heating-cooling cycles to evaluate its reliability and potential defects due to thermal fatigue. The characterization results of the composite were in close agreement before and after the thermal cycling, indicating its potential for practical applications. The electro-thermal conversion measurement findings indicate that the ss-PCM3 can achieve a conversion ability of 61.89 % when operated at 4.8 V. Several potential applications for this composite include energy-efficient buildings, infrared thermal concealment, solar energy utilization, and heat insulation

Nanocellulose Xerogels With High Porosities and Large Specific Surface Areas

Xerogels are defined as porous structures that are obtained by evaporative drying of wet gels. One challenge is producing xerogels with high porosity and large specific surface areas, which are structurally comparable to supercritical-dried aerogels. Herein, we report on cellulose xerogels with a truly aerogel-like porous structure. These xerogels have a monolithic form with porosities and specific surface areas in the ranges of 71–76% and 340–411 m2/g, respectively. Our strategy is based on combining three concepts: (1) the use of a very fine type of cellulose nanofibers (CNFs) with a width of ~3 nm as the skeletal component of the xerogel; (2) increasing the stiffness of wet CNF gels by reinforcing the inter-CNF interactions to sustain their dry shrinkage; and (3) solvent-exchange of wet gels with low-polarity solvents, such as hexane and pentane, to reduce the capillary force on drying. The synergistic effects of combining these approaches lead to improvements in the porous structure in the CNF xerogels

Understanding sol–gel processing: Hierarchical silica monoliths towards applications in chemical reaction engineering

Hierarchically structured, porous materials in the form of macro–mesoporous silica monoliths represent ideal support materials for a variety of applications such as chemical separation, heterogeneous catalysis, thermal insulation, electrochemical processes and CO2 adsorption. They are well suited for this purpose since the macropores enable fast, advection-dominated transport through the material whilst the mesoporous skeleton provides a large external surface area for mass transfer between the macro- and the mesoporous domains, as well as a large, internal surface area for possible functionalization. In this context, this thesis focuses on the generation of hierarchy in sol–gel based porous silica materials, as well as the determination and interpretation of their properties with respect to applications in reaction technology. For the entire preparation process each individual step is fine-tuned in terms of practicability, time-effectiveness and simplicity to obtain robust and straightforward synthetic routes towards stable, hierarchically structured sol–gel monoliths. The understanding of the chemical and physical processes involved in these steps allows the precise control of the material characteristics. Novel experimental methods and strategies are presented, which minimize the laboratory workload and additionally highlight the superior properties of these materials. The potential of hierarchically structured sol–gel monoliths is demonstrated by applications in heterogeneous catalysis and biocatalysis. In the following, the respective concepts of this thesis are briefly summarized. Chapter 1 examines the concept of hierarchy itself and highlights the advantages of a hierarchically structured pore system in comparison with monomodal pore systems, especially with regard to its mass transport properties. A model system based on silica membranes is presented, which discloses the advantages and disadvantages of purely mesoporous, purely macroporous and hierarchical pore structures. The monolithic support materials are synthesized using the sol–gel process, as this technique is considered as highly variable in the generation of different pore structures and sizes and it enables post-synthetic functionalization of the silica surface. In order to prepare mechanically stable and comparable sol–gel membranes, a novel and simplified drying method is presented. By varying the synthesis compositions, synthesis conditions and post-synthetic treatments, a variety of silica membranes are produced, which differ only in their porous properties. Monomodal structured materials with mean pore sizes in the range from 20 to 40 nm (mesoporous) and 350 to 3250 nm (macroporous) are produced as well as hierarchically structured materials combining these pore size ranges. All surfaces are functionalized post-synthetically with the bifunctional reagent 3-(gylcidoxypropyl)-dimethylmethoxysilane to realize the covalent immobilization of the enzyme acethylcholinesterase (AChE) under ring opening of the epoxy group of the silane. In the following, the three different pore systems are compared in terms of their enzyme loading capacity and their response times. Due to the pore-size-dependent specific surface area, the loading capacities of the representative pore systems differ significantly, resulting in a loading of 4.1 µg of AChE per membrane with a macropore size of 3250 nm and an AChE loading of 38.5 µg per membrane for a mesopore size of 20 nm, whereby the hierarchically structured pore system with equivalent pore size ranges has a loading capacity of 15.5 µg AChE per membrane. The response time of the enzyme-catalyzed substrate degradation reaction of acetylcholine to choline and acetic acid is used to determine the apparent reaction rate, which is used to describe the efficiency of the individual pore structure and thus allows conclusions on intrinsic diffusion limitations. These investigations are conducted for all three pore systems at a constant enzyme loading to ensure comparability. At a loading of 4.1 µg AChE per membrane, the purely macroporous pore system exhibits a slight advantage over the hierarchically structured material, as it causes the fastest reaction, which is due to the lowest mass transfer limitations. By increasing the enzyme loading to 12.9 µg per membrane, it is evident that the hierarchically structured pore system shows a significant reduction of the response time, and thus is superior to the purely mesoporous material, as it combines the advantages of both monomodal systems, the improved mass transport and the higher enzyme loading. In conclusion, this study demonstrates systematic investigations to highlight the advantages of pore space hierarchy, which is characterized by combined functionality and transport efficiency. Chapter 2 presents the urea-controlled synthesis of hierarchically structured silica monoliths, their transfer into a suitable column system and subsequent functionalization of the support surface in order to use them as flow microreactors. The hierarchy is generated by polymer-induced phase separation, which is an important step in the sol–gel process. The silica gels were synthesized using urea to create a mesoporous domain and to control the macroporous system. The influence of urea on the mean mesopore size is based on the base releasing property of urea by decomposition to ammonia and carbon dioxide under elevated temperature and has already been described in the literature. The resulting raise in pH increases the solubility of silica, whereby dissolution and deposition processes ensure that the initially microporous silica skeleton is expanded, resulting in a mesoporous domain. A common scientific approach is to add urea to the aqueous starting solution for a direct incorporation into the hydrogel to ensure a homogeneous pore expansion in the hydrothermal treatment. However, it is found that urea also has a strong influence on the macropore size and skeleton thickness of the obtained sol–gel monolith. With increasing urea content, the average macropore size is significantly reduced down to the submicron range. Urea influences the time sequences of gelation and phase separation due to an increase in the pH of the sol, which already occurs before substantial decomposition and additionally influences its polarity. Therefore, the gelation point occurs at an earlier stage of spinodal decomposition, which fixes a smaller macropore system. The synthesized monoliths with macropores in the submicron range are well-suited for the investigation of intrinsic reaction kinetics, since external and internal diffusion limitations are eliminated and hydrodynamic backmixing is reduced to a degree that a hydrodynamic plug flow behavior can be reached. For the application as flow microreactor, a novel cladding procedure is presented, which enables seamless housing of the sol–gel monolith in stainless steel tubing to withstand column backpressures >100 bar. By a special stop-flow functionalization method, aminopropyl groups are coated onto the silica surface generating a catalytically active column. The acquisition of reaction data for the reaction kinetics of the Knoevenagel condensation between benzaldehyde and ethyl cyanoacetate to trans-α-ethyl cyanocinnamate, which is a well-known test reaction for basic catalysts, is realized with a two-dimensional HPLC setup. The first dimension automates the adjustment of the desired reaction parameters for the microreactor and the second dimension allows the complete quantification of the reaction data by online HPLC. The entire reaction kinetics of the amino-catalyzed Knoevenagel condensation for five different reaction times at seven different reaction temperatures each is recorded in only about 400 minutes. The reaction data results in a pseudo first order reaction kinetics, which is due to the two-step reaction mechanism. The reaction data reveal a reaction behavior under quasi-homogeneous conditions, which confirms the absence of any transport limitations. In conclusion, hierarchically structured sol–gel monoliths are synthesized using urea as pore size controlling agent to obtain catalytic microreactors with a high active surface area, which allow for the investigation of intrinsic reaction kinetics without transport limitations. Chapter 3 describes a new approach for the preparation of hierarchically structured, sulfonic acid modified silica monoliths based on the sol–gel process, whereby the functionalization is introduced into the pore system in situ during gelation. By using the co-condensation method, an alkoxysilane with a propylthiol function, ((3-mercaptopropyl)trimethoxysilane, MPTMS), is added together with the unfunctionalized silica precursor for sol formation. The synthesis of such organic-inorganic hybrid materials is widely used in the literature, but not for hierarchically structured sol–gel materials, where the hierarchy is generated via polymer-induced phase separation. Here, the incorporated polymer is usually removed from the material at high temperatures by an additional step, called calcination, to obtain the pure silica product. This treatment pyrolyzes any organic matter, which would also result in a loss of incorporated functionality. Therefore, this study presents an extraction of the polymer to avoid the loss of the covalently bound functionalization on the support surface, which simultaneously converts the introduced thiol groups into sulfonic acid functions, resulting in a time-efficient synthesis route. For this purpose, an extracting agent consisting of hydrogen peroxide and nitric acid is used. Macropore size control is demonstrated by the variation of polymer and functionalization reagent compositions, whereby the functionalization reagent has a significant influence on the onset of phase separation and consequently on the final macropore size. Furthermore, the widening process to form the mesoporous domain is strongly affected, resulting in very narrow mesopore distributions <10 nm with specific surface areas of up to 576 m2 g–1. The efficiency of the extraction procedure and the successful generation of sulfonic acid modified silica is extensively investigated and characterized to evaluate the presented approach. It is shown that, as the amount of MPTMS increases, the sulfur content and thus the loading of homogeneously distributed sulfonic acid groups is increased up to 1.2 mmol g–1 without significant loss due to the extraction. The polymer, however, is removed to a high degree during the extraction process. The covalent binding of the functionalization and the successful oxidation of the sulfur to form sulfonic acid functions is demonstrated by IR as well as 13C and 29Si MAS NMR spectroscopy. Moreover, investigations using inverse gas chromatography allow to investigate surface interactions and acid strength. The functionalized organic-inorganic hybrid monoliths have significantly higher surface energies, with the specific (polar) component being more dominant as the dispersive component. Furthermore, it is shown that these materials gain acid strength, which is generated by the incorporated sulfonic acid groups. In summary, a novel and efficient synthesis route for the preparation of hierarchically structured sol–gel monoliths with homogeneously distributed sulfonic acid functionalization is introduced. In conclusion, this thesis improves the understanding of the individual steps of the sol–gel process for the preparation of hierarchically structured silica-based materials. These results are presented in the context of different possible applications, since their variability allows them to be adapted to diverse requirements and problems

Synthesis of aerogels, nanocomposites and lightweight silica aerogel superinsulation nanocomposites by ambient pressure drying method

Ph. D ThesisThis thesis mainly investigates the improvement of the new ambient pressure approach used to synthesise aerogels by using a solvent comprising of sodium bicarbonate and water instead of a low surface tension solvent. Firstly, to improve the efficiency of thermal insulation, the sodium bicarbonate approach is utilised to synthesise cost effective ceramic blanket silica aerogels (CBSA) and short ceramic fibres silica aerogel composites (CSSA). To reduce the manufacturing cost and scalable of silica aerogels, we propose applying the sodium bicarbonate approach to synthesis silica aerogels from sodium silicate (water glass) precursor. In addition, the approach is used to synthesise alumina-based aerogel (dawsonite-sodium aluminium carbonate hydroxide) from Aluminium sec-butoxide precursor (ASB). To mimic the structure and thickness of the wings of the damselfly, which was the main source of inspiration for this study, multi-layered silica aerogel films with a thickness of 0.3 mm were synthesised using the bicarbonate approach. Finally, wavy nickel nanowires (NiNWs) were synthesise and immobilised on mesoporous silica (SiO2) aerogels by the sol-gel method. In addition, nickel nanoparticles (NiNPs) were immobilised in silica aerogels to do a comparative study between the catalytic activity of immobilised NiNWs and NiNPs in silica aerogels for CO2 hydration reaction (CHR) in gaseous phase. Dynamic vapour sorption (DVS) analysis is used for that purpose. The analysis is performed at levels of 50% CO2 and 50% H2O vapour for SiO2 aerogels, immobilised nickel nanoparticles (NiNPs) on silica aerogels and NiNWs-SiO2 aerogels composites. Transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), uniaxial compression test, Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods are used to characterise the synthesised materials

Understanding Structure/Process-Property Relationships to Optimize Development Lifecycle in Yttria-Stabilized Zirconia Aerogels for Thermal Management

Aerogels are mesoporous materials with unique properties, including high specific surface area, high porosity, low thermal conductivity, and low density, increasing these materials’ effectiveness in applications such as catalyst supports, sorption media, and electrodes in solid oxide fuel cells. Zirconia (ZrO2) aerogels have special interest for high-temperature applications due to the high melting point of ZrO2 (2715°C) and stability between 600°C and 1000°C, where other aerogel systems often begin to sinter and densify. These properties and unique pore structure make zirconia aerogels advantageous as thermal management systems, especially in aeronautics and aerospace applications. However, to be effective in high-temperature applications, the aerogel formulation must be optimized so that pore collapse and subsequent surface area decrease are mitigated following high-temperature exposure. By utilizing surfactant templates, it is anticipated that the mesoporous structure and high surface area of yttria-stabilized zirconia (YSZ) aerogels will be retained following exposure to high temperatures, increasing the thermal stability and efficiency of YSZ aerogels as thermal management systems. To experimentally consider the impact of synthetic variables on aerogels, surfactants are used as templating agents to influence the pore structure and surface area of YSZ aerogels. Additionally, due to the large number of parameters associated with aerogel synthesis and processing, a developed aerogel graph database and a machine learning predictive model are applied to examine the complex relationships between aerogel synthesis, processing, and final properties, specifically BET surface area. Sub-graphs of the developed aerogel graph database are used to visually determine the impact of specific variables on the aerogel surface area, while the predictive model maps from aerogel synthetic and processing conditions to predict the final property, BET surface area, with precision. These digital design tools could reduce experimental dimensionality, time, and resources, enabling the successful synthesis of high surface area aerogels

Linear and nonlinear optical properties of silica aerogel

Scattering media have traditionally been seen as a hindrance to the controlled transport of light through media, creating the familiar speckle pattern. However such matter does not cause the loss of information but instead performs a highly complex deterministic operation on the incoming flux. Through sculpting the properties of the incoming wavefront, we can unlock the hidden characteristics of these media, affording us far more degrees of freedom than that which is available to us in traditional ballistic optics. These additional degrees of freedom have allowed for the creation of compact sophisticated optical devices based only on the deterministic nature of light scattering. Such devices include diffraction-limit-beating lenses, polarimeters, spectrometers, and some which can transmit entire images through a scattering substance. Additional degrees of freedom would allow for the creation of even more powerful devices, in new working regimes. In particular, the application of related techniques where the scattering material is actively modified is limited. This thesis is concerned with the use of optothermal nonlinearity in random media as a way to provide an additional degree of control over light which scatters through it. Specifically, we are concerned with silica aerogel as a platform for this study. Silica aerogel is a lightweight skeletal structure of silica fibrils, which results in a material which is up to 99.98 % by volume. This material exhibits a unique cocktail of properties of use such as near unitary refractive index, an order of magnitude lower thermal conductivity, and high optothermal nonlinearity. The latter two of these properties allow for the creation of localised steep thermal gradients, proportionally affecting the low refractive index significantly. Additionally through differing fabrication steps, the opacity, and as a result, we can adjust the scattering strength. In line with the development of light deterministic light scattering techniques in linear media, we develop through the use of pump-probe setups, a framework for the development of a similar line of techniques in nonlinear scattering media. We show that we can reversibly control the far-field propagation of light in weakly scattering silica aerogel. Following this, we show that nonlinear perturbation can be used to extend and modify the optical memory effect, where slight adjustments in scattering direction maintain the overall correlation of the scattered profile. Finally, we measure the nonlinear transmission matrix, a complete description of how any wavefront would pass through at a particular point in a scattering media, and how that scattering can be modified through the application of an optothermal nonlinearity. Extending the tool of scattering media into the nonlinear regime helps pave the way toward the next set of advances in the field of light scattering control."This work was supported by the Engineering and Physical Sciences Research Council [grant number EP/M508214/1]" -- Fundin

Fabrication of hydrophobic polymethylsilsesquioxane aerogels by a surfactant-free method using alkoxysilane with ionic group

<p>Phase separation control is an important factor to prepare a porous monolith by an aqueous sol–gel reaction. Here, we report a surfactant-free synthesis method to obtain hydrophobic polymethylsilsesquioxane aerogels by copolymerizing a cationic-functionalized alkoxysilane <i>N</i>-trimethoxysilylpropyl-<i>N</i>,<i>N</i>,<i>N</i>-trimethylammonium chloride. The resultant materials have low-density, high visible-light transmittance, and high thermal insulating equivalent to those of prepared under the presence of surfactant.</p

A Co-Precursor Approach Coupled with a Supercritical Modification Method for Constructing Highly Transparent and Superhydrophobic Polymethylsilsesquioxane Aerogels

Polymethylsilsesquioxane (PMSQ) aerogels obtained from methyltrimethoxysilane (MTMS) are well-known high-performance porous materials. Highly transparent and hydrophobic PMSQ aerogel would play an important role in transparent vacuum insulation panels. Herein, the co-precursor approach and supercritical modification method were developed to prepare the PMSQ aerogels with high transparency and superhydrophobicity. Firstly, benefiting from the introduction of tetramethoxysilane (TMOS) in the precursor, the pore structure became more uniform and the particle size was decreased. As the TMOS content increased, the light transmittance increased gradually from 54.0% to 81.2%, whereas the contact angle of water droplet decreased from 141° to 99.9°, ascribed to the increase of hydroxyl groups on the skeleton surface. Hence, the supercritical modification method utilizing hexamethyldisilazane was also introduced to enhance the hydrophobic methyl groups on the aerogel’s surface. As a result, the obtained aerogels revealed superhydrophobicity with a contact angle of 155°. Meanwhile, the developed surface modification method did not lead to any significant changes in the pore structure resulting in the superhydrophobic aerogel with a high transparency of 77.2%. The proposed co-precursor approach and supercritical modification method provide a new horizon in the fabrication of highly transparent and superhydrophobic PMSQ aerogels