Drying Control Chemical Additive Research Articles

Preparation of alumina/silica/exfoliated graphite nanoplatelets composite aerogels and its application as supporting material for erythritol based form-stable phase change materials

Alumina-silica (Al-Si) and Al-Si/exfoliated graphite nanoplatelets (xGnPs) composite aerogels were prepared via a novel epoxide method followed by ambient pressure drying (APD) and heat treatment. Epichlorohydrin (ECH) was applied as proton scavenger to control the hydrolysis of AlCl3·6H2O to obtain the composite gel. Then the ring-opening products of ECH acted as drying control chemical additives to prevent the gel from high volume shrinkage during APD and was then removed by heat treatment at 800 °C to obtain composite aerogels. Doping the Al-Si aerogel with xGnPs was in favor of obtaining composite aerogels with high specific area and low volume shrinkage. The specific surface area of the prepared Al-Si/xGnP aerogels could attain 350 m2/g with a pore volume of 1.156 cm3/g. The synthesized aerogels were then vacuum infiltrated with erythritol (ET) to obtain form-stable phase change materials (FSPCMs). The loading of ET in the prepared FSPCMs attained 90 wt%, and the latent heat storage capacity attained 279 J/g. In addition, the thermal conductivity of the FSPCMs could be enhanced to 49.8 % higher than pristine ET with the help of 0.6 wt% xGnPs, and the supercooling of ET in the FSPCMs could be suppressed to 59 °C. Moreover, the FSPCMs supported by Al-Si/xGnP aerogels possessed greatly enhanced light-to-thermal conversion efficiency than the pristine ET and the Al-Si aerogel supported FSPCM. More importantly, the FSPCMs possessed very good long-term thermal reliability.

Silica xerogel and iron doped silica xerogel synthesis in presence of drying control chemical additives

Silica xerogels were prepared by sol-gel method in presence of glycerol and oxalic acid as drying control chemical additivites (DCCAs). They were characterized with TGA, XRD, TEM, nitrogen adsorption/desorption isotherm, XPS, ICP, FTIR and UV–vis DR analyses. TGA results showed that the use of DCCAs provided a controlled release of water, thereby preventing structure collapse which was supported by nitrogen adsorption technique. DCCAs caused an increase in microporosity of xerogels, while some decreases in surface area and total pore volume values were occurred. Amorphous silica structure and silica density decreasing effect of DCCAs were observed from XRD patterns. While XPS, XRD and UV–vis DR analyses showed the presence of iron species in Fe2O3 form, they were found to settle as nanoparticles from TEM images. ICP results showed that iron incorporated to the xerogel structure as percent of 83. Iron loading caused significant increases in all textural properties. FTIR spectra exhibited bands representing the silica structure and DCCA-silica interaction.

Aerogel-like silica powders by combustion of sol-gel derived alcogels

Drying of sol-gel alcogels at ambient conditions leads to shrinkage and cracking due to differential capillary pressure making it difficult to obtain low-density materials. Capillary pressure depends on the surface tension of the liquid, which decreases as the temperature increases and becomes zero at a critical temperature in case of commonly used primary alcohols. Therefore, drying at supercritical conditions is often used to obtain silica aerogels from alcogels. In this case multiple solvent exchange steps are required to remove the residual water beforehand. We propose and show experimentally that the combination of the temperature-dependent behavior of the surface tension of alcohols and the nanoporous nature of sol-gel derived alcogels provides a quick and simple pathway to obtain low-density silica materials without hydrophobization, drying control chemical additives, solvent exchange nor supercritical drying apparatus. We demonstrate that, similarly to the many phenomena that require high pressure to occur in bulk phases being observed in nanopores at ambient pressure, simple combustion of sol-gel derived alcogels can result in pressures and temperatures in gel pores that decrease the surface tension of the alcohol sufficiently to produce granular silica with markedly lower density as compared to xerogels produced at room temperature. Such properties are obtained despite the inevitable high-temperature annealing that accompanies combustion. The method yields SiO2 granules with uniform size (~3 mm), low density (down to 0.358 g/cm3), high porosity (up to 79.2%), average pore size in the range of 9–14 nm and specific surface area of up to 832 m2/g.

Synthesis and properties of SnO2 aerogels via ambient pressure drying of sol–gel

Abstract

Performance manipulations of a composite membrane of low thermal conductivity for seawater desalination

Membranes of low thermal conductivity (thermal insulation) but high moisture permeability are highly desired in membrane-based humidification-dehumidification (MHDD) seawater desalination technology. The porous membranes with thermal insulation could decrease the heat losses in humidification, thus the system energy performance can be improved. To satisfy this purpose, in this study, a PVDF/SiO2 (Si aerogel) composite bilayer membrane of low heat conductivity is prepared and analyzed. To solve the problem of cracking in forming SiO2 layer, the casting solution (TEOS, H2O) is doped with a drying control chemical additive (DCCA) N, N-dimethylformamide (DMF) to decrease the surface tensions in the gel. Then performance analysis of the developed membranes are conducted and the membranes are characterized. The results found that the heat conductivity is decreased from 0.08199 W m−1 K−1 to 0.04487 W m−1 K−1, with a penalty of effective moisture diffusivity sacrificed from 4.93 × 10−6 m2/s to 2.42 × 10−6 m2/s. They are superior to the available SiO2 blend insulation membranes. Further, the membranes can be optimized by facile structural manipulations such as adjusting the SiO2 layer thickness to realize the desired performance while balancing the paradox between the permeability and heat conductivity. This is the first time that heat conductivity of membranes can be manipulated for seawater desalination.

Effect of DMF addition on the phase-chemical structure of Pd-doped methyl-modified silica membrane materials calcined in air atmosphere

ABSTRACTPd-doped methyl-modified SiO2 membrane materials were prepared by sol-gel technique with tetraethylorthosilicate (TEOS) and methyltriethoxysilane (MTES) as silicon precursors, PdCl2 as palladium source. N, N-dimethylformamide (DMF) was used as a drying control chemical additive (DCCA). The samples were characterized by X-ray Diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and thermogravimetric (TG)-differential thermogravimetric (DTG) measurement. The effect of DMF addition on the phase and chemical structures of Pd-doped SiO2 membrane materials calcined at 350°C was discussed. The results showed that the diffraction peaks of metallic Pd0 and PdO appeared in the XRD curve of Pd-doped SiO2 membrane material without DMF, while only the metallic Pd0 diffraction peak can be explicitly observed in the Pd-doped SiO2 membrane material with DMF. DMF has no significant influence on the chemical structure of Pd-doped SiO2 membrane materials but can delay the oxidation of metallic Pd0 to PdO in the air.

Role of oxalic acid in structural formation of sodium silicate-based silica aerogel by ambient pressure drying

Recently, the demand of sodium silicate-based silica aerogels has decreased due to its inadequate physical properties when compared to those made by other silicon alkoxides. To avoid this problem, introduction of a drying control chemical additive (DCCA) in the sol has received great attention. DCCA is one of the additives that can control the rate of hydrolysis and the condensation reaction in sol state by the formation of hydrogen bonds between DCCA and silanol groups of silica sol. A control over these reactions results in a uniform pore size distribution, which, in turn, decreases the drying stress with a decrease in pore size, in accordance with the Young–Laplace equation. Therefore, the structure of the silica aerogel can be maintained by minimizing the volume shrinkage due to drying stress. In this research, oxalic acid was first used as DCCA in the formation of sodium silicate-based silica aerogel by ambient pressure drying. The physical properties of these silica aerogels can be changed by changing the molar ratio of oxalic acid/Na2SiO3 in the sol state. When the oxalic acid:Na2SiO3 molar ratio was 15 × 10−4 in the sol state, aerogels with high specific surface area (623.2 m2/g), pore volume (4.271 cm3/g), average pore diameter (27.41 nm), high porosity (94.3%), high contact angle (144.09°), and high optical transmittance (75%) were obtained.

Preparation of sol–gel silica samples modified with drying control chemical additives

In this work we investigate the effect of different drying control chemical additives (DCCAs) on the sol–gel processing of silica. Formamide, N,N-dimethylformamide, and propylene carbonate were used as DCCAs in this study. The sols were prepared by the hydrolysis and condensation of tetraethyl orthosilicate in the presence of water, nitric acid, and ethanol. After gelation, the samples were heat treated in air at temperatures from 110 to 1150°C. The structural evolution of these materials was evaluated by Fourier transform infrared spectroscopy (FTIR), nitrogen adsorption tests, and helium pycnometry. The band attributed to silanol groups (≈960cm−1) decreased in intensity and shifted towards larger wavenumbers as reaction time progressed. Bands ascribed to Si–O bonds also shifted towards larger wavenumbers with reaction time. These findings suggest the formation of a more rigid silica framework as time progressed. This behavior was more pronounced for sols prepared using formamide and N,N-dimethylformamide. The heat treatment of DCCA-containing gels at temperatures up to 800°C gave rise to materials with a strong and interconnected framework, showing a high concentration of silanols on the surface. The presence of such groups could enable the chemical modification of these materials upon a post-synthesis treatment.

Rapid synthesis of sodium silicate based hydrophobic silica aerogel granules with large surface area

In this study, we tried to synthesize ultralow density and super hydrophobic silica aerogel with extra high specific surface area, by using absolutely cost effective processing from sodium silicate. Hydrosol was obtained through ion exchange. To reduce the processing time, the gels were mechanically agitated to granules before solvent exchange. N,N-Dimethylformamide (DMF), serving as drying control chemical additive (DCCA), was introduced to reduce shrinkage. When the molar ratio of Si in sodium silicate to DMF is 0.31, the pore size distribution is the narrowest. Before dried under ambient pressure, the gels were surface modified by Trimethylchlorosilane (TMCS) in order to make sure the aerogels are hydrophobic. And the proper molar ratio of TMCS to pore water is 0.0233 which is much lower than that in previous experiments. The resulting aerogels have well-developed mesoporous structure with extremely high specific surface area (817m2/g) and super hydrophobicity (contact angle of 165°).

Synthesis and characterization of silica aerogels dried under ambient pressure bed on water glass

In this paper, we report the experimental results on the synthesis of water glass based silica aerogels, which were dried under ambient pressure. Water glass was hydrolyzed and condensed in water using HCl as the catalyst. To minimize shrinkage during drying process, N,N-dimethylformamide (DMF), acting as drying control chemical additive (DCCA), was introduced. Before the ambient pressure drying, solvent exchange and surface modification were completed. In order to get hydrophobic aerogel, trimethylchlorosilane (TMCS) was used to modify the hydrophilic hydrogel surface. Here a large amount of TMCS can be saved compared with that in single step solvent exchange/surface modification method. The effects of DMF on the physical and textural properties of the resulting aerogels were investigated. When the molar ratio of Si in water glass to DMF is 2.23, the synthesized silica aerogels have better properties. Characterized by FT-IR, SEM, BET, etc., the resulting aerogels have well-developed mesoporous structure (mean pore size of ~15nm) with super hydrophobicity (contact angle of 161°) and excellent absorption capacity of organic liquids.

The role of the DCCAs in the preparation of crack-free Xerogel

Sol–gel chemistry is an efficient tool to control the morphology and reactivity of solids. Drying is the crucial step due to the capillary forces exerted during the classic ambient drying process, which induces the collapse of the wet gel porosity. The use of N, N-dimethylformamide (DMF), a newly found DCCA were found to be effective in the sol–gel synthesis of crack-free silica gel monolith from tetraethoxysilane (TEOS) via acid catalyzed. We investigate the effect of N, N- dimethylformamide as DCCAs in simple inorganic silica obtained from tetraethoxysilane (TEOS) and the effects drying control chemical additives (DCCA) on gelatin time and physical properties of the dry gel were examined in an acidic silica sol-gel process.

Fabrication of thin-film gadolinia-doped ceria (GDC) interdiffusion barrier layers for intermediate-temperature solid oxide fuel cells (IT-SOFCs) by chemical solution deposition (CSD)

A dense gadolinia-doped ceria (GDC) interdiffusion barrier layer as thin as 300nm was successfully fabricated on a rigid anode/electrolyte bilayer substrate using the chemical solution deposition (CSD) process for intermediate temperature solid oxide fuel cells (SOFCs). Drying-related macro-defects were removed by employing drying control chemical additives (DCCA), which effectively relieved drying stresses. The major process flaws caused by the constraining effects of the rigid substrate were completely eliminated by the addition of GDC nanoparticles into the chemical solution, which suppressed the generation of microstructural anisotropy by mitigating the predominant bi-axial substrate constraints. As a consequence, a thin film GDC interlayer was successfully deposited with a high volumetric density, effectively preventing the chemical interaction between the electrolyte and cathode during the fabrication process and subsequent operation. The cell test and microstructural analysis confirmed excellent electrochemical performance and structural and chemical stability. The CSD process presented in this paper is considered to be a promising technology for the practical preparation of GDC thin film barrier layers for intermediate temperature SOFCs based on the film quality, processing costs and potential for large-scale production.

Improvement in Mechanical Property of SiO2 Films Prepared by Sol-Gel

Silica films prepared by the base catalyzed sol–gel process show poor mechanical property. In this study, silica anti-reflective films with good mechanical property have been prepared by the catalyzed sol–gel process with dimethyl formamide added as drying control chemical additives. The silica sol was first dip deposited onto substrate to form films on both sides of the substrate and then subjected to thermal treatment. After thermal treatment, the film was achieved due to the formation of porous structure in the resultant film as a result of decomposing tetraethylorthosilicate and dimethyl formamide. Nano-indenter measurement shows that the Young’s modulus and hardness of base catalyzed films are 10-20GPa and 0.25-0.55GPa, respectively. But the films added with dimethyl formamide are much better than those films derived from base-catalyzed silica sols, the Young’s modulus and hardness are 32-39GPa and 1.22-1.47GPa, respectively. The good mechanical properties make such films potential in both military and civil applications.

Improved performance of Mo-V-Te-Nb-O x catalysts prepared from a solution containing drying control chemical additives in propane oxidation to acrylic acid

Mo1.0V0.4Te0.25Nb0.24O x catalyst was prepared by hydrothermal synthesis in a solution containing a drying control chemical additive (DCCA) such as propionic acid (PA) or formamide (FA). The performance of the prepared catalysts in propane oxidation for producing acrylic acid (AA) was improved because the catalyst showed an increase in both the activity and AA selectivity. The activity was increased due to an increase in the surface area and in the oxygen mobility. The AA selectivity was increased due to a decrease in the number of acidic sites, which were responsible for the unselective and deep oxidation of intermediates. The catalyst performance was degraded when excess amounts of DCCA were used in the preparation step because the surface area decreased, due to the residues of DCCA, and the surface density of acidic sites increased.

Preparation of Cu-SiO2 composite aerogel by ambient drying and the influence of synthesizing conditions on the structure of the aerogel

A copper-doped silica composite aerogel with high specific surface area was prepared using a sol-gel method at ambient pressure. A drying control chemical additive (DCCA) N,N-dimethylformamide (DMF) was introduced to the composite sol of tetraethyl orthosilicate (TEOS) and copper nitrate (Cu(NO3)2·3H2O) during the synthesizing process. The influence of the preparation conditions including Cu loading, catalyst concentration and heat treatment on the structure of copper-doped silica was investigated. The results showed that the obtained aerogel particles were uniformly distributed. The pore diameter was in a range of 2 to 15 nm. Heavier Cu loading benefited the formation of CuO crystalline, and reduced the specific surface area and pore diameter. When the catalyst concentration was high, the aggregation of Si-O network was reduced with the increase of it. The composite aerogel exhibited a good thermal stability after the heat treatment at high temperature.

Fluorescence enhancement in rare earth doped sol-gel glass by [formula omitted] dimethylformamide as a drying control chemical additive

Studies of terbium fluorescence intensity as a function of annealing temperature reveal the cause of increased fluorescence yields observed in rare earth doped sol-gel silicates prepared using N,N-dimethylformamide (DMF) as a drying control chemical additive (DCCA). While gels prepared with DMF actually have lower fluorescence yields than gels prepared without DMF at lower annealing temperatures, DMF gels can be annealed at much higher temperatures while retaining high optical quality. At these higher temperatures, terbium fluorescence yields increase dramatically as the sol-gel network undergoes densification, closing the pores of the network and eliminating the fluorescence quenching silanols on pore surfaces. DMF is therefore found to enhance the fluorescence properties of rare earth sol-gel glasses by reducing micro-fracturing and facilitating network densification. Further investigations are underway to determine the effectiveness of other promising DCCAs, such as glycerol, and to explore the possibility of exploiting the solubility properties of DCCAs to improve rare earth dopant dispersion.

Synthesis of Zirconia Aerogels by Ambient Pressure Drying with Propylene Oxide Addition

ZrO_2 aerogels were successfully synthesized by the sol-gel process and ambient pressure drying using ZrO(NO_3)_2·5H_2O as a precursor,1,2-propylene oxide(PO) as a gelation agent,and formamide (FA) as a drying control chemical additive(DCCA).The prepared zirconia aerogel samples were characterized using differential thermal analysis(DTA),scanning electron microscopy(SEM),and nitrogen adsorption/desorption analysis.The results showed that the zirconia aerogels prepared by the propylene oxide addition method had a nanoscale porous network structure,as well as supercritical dried ones,with a low bulk density(202.08 kg·m~(-3)) and a high surface area(645.0 m~2·g~(-1)).Propylene oxide can induce gelation through its nucleophilic property and its irreversible ring-opening reaction,and thus the sol-gel process and the state of gel can be controlled.

Effects of types of drying control chemical additives on the morphologies and electrochemical properties of Li 4Ti 5O 12 anode powders prepared by spray pyrolysis

The various types of drying control chemical additives are applied to control the morphological and electrochemical characteristics of the Li 4Ti 5O 12 powders prepared by spray pyrolysis. The Li 4Ti 5O 12 powders obtained from the spray solutions with formamide, N, N-dimethylformamide, N-methylacetamide and 1,4-dioxane have dense structures and fine sizes. On the other hand, the Li 4Ti 5O 12 powders obtained from the spray solution with glycerol have hollow structure and large size. The Li 4Ti 5O 12 powders obtained from the spray solutions with formamide, N, N-dimethylformamide and 1,4-dioxane have high initial discharge capacities of 167, 166 and 163 mAh/g at a constant current density of 0.1 C. The discharge capacity of the Li 4Ti 5O 12 powders obtained from the spray solution with formamide is 95% of the initial discharge capacity after 60 cycles.

Electrochemical properties of LiNi0.8Co0.2−xAlxO2 (0≤x≤0.1) cathode particles prepared by spray pyrolysis from the spray solutions with and without organic additives

Fine-sized LiNi0.8Co0.2−xAlxO2 (0≤x≤0.1) cathode particles were prepared by spray pyrolysis from the spray solutions with and without organic additives. Citric acid, ethylene glycol, and Drying Control Chemical Additive (DCCA) were used as organic additives and improved the morphologies and electrochemical properties of the cathode particles. The LiNi0.8Co0.2−xAlxO2 (0≤x≤0.1) cathode particles obtained from the spray solutions with organic additives were of micro size and had slightly aggregated morphologies. The initial discharge capacities of the LiNi0.8Co0.2−xAlxO2 (0≤x≤0.1) cathode particles obtained from the spray solutions without organic additive changed from 169 mAhg−1 to 190 mAhg−1 when the x changed from 0 to 0.1. However, the initial discharge capacities of the cathode particles obtained from the spray solutions with organic additives changed from 196 mAhg−1 to 218 mAhg−1. The initial discharge capacity of the LiNi0.8Co0.15Al0.05O2 cathode particles obtained from the spray solution with organic additives was maintained after the 20th cycle at a current density of 0.1 C.

Synthesis of SiO2–SnO2 gels in water free conditions

A series of SiO2–SnO2 samples with various Sn/Si molar ratios (0.05–1.0) have been synthesized by the sol–gel technique from (Tetraethylorthosilicate) TEOS and Sn(CH3COO)4 precursors in water free conditions. The synthesis applied allowed obtaining the final product in monolithic (nonfractured upon drying) form with no use of drying control chemical additives. All samples are characterized by thermal analysis, XRD, and FTIR. The low temperature nitrogen adsorption measurements indicate the presence of both micro and mesopores. The samples containing less than 20 wt% of SnO2 show much higher surface area than SiO2 gel. The appearance of new bands at 1,048 and 882 cm−1 in the FTIR spectra could be related to stretching vibrations of the three dimensional Si–O–Sn network, which suggests that tin component has replaced silicon atoms in Si–O–Si structure.