Low-cost and fast synthesis of nanoporous silica cryogels for thermal insulation applications

Abstract

Nanoporous silica cryogels with a high specific surface area of 1095 m² g^?1 were fabricated using tert-butyl alcohol as a reaction solvent, via a cost-effective sol–gel process followed by vacuum freeze drying. The total time of cryogel production was reduced markedly to one day. The molar ratio of solvent/precursor, which was varied from 5 to 13, significantly affected the porous structure and thermal insulating properties of the cryogels. The silica cryogels with low densities in the range of 0.08–0.18 g cm^?3 and thermal conductivities as low as 6.7 mW (m·K)^?1 at 100 Pa and 28.3 mW (m·K)^?1 at 10⁵ Pa were obtained using this new technique.     

Thermal Conductivity of Magnesium Oxide From Absolute,Steady-State Measurements

The thermal conductivity of polycrystalline magnesium oxide has been measured over the temperature range from 400 K to 1300 K using a modified guarded-hot-plate design. Three different thicknesses of specimens having 93 % of theoretical density were tested to verify the operation, accuracy, and reproducibility of our apparatus. The measured thermal conductivity ranges from 30 W · m⁻¹ · K⁻¹ down to 8 W · m⁻¹ · K⁻¹ and has an inverse-temperature functionality. The results agree well with literature values for this material.     

Supermacroprous chitosan–agarose–gelatin cryogels: in vitro characterization and in vivo assessment for cartilage tissue engineering

The study focuses on the synthesis of a novel polymeric scaffold having good porosity and mechanical characteristics synthesized by using natural polymers and their optimization for application in cartilage tissue engineering. The scaffolds were synthesized via cryogelation technology using an optimized ratio of the polymer solutions (chitosan, agarose and gelatin) and cross-linker followed by the incubation at sub-zero temperature (−12°C). Microstructure examination of the chitosan–agarose–gelatine (CAG) cryogels was done using scanning electron microscopy (SEM) and fluorescent microscopy. Mechanical analysis, such as the unconfined compression test, demonstrated that cryogels with varying chitosan concentrations, i.e. 0.5–1% have a high compression modulus. In addition, fatigue tests revealed that scaffolds are suitable for bioreactor studies where gels are subjected to continuous cyclic strain. In order to confirm the stability, cryogels were subjected to high frequency (5 Hz) with 30 per cent compression of their original length up to 1 × 10⁵ cycles, gels did not show any significant changes in their mass and dimensions during the experiment. These cryogels have exhibited degradation capacity under aseptic conditions. CAG cryogels showed good cell adhesion of primary goat chondrocytes examined by SEM. Cytotoxicity of the material was checked by MTT assay and results confirmed the biocompatibility of the material. In vivo biocompatibility of the scaffolds was checked by the implantation of the scaffolds in laboratory animals. These results suggest the potential of CAG cryogels as a good three-dimensional scaffold for cartilage tissue engineering.     

A Double-Primary Dead-Weight Tester for Pressures (35–175) kPa in Gage Mode

Primary pressure standards in the atmospheric pressure range are often established using mercury manometers. Less frequently, controlled-clearance dead-weight testers in which one component (normally the piston) has been dimensionally measured have also been used. Recent advances in technology on two fronts i) the fabrication of large-diameter pistons and cylinders with good geometry; and ii) the ability to measure the dimensions of these components, have allowed some dead-weight testers at NIST to approach total relative uncertainties (k = 2) in dimensionally-derived effective areas near 5 × 10⁻⁶. This paper describes a single piston/cylinder assembly (NIST-PG201WC/WC) that serves as both a primary gage in which both piston and cylinder are measured dimensionally and a controlled-clearance primary gage (employing the Heydemann-Welch method). Thus it allows some previous assumptions about the modeling of dead-weight testers to be checked. For the gage described in this paper the piston/cylinder clearance obtained from the two analyses have relative differences of 4 × 10⁻⁶ to 7 × 10⁻⁶ over the pressure range 35 kPa to 175 kPa. Some implications of these results will be discussed. From the dimensional characterizations and auxiliary measurements we have determined that the effective area for this gauge at 20 °C is: A_eff,20 = 1961.0659mm²(1 + 3.75 × 10^?12P/Pa + 3.05 × 10^?12P_J/Pa), where P is the system pressure and P_J is a control pressure. The estimated relative uncertainty in effective area is 8.2 × 10⁻⁶ +1.4 × 10⁻¹¹ P/Pa (k = 2). The temperature coefficient for the area was measured and found to be (9.06 ± 0.04) × 10⁻⁶/K. Thus using the gage at a reference temperature of 23 °C yields an effective area: A_eff,23 = 1961.1192mm²(1 + 3.75 × 10^?12P/Pa + 3.05 × 10^?12P_J/Pa), with almost no increase in the uncertainty over that at 20 °C.     

Thermally conductive polyamide 6/carbon filler composites based on a hybrid filler system

We explored the use of a hybrid filler consisting of graphite nanoplatelets (GNPs) and single walled carbon nanotubes (SWCNTs) in a polyamide 6 (PA 6) matrix. The composites containing PA 6, powdered GNP, and SWCNT were melt-processed and the effect of filler content in the single filler and hybrid filler systems on the thermal conductivity of the composites was examined. The thermal diffusivities of the composites were measured by the standard laser flash method. Composites containing the hybrid filler system showed enhanced thermal conductivity with values as high as 8.8 W (m · K)⁻¹, which is a 35-fold increase compared to the thermal conductivity of pure PA 6. Thermographic images of heat conduction and heat release behaviors were consistent with the thermal conductivity results, and showed rapid temperature jumps and drops, respectively, for the composites. A composite model based on the Lewis–Nielsen theory was developed to treat GNP and SWCNT as two separate types of fillers. Two approaches, the additive and multiplicative approaches, give rather good quantitative agreement between the predicted values of thermal conductivity and those measured experimentally.     

Thermal-Conductivity Apparatus for Steady-State,Comparative Measurement of Ceramic Coatings

An apparatus has been developed to measure the thermal conductivity of ceramic coatings. Since the method uses an infrared microscope for temperature measurement, coatings as thin as 20 μm can, in principle, be measured using this technique. This steady-state, comparative measurement method uses the known thermal conductivity of the substrate material as the reference material for heat-flow measurement. The experimental method is validated by measuring a plasma-sprayed coating that has been previously measured using an absolute, steady-state measurement method. The new measurement method has a relative standard uncertainty of about 10 %. The measurement of the plasma-sprayed coating gives 0.58 W·m⁻¹·K^−l which compares well with the 0.62 W·m⁻¹·K^−l measured using the absolute method.     

Amino‐functionalised mesoporous silica microspheres for immobilisation of Candida antarctica lipase B – application towards greener production of 2,5‐furandicarboxylic acid

In the present study, amino‐functionalised mesoporous silica microspheres were utilised as support for the covalent immobilisation of Candida antarctica lipase B (CaLB) for the subsequent production of 2,5‐furandicarboxylic acid (FDCA) from 2,5‐diformylfuran (DFF). Under the optimised operating conditions of pH 6.5, particle/enzyme ratio of 1.25:1.0 and glutaraldehyde concentration of 4 mM, a maximum CaLB immobilisation yield of 82.4% on silica microspheres was obtained in 12.25 h. The immobilised CaLB was used for the synthesis of alkyl esters, which were utilised along with hydrogen peroxide for FDCA synthesis. The biocatalytic conversion of 30 mM DFF dictated a 77–79% FDCA in 48 h at 30°C; where the turnover number and turnover frequency of immobilised CaLB were 6220.73 mol mol⁻¹ and 129.59 h⁻¹, respectively, for ethyl acetate, against 6297.65 mol mol⁻¹ and 131.2 h⁻¹, respectively, for ethyl butyrate. Upon examining the operational stability, the immobilised CaLB exhibited high stability till five cycles of FDCA production.Inspec keywords: mesoporous materials, organic compounds, biotechnology, silicon compounds, renewable materials, catalysis, catalysts, enzymes, thermal stability, biofuelOther keywords: FDCA production, amino‐functionalised mesoporous silica microspheres, greener production, 2,5‐furandicarboxylic acid, covalent immobilisation, 2,5‐diformylfuran, CaLB immobilisation, Candida antarctica lipase B immobilisation, ethyl butyrate, time 48.0 hour, temperature 30.0 degC, time 12.25 hour     

Preparation and Calibration of Carrier-Free 209Po Solution Standards

Carrier-free ²⁰⁹Po solution standards have been prepared and calibrated. The standards, which will be disseminated by the National Institute of Standards and Technology as Standard Reference Material SRM 4326, consist of (5.1597 ±0.0024) g of a solution of polonium in nominal 2 mol · L⁻¹ hydrochloric acid (having a solution density of (1.031±0.004) g · mL⁻¹ at 22 °C) that is contained in 5 mL flame-sealed borosilicate glass ampoules, and are certified to contain a ²⁰⁹Po alpha-particle emission rate concentration of (85.42±0.29) s⁻¹ · g⁻¹ (corresponding to a ²⁰⁹Po activity concentration of (85.83 ±0.30) Bq · g⁻¹) as of the reference time of 1200 EST 15 March 1994. The calibration was based on 4πα liquid scintillation (LS) measurements with two different LS counting systems and under wide variations in measurement and sample conditions. Confirmatory measurements by 2πα gas-flow proportional counting were also performed. The only known radionuclidic impurity, based on α- and photon-emission spectrometry, is a trace quantity of ²⁰⁸Po. The ²⁰⁸Po to ²⁰⁹Po impurity ratio as of the reference time was 0.00124 ±0.00020. All of the above cited uncertainty intervals correspond to a combined standard uncertainty multiplied by a coverage factor of k = 2. Although ²⁰⁹Po is nearly a pure α emitter with only a weak electron capture branch to ²⁰⁹Bi, LS measurements of the ²⁰⁹Po a decay are confounded by an a transition to a 2.3 keV (J^π= 1/2⁻) level in ²⁰⁵Pb which was previously unknown to be a delayed isomeric state.     

Mechanochemically synthesized CsH2PO4–H3PW12O40 composites as proton-conducting electrolytes for fuel cell systems in a dry atmosphere

Cesium dihydrogen phosphate (CsH₂PO₄, CDP) and dodecaphosphotungstic acid (H₃PW₁₂O₄₀·nH₂O, WPA·nH₂O) were mechanochemically milled to synthesize CDP–WPA composites. The ionic conductivities of these composites were measured by an ac impedance method under anhydrous conditions. Despite the synthesis temperatures being much lower than the dehydration and phase-transition temperatures of CDP under anhydrous conditions, the ionic conductivities of the studied composites increased significantly. The highest ionic conductivity of 6.58×10⁻⁴ Scm⁻¹ was achieved for the 95CDP·5WPA composite electrolyte at 170 °C under anhydrous conditions. The ionic conduction was probably induced in the percolated interfacial phase between CDP and WPA. The phenomenon of high ionic conduction differs for the CDP–WPA composite and pure CDP or pure WPA under anhydrous conditions. The newly developed hydrogen interaction between CDP and WPA supports anhydrous proton conduction in the composites.     

Electronic conduction in La-based perovskite-type oxides

A systematic study of La-based perovskite-type oxides from the viewpoint of their electronic conduction properties was performed. LaCo_0.5Ni_0.5O_3±δ was found to be a promising candidate as a replacement for standard metals used in oxide electrodes and wiring that are operated at temperatures up to 1173 K in air because of its high electrical conductivity and stability at high temperatures. LaCo_0.5Ni_0.5O_3±δ exhibits a high conductivity of 1.9 × 10³ S cm⁻¹ at room temperature (R.T.) because of a high carrier concentration n of 2.2 × 10²² cm⁻³ and a small effective mass m∗ of 0.10 m_e. Notably, LaCo_0.5Ni_0.5O_3±δ exhibits this high electrical conductivity from R.T. to 1173 K, and little change in the oxygen content occurs under these conditions. LaCo_0.5Ni_0.5O_3±δ is the most suitable for the fabrication of oxide electrodes and wiring, though La_1−xSr_xCoO_3±δ and La_1−xSr_xMnO_3±δ also exhibit high electronic conductivity at R.T., with maximum electrical conductivities of 4.4 × 10³ S cm⁻¹ for La_0.5Sr_0.5CoO_3±δ and 1.5 × 10³ S cm⁻¹ for La_0.6Sr_0.4MnO_3±δ because oxygen release occurs in La_1−xSr_xCoO_3±δ as elevating temperature and the electrical conductivity of La_0.6Sr_0.4MnO_3±δ slightly decreases at temperatures above 400 K.     

Calcium Fluoride Precipitation and Deposition From 12 mmol/L Fluoride Solutions With Different Calcium Addition Rates

The effects of different Ca-addition rates on calcium fluoride (CaF₂) precipitation and deposition were investigated in 12 mmol/L sodium fluoride solutions to which 0.1 mol/L calcium chloride solution was continuously added at average rates of (5, 7.5, 10, 12.5, 15 or 20) mmol L⁻¹ min⁻¹. The changes in ionic fluoride and calcium concentrations, as well as turbidity, were continuously recorded by F and Ca electrodes, and a fiber optic based spectrophotometer, respectively. The F⁻ concentration decreased and turbidity increased with time indicating precipitation of CaF₂. For the systems with Ca-addition rates of (5, 7.5, 10, 12.5, 15, and 20) mmol L⁻¹ min⁻¹, the 1 min CaF₂ depositions in the model substrate (cellulose filter paper, pores 0.2 µm) expressed as mean ± SD of deposited F per substrate surface area were (3.78 ± 0.31, 11.45 ± 0.89, 9.31 ± 0.68, 8.20 ± 0.56, 6.63 ± 0.43, and 2.09 ± 0.28) µg/cm², respectively (n = 10 for each group). The 1-min F depositions did not show positive correlation to Ca-addition rates. The lowest 1-min F deposition was obtained in the systems with the highest Ca-addition rate of 20 mmol L⁻¹ min⁻¹ for which CaF₂ precipitation rate reached the maximum value of 0.31 mmol L⁻¹ s⁻¹ almost immediately after beginning of reaction (6 s). The largest 1-min F depositions were obtained from the systems with Ca addition rates of (7.5 to 12.5) mmol L⁻¹ min⁻¹ in which CaF₂ precipitation rates continuously increased reaching the maximum values of (0.13 to 0.20) mmol L⁻¹ s⁻¹ after (18 to 29) s, respectively. The 1-min F depositions were greatly enhanced in comparison with the control F solutions that did not have continuous Ca-addition. This indicates that continuous Ca addition that controls the rate of CaF₂ formation could be a critical factor for larger F depositions from F solutions. The efficacy of conventional F mouthrinses could be improved with addition of a substance that continuously releases Ca.     

A new route for preparation of sodium-silicate-based hydrophobic silica aerogels via ambient-pressure drying

An in-depth investigation into the synthesis of hydrophobic silica aerogels prepared by the surface derivatization of wet gels followed by subsequent drying at ambient pressure is reported. The following sol–gel parameters were examined for their effect on the physical properties of the derived aerogels: number of gel washings with water, percentage of hexane or methanol in silylating mixture, molar ratio of tartaric acid: Na₂SiO₃, gel aging period, weight% of silica, trimethylchlorosilane (TMCS) percentage, and silylation period. These parameters were varied from 1 to 4, 0 to 100%, 0.27 to 1.2, 0 to 4 h, 1.5 to 8 wt.%, 20 to 40% and 6 to 24 h, respectively. The properties of hydrophobic silica aerogels synthesized by this new route were investigated in terms of bulk density, percentage volume shrinkage, percentage porosity, thermal conductivity and contact angle with water, and by Fourier transform infrared spectroscopy (FTIR). The as-prepared hydrophobic silica aerogels exhibited high temperature stability (up to approximately 435 °C) as measured by thermogravimetric/differential thermal analysis (TGA-DTA). The optimal sol-gel parameters were found to be a molar ratio of Na₂SiO₃:H₂O : tartaric acid : TMCS of 1 : 146.67 : 0.86 : 9.46, an aging period of 3 h, four washings with water in 24 h and the use of a 50% hexane- or methanol-based silylating mixture. Aerogels prepared with these optimal parameters were found to exhibit 50% optical transparency in the visible range, 84 kg m⁻³ density, 0.090 W mK⁻¹ thermal conductivity, 95% porosity and a contact angle of 146° with water.     

Preparation of macroporous zirconia monoliths from ionic precursors via an epoxide-mediated sol-gel process accompanied by phase separation

Monolithic macroporous zirconia (ZrO₂) derived from ionic precursors has been successfully fabricated via the epoxide-mediated sol-gel route accompanied by phase separation in the presence of propylene oxide (PO) and poly(ethylene oxide) (PEO). The addition of PO used as an acid scavenger mediates the gelation, whereas PEO enhances the polymerization-induced phase separation. The appropriate choice of the starting compositions allows the production of a macroporous zirconia monolith with a porosity of 52.9% and a Brunauer–Emmett–Teller (BET) surface area of 171.9 m² · g⁻¹. The resultant dried gel is amorphous, whereas tetragonal ZrO₂ and monoclinic ZrO₂ are precipitated at 400 and 600 °C, respectively, without spoiling the macroporous morphology. After solvothermal treatment with an ethanol solution of ammonia, tetragonal ZrO₂ monoliths with smooth skeletons and well-defined mesopores can be obtained, and the BET surface area is enhanced to 583.8 m² · g⁻¹.     

Neutron Lifetime Experiment Based on an Accordion-Like UCN Storage Volume Coated With “Low Temperature Fomblin”

A new type of per-fluorinated polymer, “Low Temperature Fomblin,” has been tested as a wall coating in an ultracold neutron (UCN) storage experiment using a gravitational storage system. The data show a UCN reflection loss coefficient η as low as ≈ 5 × 10⁻⁶ in the temperature range 105 K to 150 K. We plan to use this oil in a new type of neutron lifetime measurement, where a bellows system (“accordion”) enables to vary the trap size in a wide range while the total surface area and distribution of surface area over height remain constant. These unique characteristics, in combination with application of the scaling technique developed by W. Mampe et al. in 1989, ensure exact linearity for the extrapolation from inverse storage lifetimes to the inverse neutron lifetime. Linearity holds for any energy dependence of loss coefficient µ(E). Using the UCN source at the Institut Laue Langevin we expect to achieve a lifetime precision below ±1 s.     

Biosynthesis of gold nanoparticles using fungus Trichoderma sp. WL‐Go and their catalysis in degradation of aromatic pollutants

An efficient green method of gold nanoparticles (AuNPs) biosynthesis was achieved by cell‐free extracts of fungus Trichoderma sp. WL‐Go. Based on UV–Vis spectra, AuNPs biosynthesised by cell‐free extracts with 90 mg/l protein exhibited a characteristic absorption band at 556 nm and was stable for 7 days. Transmission electron microscopy images revealed that the as‐synthesised AuNPs were spherical and pseudo‐spherical, and the average size was calculated to be 9.8 nm with a size range of 1–24 nm. The AuNPs illustrated their good catalytic activities for reduction of nitro‐aromatics (2‐nitrophenol, 3‐nitrophenol, 4‐nitrophenol, 2‐nitroaniline, 3‐nitroaniline) with catalytic rate constants of 7.4 × 10⁻³ s⁻¹, 10.3 × 10⁻³ s⁻¹, 4.9 × 10⁻³ s⁻¹, 5.8 × 10⁻³ s⁻¹, 15.0 × 10⁻³ s⁻¹, respectively. Meanwhile, the AuNPs also showed excellent catalytic performance in decolourisation of azo dyes with decolourisation efficiency from 82.2 to 97.5%. This study provided a green gentle method for AuNPs synthesis as well as exhibiting efficient catalytic capability for degradation of aromatic pollutants.Inspec keywords: catalysts, dyes, particle size, reduction (chemical), nanobiotechnology, nanofabrication, ultraviolet spectra, gold, transmission electron microscopy, nanoparticles, proteins, catalysis, visible spectra, pollution control, microorganismsOther keywords: nitro‐aromatics, catalytic rate constants, decolourisation efficiency, green gentle method, efficient green method, gold nanoparticles biosynthesis, cell‐free extracts, UV–Vis spectra, characteristic absorption band, transmission electron microscopy images, as‐synthesised AuNPs, catalytic performance, protein, catalytic activities, efficient catalytic capability, fungus Trichoderma sp. WL‐Go, aromatic pollutants degradation, 2‐nitrophenol, 3‐nitrophenol, 4‐nitrophenol, 2‐nitroaniline, 3‐nitroaniline, azo dye decolourisation, Au     

Experimental evaluation of the viscoelasticity of porcine vitreous

This study aims to estimate the material properties of the porcine vitreous while testing it in close to its natural physiological conditions. Eighteen porcine eyes were tested within 48 h post-mortem. A custom-built computer-controlled test rig was designed to support, load and monitor the behaviour of eye globes while being subjected to dynamic rotation cycles mimicking saccade eye movement. Specimens were glued to the base of a container, surrounded by gelatin, frozen and cut in half to expose the vitreous. After thawing, the container was subjected to concentric dynamic rotations of up to 5°, 10° or 15°, while taking 50 MP photos of the specimen every 2 ms. The images were analysed by a digital image correlation algorithm to trace the movement of marked points on the vitreous surface with different radii from the centre of the posterior chamber. The initial camera image was used in building a finite-element model of the test set-up, which was used in an inverse analysis exercise to estimate the material properties of the vitreous. Angular displacements of the monitored points were up to 3.3°, 4.1° and 3.9° in response to eye rotations of 5°, 10° and 15°, respectively. With the experimental relationships between eye rotation and angular displacements used as target behaviour, the inverse analysis exercise estimated the initial shear modulus, the long-term shear modulus and the viscoelastic decay constant of the porcine vitreous as 2.10 ± 0.15 Pa, 0.50 ± 0.04 Pa and 1.20 ± 0.09 s⁻¹, respectively. Consideration of the viscoelasticity of the vitreous was essential to represent its experimental behaviour. Testing the vitreous in close to its normal physiological conditions produced estimations of the initial shear modulus and long-term shear modulus that were, respectively, smaller and larger than reported values (Zimberlin et al. 2010 Soft Matter 6, 3632–3635. (doi:10.1039/b925407b), Liu et al. 2013 J. Biomech. 46, 1321–7. (doi:10.1016/j.jbiomech.2013.02.006), Rossi et al. 2011 Invest. Ophthalmol. Vis. Sci. 52, 3994–4002. (doi:10.1167/iovs.10-6477)).     

Thermal Conductivity Measurement of an Electron-Beam Physical-Vapor-Deposition Coating

An industrial ceramic thermal-barrier coating designated PWA 266, processed by electron-beam physical-vapor deposition, was measured using a steady-state thermal conductivity technique. The thermal conductivity of the mass fraction 7 % yttria-stabilized zirconia coating was measured from 100 °C to 900 °C. Measurements on three thicknesses of coatings, 170 μm, 350 μm, and 510 μm resulted in thermal conductivity in the range from 1.5 W/(m·K) to 1.7 W/(m·K) with a combined relative standard uncertainty of 20 %. The thermal conductivity is not significantly dependent on temperature.     

Interlaboratory Comparison of Magnetic Thin Film Measurements

A potential low magnetic moment standard reference material (SRM) was studied in an interlaboratory comparison. The mean and the standard deviation of the saturation moment m_s, the remanent moment m_r, and the intrinsic coercivity H_c of nine samples were extracted from hysteresis-loop measurements. Samples were measured by thirteen laboratories using inductive-field loopers, vibrating-sample magnetometers, alternating-gradient force magnetometers, and superconducting quantum-interference-device magnetometers. NiFe films on Si substrates had saturation moment measurements reproduced within 5 % variation among the laboratories. The results show that a good candidate for an SRM must have a highly square hysteresis loop (m_r/m_s > 90 %), H_c ≈ 400 A·m⁻¹ (5 Oe), and m_s ≈ 2 × 10⁻⁷ A·m² (2 × 10⁻⁴ emu).     

Relation Between Wire Resistance and Fluid Pressure in the Transient Hot-Wire Method

The resistance of metals is a function of applied pressure, and this dependence is large enough to be significant in the calibration of transient hot-wire thermal conductivity instruments. We recommend that for the highest possible accuracy, the instrument’s hot wires should be calibrated in situ. If this is not possible, we recommend that a value of γ, the relative resistance change with pressure, of −2×10⁻⁵ MPa⁻¹ be used to account for the pressure dependence of the platinum wire’s resistance.