Visible,near-infrared and infrared optical properties of silica aerogels

Silica aerogel is an excellent thermal insulation material with a low thermal conductivity and a high porosity and has attracted great concern in applications. This paper was to experimentally investigate the optical properties of optically thick silica aerogel in the visible, near-infrared and infrared spectrum region. The fiber-loaded silica aerogel sample was prepared through sol–gel technique and supercritical drying process. Silica fibers were added into the aerogel during the preparation procedure to strength the skeleton of aerogel. As a comparison with the fiber-load silica aerogel, a silica fiber composite sample with the same chemical component and different physical structure was also prepared. A simplified two-flux model neglecting the boundary effect was used to describe the radiation propagation characteristics inside the samples. The spectral normal-hemispherical reflectances, transmittances, and normal emittances of silica aerogel and silica fiber samples were measured and compared in the wavelengths of 0.38–15 μm. Then the spectral optical constants of samples were determined using the experimental data. The spectral absorption and scattering coefficients of silica aerogel were within (0.01 cm⁻¹, 31.0 cm⁻¹) and (1.4 cm⁻¹, 25.8 cm⁻¹). The results showed that the spectrum region where the scattering coefficient is low usually corresponds to a high absorption coefficient. In addition, the total radiation properties of samples were predicted at high temperatures. The analysis of optical properties of silica aerogel is necessary to provide valuable data in applications.     

Lightweight and thermally insulating aerogel glass materials

Glass represents an important and widely used building material, and crucial aspects to be addressed include thermal conductivity, visible light transmittance, and weight for windows with improved energy efficiency. In this work, by sintering monolithic silica aerogel precursors at elevated temperatures, aerogel glass materials were successfully prepared, which were characterized by low thermal conductivity [k ≈ 0.17–0.18 W/(mK)], high visible transparency (T _vis ≈ 91–96 % at 500 nm), low density (ρ ≈ 1.60–1.79 g/cm³), and enhanced mechanical strength (typical elastic modulus E _r ≈ 2.0–6.4 GPa). These improved properties were derived from a series of successive gelation and aging steps during the desiccation of silica aerogels. The involved sol → gel → glass transformation was investigated by means of thermo-gravimetric analysis, scanning electron microscopy, nanoindentation, and Fourier transform infrared spectroscopy. Strategies of improving further the mechanical strength of the obtained aerogel glass materials are also discussed.     

Effect of Al particle size on the reaction kinetics and densification of TiAl intermetallics

The present investigation deals with response of the particle size of aluminum on the reactive sintering of Ti–Al intermetallics and subsequently on their reaction kinetics and densification behavior. Aluminum powders of initial average particle size of 44 μm were milled for various durations in a planetary ball mill to produce average particles sizes of 100, 28 and 7 μm. These aluminum powders of various particle sizes i.e. 100, 28 and 7 μm were mixed with titanium powder of average particles size of 44 μm in the ratio of 1:1 corresponding to the Ti–Al intermetallic composition. The reactive sintering temperatures of the mixtures were determined by DTA and the effect of change in particle Al particle size has been determined for the activation energy ofthe self-propagating reaction. The effect of Al particle size on the sintering was determined by studying density and microstructure.     

Synthesis and properties of submicron range CaSO4:Eu particles

CaSO₄:Eu with particle size in submicron range was synthesized. Radiation induced Eu³⁺↔Eu²⁺ conversion as well as thermal conversion was studied. The samples showed thermal conversion above 400 °C. However, no radiation induced conversion in submicron range particles was observed. Particles heated above 400 °C coalesce and when heated at 925 °C bigger particles of 20 μm size were formed. Optical microscopy of these particles reveals red inclusion of about 5 μm inside CaSO₄ particle. It is speculated that the red inclusion is CaS:Eu²⁺.     

Influence of phosphorus on the development of nickel alloy/h-BN-based self-lubricating composites processed by powder metallurgy

The purpose of this work was to evaluate the influence of phosphorus quantity of the matrix of a nickel alloy on microstructure and hardness of nickel composite with hexagonal boron nitride – h-BN. The work was carried out using two nickel alloys with varying quantities of phosphorus (0.3 and 0.75 wt.%). The quantity of particles of solid lubricant added to the alloy was 10 vol.% of h-BN. The samples were compacted at 600 MPa and sintered at 1150?°C for 60 min in plasma. The results showed that the composite with 0.3 wt.% of phosphorus had hardness of 236 HV 0.025 (±34,25) and the composite with 0.75 wt.% of phosphorus had hardness of 326 HV 0.025 (±38,90), and formation of pellets of h-BN with a size between 50 and 100 μm and low porosity due to the formation of liquid phase of phosphide during sintering which carries the lubricant fine particles of approximately 10 μm dispersed in the microstructure during pore coalescence. As a result, the quantity of phosphorus present may influence the microstructural properties (pore number and size and distribution of particles of solid lubricant) and hardness which are so important for the development of self-lubricating composites, particularly, the nickel alloy/h-BN composites.     

An investigation into the influence of zinc precursor on the microstructural,photoluminescence, and gas-sensing properties of ZnO nanoparticles

Carbon/silica-nanostructured microfibers were synthesized via electrospinning method using phenol-formaldehyde resin and tetraethyl orthosilicate as carbon and silica precursor with triblock copolymer Pluronic P123 as soft template. The prepared samples show uniform microfiber structure with ~1 μm in diameter and dozens of microns in length. Additionally, the mesopores in the material is about 2–6 nm. When the silica component was removed by HF, the porous carbon microfibers (PCMFs) were obtained. In addition, after the carbon/silica composites were calcined in air, the porous silica microfibers (PSiMFs) were obtained, revealing the converse porous nanostructure as PCMFs. It is a simple way to prepare PCMFs and PSiMFs with silica and carbon as the template to each other. Additionally, PCMFs possess an ultrahigh specific surface area (2,092 m² g^?1) and large pore volume. The electrochemical performance of the prepared PCMF material was investigated in 6.0 M KOH electrolyte. The PCMF electrode exhibits a high specific capacitance (252 F g^?1 at 0.5 A g^?1). Then, superior cycling stability (97 % retention after 4,000 cycles) mainly is due to its unique nanostructure.     

Synthesis and Characterization of Magnetic Polymer Microspheres

In this study, amino group–modified magnetic polymer microspheres were synthesized in a well‐shaped spherical form with a size range of 0.6 μm to 0.8 μm by mini‐emulsion polymerization. The morphology, composition, and thermal properties of the magnetic polymer microspheres were characterized by infrared (IR) spectra, transmission electron microscopy (TEM) photographs, particle size analysis, and thermogravimetric analysis (TGA) spectra. The results demonstrate that acrylic acid used for stabilizing the reaction system greatly affects the particle size, magnetite contents, and thermal stability of magnetic polymer microspheres.     

Particle size effect on thermal conductivity of AlN films with embedded diamond particles

Fabricating composite thin films is an effective and economic solution to improve the thermal performance of the films. The diamond particles of different sizes were successfully embedded in AlN thin films by a chemical solution approach, which was confirmed by scanning electron microscope, x-ray diffraction analysis and x-ray photoelectron spectroscopy. The thermal properties of the films embedded with different diamond particles were studied by using a 3-omega method, which was observed to be strongly dependent on the particle size. A 19 % enhancement in thermal conductivity can be achieved by embedding diamond particles of 1-μm radius in AlN thin films. However, the thermal conductivity decreases after embedding with 10-nm radius diamond particles. The results are discussed with high volume model, which confirms that the interface thermal resistance between the embedded materials and the films plays an important role in determining the thermal conductivity of the as-grown carbon material embedded AlN films.     

Effect of particle size in fluidization of polyethylene particle mixtures on the extent of bed electrification and wall coating

In this work the effects of polyethylene fluidizing particle size (smaller than 400 μm) on the degree of fluidized bed electrification and wall coating formation was studied. Experiments were conducted in a stainless steel, 0.15 m diameter column, under ambient conditions. Polyethylene resin as received (20–1500 μm) as well as mono-sized and binary mixture of large (600–710 μm) and small (212–300 & 300–425 μm) polyethylene particles were fluidized while their mass, net specific charge and size distribution in the bulk of the bed and the wall coating were measured. For the binary mixture the fraction of the small particles examined were 5%–10% and 20%. The extent of wall coating varied between different cases tested with the mono-sized large particles resulting in the most amount coating. It was found that as the fraction of the small particles in the binary mixture was increased, these particles formed majority of the wall coating. At the mass fraction of 20%, the extent of wall coating and its net specific charge were similar to that of resin as received. Overall results implied that the magnitude of the smaller sized particles within the resin played an important role in the degree of particles electrostatic charging and the extent of the particles adhesion to the column wall. Small particles were found to generate a much larger net specific charge which although resulted in them coating the column wall but prevented the coating layer growth.     

Mg粉粒度对MgB2超导体宏观特征和显微结构的影响

研究了Mg粉粒度对MgB2超导体宏观特征和显微结构的影响规律。将不同粒度的Mg粉和B粉分别按1.03∶2的比例混合、压制成型后,在流通高纯氩气的条件下于800℃烧结1h,制备出MgB2块材。利用阿基米德方法、X射线衍射仪和扫描电子显微镜等分别测试样品的密度、物相组成和显微结构等。结果表明,Mg粉粒度对MgB2超导体的物相组成没有影响;随着Mg粉粒度的增加,MgB2超导体的最大气孔尺寸、质量损失率、体积膨胀系数都逐渐增大,但其密度逐渐降低。     

Polyimide/hollow silica sphere hybrid films with low dielectric constant

Abstract

Polyimide (PI)/hollow silica (HS) sphere hybrid films with low dielectric constant values (low-k) were synthesized via thermal imidization process using pyromellitic dianhydride (PMDA)/4,4′-oxydianiline (ODA) as the polymer matrix and HS spheres as inorganic particles with the closed air voids. The monodispersed HS spheres were synthesized via a one-step process, which means that the formation of silica shells and dissolution of the core particles (polystyrene particles) occurs in the same medium. The HS particles have uniform size of ca. 1.5 μm in diameter and ca. 100 nm in shell thickness. PI/HS sphere hybrid films synthesized using mixture of polyamic acid (PAA) and HS spheres prepared via one-pot process, which means that the production of PAA and HS spheres mixture occurs with the polymerization of PMDA and ODA in the same bottle. HS spheres of two different kinds (pristine HS spheres (PHS spheres) and amine-modified HS spheres (AHS spheres)) were used for the preparation of the hybrid films. With the varying contents of AHS spheres in the range of 1–10 wt%, the dielectric constants of the PI/AHS sphere hybrid films were reduced from 3.1 of pure PI to 1.81 by incorporating 5 wt% AHS. The dielectric constants of the PI/PHS sphere hybrid films were reduced to 1.86 by incorporating 5 wt% PHS. Organic–inorganic hybrid porous polyimides may be expected as prime candidates for polymeric insulators due to their high thermal stability, good mechanical properties, solvent resistance, and low-k.     

Effects of pore topology and iron oxide core on doxorubicin loading and release from mesoporous silica nanoparticles

Mesoporous silica nanoparticles (MSNs) have a network of pores that give rise to extremely high specific surface areas, making them attractive materials for applications such as adsorption and drug delivery. The pore topology can be readily tuned to achieve a variety of structures such as the hexagonally ordered Mobil Crystalline Material 41 (MCM-41) and the disordered “wormhole” (WO) mesoporous silica (MS) structure. In this work, the effects of pore topology and iron oxide core on doxorubicin loading and release were investigated using MSNs with pore diameters of approximately 3 nm and sub-100 nm particle diameters. The nanoparticles were loaded with doxorubicin, and the drug release into phosphate-buffered saline (PBS, 10 mM, pH 7.4) at 37 °C was monitored by fluorescence spectroscopy. The release profiles were fit using the Peppas model. The results indicated diffusion-controlled release for all samples. Statistically significant differences were observed in the kinetic host–guest parameters for each sample due to the different pore topologies and the inclusion of an iron oxide core. Applying a static magnetic field to the iron oxide core WO-MS shell materials did not have a significant impact on the doxorubicin release. This is the first time that the effects of pore topology and iron oxide core have been isolated from pore diameter and particle size for these materials.

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Graphical abstract Comparison of the release of doxorubicin from WO and iron oxide@WO particles into PBS

     

Single particle size and fluorescence spectra from emissions of burning materials in a tube furnace to simulate burn pits

A single-particle fluorescence spectrometer (SPFS) and an aerodynamic particle sizer were used to measure the fluorescence spectra and particle size distribution from the particulate emissions of 12 different burning materials in a tube furnace to simulate open-air burning of garbage. Although the particulate emissions are likely dominated by particles <1 μm diameter, only the spectra of supermicron particles were measured here. The overall fluorescence spectral profiles exhibit either one or two broad bands peaked around 300–450 nm within the 280–650 nm spectral range, when the particles are illuminated with a 263-nm laser. Different burning materials have different profiles, some of them (cigarette, hair, uniform, paper, and plastics) show small changes during the burning process, and while others (beef, bread, carrot, Styrofoam, and wood) show big variations, which initially exhibit a single UV peak (around 310–340 nm) and a long shoulder in visible, and then gradually evolve into a bimodal spectrum with another visible peak (around 430–450 nm) having increasing intensity during the burning process. These spectral profiles could mainly derive from polycyclic aromatic hydrocarbons with the combinations of tyrosine-like, tryptophan-like, and other humic-like substances. About 68 % of these single-particle fluorescence spectra can be grouped into 10 clustered spectral templates that are derived from the spectra of millions of atmospheric aerosol particles observed in three locations; while the others, particularly these bimodal spectra, do not fall into any of the 10 templates. Therefore, the spectra from particulate emissions of burning materials can be easily discriminated from that of common atmospheric aerosol particles. The SFFS technology could be a good tool for monitoring burning pit emissions and possibly for distinguishing them from atmospheric aerosol particles.     

Layering laser-assisted sintering of functional graded porous PZT ceramoplasts

Within the scope of the present study, the three-dimensional (3D) samples from poly(vinylidene fluoride) plus lead zirconate titanate (PVDF+PZT) and silica+PZT powdered compositions were successfully prepared by the selective laser sintering (SLS) process. The optimal regimes for the layer-by-layer fabrication of 3D samples were determined both for wavelengths of 10.6 and 1.06 μm. The sample structure and element composition were characterized by the use of the scanning electronic microscope in combination with the energy dispersive X-ray spectrometry (EDX) analysis. It was shown that after the SLS process, the initial perovskite phase did not undergo any significant structural changes. The results of characterization tests testified to the decease of the sample density and opened porosity by the 2–5 times as compared to the solid PZT that seems to be useful for acoustic applications. A comparative estimate of the PVDF destruction process was fulfilled for the laser wavelength of 10.6 and 1.06 μm, by using infrared spectroscopy, photo calorimetrical and chromatography measurements, sol-gel and viscosity analysis. It was found that the structuring velocity had diametrically opposite behavior for these wavelengths: it grows with the laser power increase for λ = 10.6 μm and, vice versa, it falls for λ = 1.06 μm.     

Transition metal doping of amorphous silica particles

Amorphous, homogeneously doped particles with 1–10 atomic percent (at%) nickel or zinc, smaller than 100 nm, were synthesized. Additionally, 20 at% nickel and zinc-doped silica particles were synthesized, although the particle size was > 300 nm. The coordination state of the metal in solution did not change with the ammonia or water concentrations used. Particle size was found to decrease with both increasing water and ammonia hydroxide concentrations. The NH₄OH concentrations of 4.5 and 6.5 M used allowed the formation of ammine–transition metal complexes and the zeta potential to remain in a stable range, allowing for spherical, nearly monodisperse particle formation at high metal dopant concentrations.     

Thermal and Flammability Properties of Polypropylene/Silica Aerogel Composites

Polypropylene (PP)/silica aerogel (SA) composites were prepared and their thermal and flammability properties were studied. The PP/SA composites with different weight percent were prepared via melt compounding method using an internal mixer. Their morphology, thermal conductivity, thermal stability and combustion behavior were characterized. The SEM images confirmed the homogenous mixing of the components. The measurement of the thermal conductivities of samples indicated that PP would be a better thermal barrier in the presence of SA. The thermal gravimetric analysis results showed that combining the silica aerogel particles into polypropylene increased the decomposition temperature. The resultant composites displayed improved flame retardancy with a significant reduction in the peak heat release rate and increase of limited oxygen index value. It can be concluded that the flame retardant mechanism of PP/SA composites is associated with two decisive factors: a coat-like char effect and a physical crosslinking effect.     

Core-loss reduction of Fe–Si–Cr crystalline alloy according to particle size in the high frequency band

We used different sizes of gas atomized Fe–Si–Cr alloy powder to produce soft magnetic composites (SMCs), this alloy has higher resistivity than existing materials used in SMCs. These powders were prepared by sieving raw materials which had an average size from less than 25 μm to over 63 μm. Our experiments show that as particle size decreases, the magnetic saturation tends to increase, the sample made from the powder with particles 25–38 μm in size recorded the highest magnetic saturation of 169.38 emu/g. Additionally, as particle size decreased, permeability increased. The sample made from powder with particles under 25 μm had a permeability of 20.7 H/m at 1 MHz. Also, the relationship between particle size and quality factor was found to be inversely proportional. Finally, the minimum core-loss was 187.26 kW/m³ at 1 MHz for the sample made from powder whose constituent particles are under 25 μm.     

An Assessment of the Use of Mesoporous Silica Materials to Improve Pulsed Dipolar Spectroscopy

Protein immobilization in mesoporous silica nanoparticles has attracted much attention due to its wide range of applications. However, it remains largely unexplored how the use of mesopores can alter the spatial distribution of encapsulated biomolecules so as to improve pulsed dipolar spectroscopy sensitivity. Here, we performed electron spin resonance measurements for three different spin-labeled biomolecules (including two different peptides and a protein) encapsulated in various types of mesoporous materials differing in textural properties such as nanochannel length (e.g., 0.2–4 μm) and average pore diameter (e.g., 6–11 nm, approximately). Our results show that biomolecules are clustered somewhat upon the encapsulation into mesopores, and that due to the clustering, instantaneous diffusion plays an important role in the spin relaxation in nanochannels. The extent of molecular clustering exhibits a clear positive correlation with the length of nanochannels, whereas it shows little correlation with pore diameters. Among the materials studied, mesoporous materials with the shortest length of nanochannels are most effective to reduce spin clustering, thus suppressing the unwanted instantaneous diffusion and enhancing spin–spin relaxation time. This study has opened a possibility of improving the quality of pulsed dipolar spectroscopy with mesoporous silica nanoparticles.     

Ten-gram-scale preparation of PTMS-based monodisperse ORMOSIL nano- and microparticles and conversion to silica particles

Monodisperse organically modified silica (ORMOSIL) particles, with an average diameter ranging from 550 nm to 4.2 μm, were prepared at low temperature at a scale of about 10 g/batch by a simple one-step self-emulsion process. The reaction mixture was composed only of water, phenyltrimethoxysilane (PTMS), and a base catalyst, without any surfactants. The size control of the particles and the monodispersity of resultant particles were achieved through the controlled supply of hydrolyzed PTMS monomer molecules, which was enabled by manipulating the reaction parameters, such as monomer concentration, type and amount of base catalyst, stirring rate, and reaction temperature. PTMS-based ORMOSIL particles were converted into silica particles by employing either a wet chemical reaction with an oleum-sulfuric acid mixture or thermal treatment above 650 °C. Complete removal of organic groups from the ORMOSIL particles was achieved by the thermal treatment while ~?74% removal was done by the chemical process used.

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Graphical abstract ?

     

超低密度SiO2气凝胶的制备及成型研究

 以正硅酸乙酯(TEOS)为硅源，采用酸碱二步催化溶胶-凝胶法，结合超临界干燥技术制备了超低密度SiO₂气凝胶,最低密度为3.4 mg/cm³；进一步结合成型工艺，在解决了模具设计和脱模技术后制备了具有不同密度的柱状和微型套筒样品，密度10～50 mg/cm³。研究了水、催化剂、稀释剂对二步溶胶-凝胶过程的影响，获得了制备低密度SiO₂气凝胶的最佳条件。利用扫描电镜、孔径分布及比表面积测试仪等对SiO₂气凝胶微结构进行了研究。结果表明，获得的超低密度SiO₂气凝胶具有较好的纳米网络，平均孔径18.9 nm，还具有高比表面积898 9 m²/g。