Interaction Between Montmorillonite and Benzidine in Aqueous Solutions IV. The Color Reaction in the Frozen State

Changes in the color of Na, Ca, and Cu-bentonite-benzidine pastes were observed for color changes upon freezing at ?5°C, ?15°C, and ?21°C. At ?5°C no color change could be detected. At ?15°C and ?21°C, when the benzidine/bentonite ratio (W/W) was in the range of approximately 0.0003 to 0.003, the pastes' colors changed from blue or pale blue to green or bluish green. Reflectance measurements of Na-bentonite-benzedine paste at ?150°C were carried out in a specially designed cell. These measurements show that in addition to the color change, the reflectivity of the frozen paste became higher upon freezing to ?150°C, presumably because of the formation of ice crystals. The explanation of the color changes is based on the assumption that the (blue) monovalent and the (yellow) divalent cations coexist in the frozen paste at different zones on the bentonite surface.     

Effect of a High-Pressure-Induced Freezing Process on the Stability of Freeze-Dried Nanocapsules

The objective of this study was to develop a freeze-drying process involving high-pressure techniques during freezing in order to optimize the stability of nanocapsules. Nanocapsules prepared by the emulsion-diffusion method were frozen at ? 20, ? 10, and ? 5°C, respectively, and dried under vacuum as a conventional freeze drying (CFd). For the high-pressure freezing process, the nanocapsules suspensions were frozen at ? 20, ? 10, and ? 5°C at 100, 200, and 300 MPa, respectively, and then dried under vacuum conditions (abbreviated as HPFd). The size and morphological analysis of freeze-dried samples were investigated by using Mastersizer®, TEM, and SEM. From these experiments, the mean nanocapsules sizes were entirely aggregated depending on increasing pressure and decreasing freezing temperature in CFd. In HPFd, the nanocapsules frozen at ? 10°C were stable depending on the increment of the pressure level. We concluded that the HPFd process was more effective for the stability of nanocapsules compared to the CFd process at freezing temperatures of ? 10°C due to high supercooling of the internal phase during the high-pressure freezing process.     

Coatings to prevent frost

The ability of hydrophobic, organic–inorganic hybrid coatings to decelerate frost propagation was investigated. Compared to a bare aluminum surface, the coatings do not significantly reduce the freezing probability of supercooled water drops. On both surfaces, the probability for ice nucleation at temperatures just below 0°C, for example at ?4°C, is low. Freezing of a single drop on aluminum leads, however, to instant freezing of the complete surface. On hydrophobic coatings, such a freezing drop is isolated; the frozen area grows slowly. At ?4°C surface temperature in a +12°C/90% relative humidity environment, on surfaces providing a water contact angle hysteresis of about 10° and receding water contact angles higher than 90°, a rate for the growth of the average radius of the frozen area of about 2 µm/s was observed. Submitting the surface to an airflow of 1 m/s led to faster frost spreading in flow direction. Although the airflow compromised the anti-ice properties to some extent, the application of the hydrophobic coating in a heat recovery ventilation experiment extended the time interval between defrosting cycles by a factor of 2.3.     

超声辅助冷冻对湿面筋蛋白中冰晶粒度分布及总水含量的影响

超声辅助食品冷冻过程是一种新的技术。本文通过对湿面筋蛋白中冰晶粒度分布及总水含量的检测,探讨超声改善冷冻食品品质的微观作用机制,为超声冷冻新技术在食品化工中的应用奠定化学工程学基础。测定了有无超声环境下冷冻的湿面筋蛋白经过不同冷冻贮藏时间后总水含量, 结果显示：随着冷冻贮藏时间的延长,超声环境下冷冻的湿面筋蛋白中总水含量比传统环境下高。用扫描电镜和压汞仪间接观察和分析了冷冻的湿面筋蛋白中冰晶的形状和粒度分布。研究表明,适宜的功率超声辅助下,冷冻过程能在湿面筋蛋白内部形成细小且分布均匀的冰晶体,减少面筋蛋白网络结构的破坏,从而改善冷冻食品的品质。     

Fabrication and properties of anisotropic polyimide aerogels with aligned tube-like pore structure

Polyimide (PI) aerogels with highly aligned tube-like pores were fabricated by unidirectional ice crystal-induced self-assembly method. During this process, the mold bottom contacted with the freezing medium, the aqueous solution of poly(amic acid) (PAA) ammonium salt in the mold was unidirectionally frozen, the ice crystals grew from the bottom to top of PAA ammonium salt (PAS) solution along the freezing direction, which endowed PI aerogels with aligned tube-like pores after sublimation of ice crystals and thermal imidization of PAS. The obtained aerogels had low densities (0.077–0.222 g cm⁻³) and high porosities (83.8–94.2%) and exhibited anisotropic morphology and properties. Their compression strength in vertical direction (parallel to freezing direction) was higher than that in horizontal direction (perpendicular to freezing direction). Their heat transport in horizontal direction was much slower than that in vertical direction; the aerogels had better thermal insulating property in horizontal direction. This facile approach contributed to prepare new type of PI aerogel materials. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48769.     

Microstructure,Tuning of Band Gap,Enhanced Green Band Emission and Antibacterial Studies of Cu,Cr Dual Doped ZnO Nanoparticles by Annealing Temperature

Zn_0.94Cu_0.04Cr_0.02O nanoparticles have been synthesized by sol–gel method and annealed at 400, 600 and 800 °C. Hexagonal wurtzite structure was not affected by the temperature but single phase was altered by high temperature (800 °C). The reduced crystallite size (19.9 nm) at 600 °C was due to the proper substitution of doping element. The enhanced crystallite size at 800 °C was due to the formation secondary phase like CuO and defect states. The broad absorption peak at 800 °C around visible region represented the oxygen related defects and Cu/Cr interstitials. The red shift of band gap and enhanced visible light absorption are useful for solar applications. Better antibacterial efficiency of nanoparticles annealed at 600 °C was due to increase of surface area by reduced particle size and modification in morphology.     

Ice crystal formation in the carbon nanotube suspension: A modelling approach

Freeze-dried porous solid foams were prepared from carbon nanotube (CNT) suspensions via either contact freezing with a heat exchanger or immersion freezing into a cryo-bath. Microstructures in the freeze-dried foam cast by ice crystals formed during the freezing step. It was found that domains of the carbon nanotubes in the freeze-dried solid foams were fairly well interconnected, and the microstructures were largely influenced by the freezing condition and freezing method. A mathematical model was proposed to simulate the observed thermal history during the freezing step; then theoretical predictions of the microstructural features were attempted. The simulated thermal history was in good agreement with experimental result. The resulting mean ice crystal sizes L* could be estimated from the calculated freezing front velocity (R) and the temperature gradient in the frozen zone (G). Interestingly, it was found that a correlation based on the power law (L*∝R^?0.2G^?0.2) was applicable to the present system for both the contact and immersion freezing methods. Though the fundamental ice crystallization phenomena were essentially the same for both freezing methods, actual temperature distribution and movement of the freezing front through the whole freezing bulk could control the morphology of ice crystal in the frozen matrix. In other words, good control of thermal flow in the freezing system would greatly contribute to rational design of microstructures of the CNT foam.     

Single Particle Measurements of the Optical Properties of Small Ice Crystals and Heterogeneous Ice Nuclei

Dust aerosol and ice crystals are two major types of nonspherical particles in the atmosphere which have significant roles in cloud-aerosol interactions and the radiative budget. The presence of dust and ice often coincide in the atmosphere because dust particles are efficient ice nuclei. The size and composition dependence of the scattering properties of dust and ice are needed to assess their individual contributions to the optical scattering of sunlight. Here we present a new measurement technique used to determine the single particle forward scattering, backscattering, and depolarization ratio (at a wavelength of 680 nm) for representative nonspherical atmospheric particles. The Texas A&M University Continuous Flow Diffusion Chamber (CFDC) was used as an ice crystal generator to produce ice crystals via both homogenous and heterogeneous nucleation mechanisms under well-controlled laboratory conditions. Optical scattering properties of mineral dusts and small ice crystals (0.6 μm to 50 μm optical diameter) were measured by the Droplet Measurement Technologies, Inc. (DMT) Cloud Aerosol Spectrometer with Polarization (CASPOL). Significant differences between the optical properties of single dusts and ice particles of the same size were observed. Differences between the optical signatures of homogeneously and heterogeneously nucleated ice crystals were not statistically significant. Our results suggest that atmospheric ice crystals can be identified and quantified independently from the dust particles on which they form based on analysis of their backscatter and depolarization signals.

Copyright 2014 American Association for Aerosol Research     

血液在可见光及近红外范围内随温度变化的动态吸收特性

人体病变组织的光热效应是皮肤病激光疗法与光学诊断的基础,深入理解血液光吸收特性随温度变化的动态规律对利用激光来治疗葡萄酒色斑(PWS)等血管性疾病具有重要意义。设计并搭建了血液动态吸收光谱实验测量系统,利用常温水浴将血液样品缓慢加热至80℃(0.5～1.0℃·min^-1),每间隔5℃为单位对血液的光吸收特性进行测量,得到了血液在可见光及近红外范围400～1000 nm内随温度变化的动态吸收光谱。实验结果表明,加热过程中血液光吸收特性变化可分为4个阶段:常温至50℃血液吸收光谱基本无变化;50～58℃吸收特性曲线变平缓, 630 nm处吸光度增加为常温下的2倍;随着温度的进一步上升,血液光吸收特性大幅度增加;65℃后略有增加直至不再变化。整个加热过程中,血液吸光度均在970 nm处存在峰值,并且吸收性能随着温度的上升有所增加,这为今后使用该波段治疗较深的顽固病变血管奠定了坚实的基础。     

Experimental Study and Modeling of Freeze-Drying in Syringe Configuration. Part I: Freezing Step

This article concerns the experimental study and the modeling of the freezing step during freeze-drying of a model lyoprotectant formulation in syringe configuration. First, ice crystal morphology observations were carried out in a cold chamber equipped with stereomicroscopy to determine the ice crystal size distribution. We observed that the ice crystal morphology was homogeneous as a function of the radial position and that, contrary to our previous observations in glass vial configuration, no significant ice morphology differences existed between the bottom and the top of syringes. Thus, it seems that the gas cooling by free convection at the walls of the syringe during the freezing step led to higher homogeneity in the ice crystal morphology. These data allowed calculation of the mean ice crystal diameter (which was supposed to represent the mean porous diameter) and then the permeability of the freeze-dried cake. Moreover, a two-dimensional axisymmetric (2D) mathematical model in cylindrical geometry of the freezing step for aqueous sucrose solution was elaborated. It has been proved that this model was able to simulate quite precisely the whole temperature profiles during the freezing step, except the exothermic peak of nucleation. In the last part of this work on syringe geometry, the impact of annealing treatments on the freeze-dried matrix morphology was investigated. Some heterogeneity of sublimation rates was observed, with significant variation of residual humidity. Additionally, the water vapor mass transfer resistance values decreased if a convenient annealing treatment (shelf temperature at ?10°C) was applied.     

Effect of Ultrasonically Induced Nucleation on the Drying Kinetics and Physical Properties of Freeze-Dried Strawberry

Power ultrasound has proved to be very useful in controlling crystallization processes since sonication can enhance both the nucleation rate and crystal growth rate by producing fresh and/or more nucleation sites. Therefore, in this study, power ultrasound was applied to control the freezing step of freeze-dried strawberry. The results showed that when power ultrasound was applied at different temperatures, it increased the nucleation temperature and shortened the characteristic freezing time. The application of power ultrasound in the freezing step increased the drying time in subsequently freeze-dried strawberry samples. This longer drying time was found to be due to increased resistance to moisture diffusion due to the formation of a network of small pores caused by sublimation of small ice crystals induced by the power ultrasound. Scanning electron microscopic images revealed that the freeze-dried sample frozen by ultrasound-assisted freezing (UAFD) had finer cellular structure compared to those frozen in other freezing conditions. UAFD samples had better textural hardness, while the rehydration capacity was lower compared to those of NRFD and NIFD samples.     

Heterogeneous Ziegler–Natta catalysts with various sizes of MgCl<Subscript>2</Subscript> crystallites: synthesis and characterization

A MgCl₂-based Ziegler–Natta catalyst was characterized using X-ray diffraction (XRD) patterns, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) and IR spectra. We focused on the XRD reflection at 2θ = 50° to determine the thickness of MgCl₂ crystals, and validated these results with TEM pictures. SEM pictures were taken in order to measure the size of the nanoparticles formed by the MgCl₂ crystals. Several compounds were synthesized for comparison and to aid interpretation of the infrared (IR) spectra. The catalysts were prepared by precipitating MgCl₂, which was used as support material and subsequently treated with TiCl₄. The thickness of the catalyst crystals was calculated from the XRD reflection at 2θ = 50°. Changing the precipitation temperature within a range from 40 to 90 °C altered the thickness of the MgCl₂ crystal plates. The maximum thickness of 7 nm was achieved at a precipitation temperature of 60 °C. The SEM pictures showed that the nanoparticles had a diameter of ~200 nm. A crystal base unit had a volume that corresponded to that of a sphere of 3.5 nm radius. Thus, we estimated that a typical catalyst particle with a diameter of 20 μm contained about one million nanoparticles, each of which consisted of about 25,000 MgCl₂ crystal units.     

Influence of Freezing Temperature on Product Parameters of Solid Dosage Forms Prepared via the Freeze‐Casting Technique

The freeze‐casting technique was used to formulate porous solid bodies containing theophylline as an active pharmaceutical ingredient, potato starch as a filler and citric acid or saccharose as binding agents. Aqueous suspensions of the ingredients were frozen at three different temperatures. The aim of this work was to investigate the influence of the freezing temperature on the ice crystal growth rate of the suspensions and the resulting porosity of the freeze‐casted samples. The impact of the freezing temperature on the solidification of the suspensions was analyzed via contact angle measurements. The rate of ice crystal growth was expressed as an overall linear growth rate. The porosity of the freeze‐casted bodies was determined by high‐pressure mercury porosimetry. A close correlation was found between the freezing temperature and the investigated product parameters. Lower cooling temperatures resulted in higher final porosities. The lowest temperature, at which final product properties could be regulated by varying the cooling regimes, was –30 °C. The influence of the freezing temperature and the impact of the additives were not remarkable at a freezing temperature lower than –30 °C. Therefore, it can be concluded that the optimum freezing temperature of the suspensions investigated in this study is in the temperature range between –20 °C and –30 °C.     

Structural,Electrical and Optical Characterization of Pure and Doped Poly (Vinyl Alcohol) (PVA) Polymer Electrolyte Films

A sodium ion conducting polymer electrolyte based on Poly (Vinyl Alcohol) (PVA) complexed with Sodium Iodide (NaI) was prepared using solution cast technique. The structural properties of composite PVA polymer electrolyte films were examined by XRD. The XRD results revealed that the amorphous domains of PVA polymer matrix increased in size with the increase of NaI salt concentration. The variation in film morphology was examined by scanning electron microscopy. FT-IR spectra studies for pure PVA and complexed films revealed the vibrational changes that occurred due to the effect of dopant salt in the polymer. DC conductivity was measured in the temperature range of 303–373° K and the conductivity was found to increase with the increase of dopant concentration as well as temperature. Optical absorption studies were made in the wavelength range 200–600 nm. The absorption edge, direct band gap, and indirect band gap values were evaluated.     

Preparation of porous TiO2 by a novel freeze casting

Highly porous and open interconnected pore structural TiO₂ were prepared by a novel freeze casting method. In the experiment, the well-dispersed aqueous slurries were first frozen, and then dried at a reduced vacuum. Since the sublimation of ice crystals developed in the freezing process, the green bodies with highly porous were obtained. The phase composition and the microstructure of the sintered samples were characterized by XRD, SEM, porosity and the pore size distribution was measured by mercury porosimetry. The results demonstrated that the PVA concentration in the slurries remarkably affect the microstructure of TiO₂ ceramics. The pore morphology of TiO₂ ceramics with 3 wt.% polyvinyl alcohol (PVA) addition was dendritic, and however, the pore morphology of TiO₂ ceramics with 6 wt.% PVA addition changed into columnar. The reason for the variation of the pore morphology was ascribed to the effect of the PVA gelation on the growth behavior of the ice crystals.     

Effect of ultrasonically induced nucleation on ice crystals’ size and shape during freezing in vials

This work aimed at determining empirical correlations between the final frozen product ice crystals’ characteristics (mean size and shape) and ultrasound assisted freezing operating conditions (supercooling and acoustic power). For this purpose 10% w/w mannitol aqueous solution samples contained in vials were frozen by means of a refrigerated shelf coupled with an ultrasonic device able to induce ice nucleation. Ice crystals’ were observed using reflected light optical microscopy over samples’ transversal and longitudinal sections in a cold chamber. A response surface analysis based on a five levels composite experimental design had been carried out. It was found that increasing supercooling and acoustic power resulted in decreasing ice crystals’ mean size and increasing their mean circularity.     

Effect of curing temperature on properties and solvent welding strength of ovalbumin

Effect of curing temperature on properties of ovalbumin has been investigated. Functional basis, crosslinking degree, tensile strength, and hardness increases with rising curing temperature. The increase of visible transmittance and the decrease of absorption accompany with increasing wavelength. The absorptive peak shows at 440–450 nm and the wavelength of the absorptive peak increases with the rising curing temperature. The relationship of joint strength with solvent welded joints of ovalbumin to their microstructure is also investigated. Ovalbumin can promote joint strength after the treatment of distilled water and curing. Comparing joint strength with fracture morphology, the smoother fracture surface morphology is related to the maximum tensile and shear joint strength, respectively. The joint strength is increasing with curing temperature and compressive stress, and the joint strength of treatment with 150°C curing temperature and 0.12 kgf/mm² compressive stresses are larger than its original tensile fracture strength of cured ovalbumin at same curing temperature. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Effects of Ultrasonic Parameters on the Crystallization Behavior of Palm Oil

The crystallization behavior of palm oil (PO) without and with the application of high-intensity ultrasound (HIU) was investigated as the function of irradiation time (20, 60, 120, and 240 s), ultrasonic intensity (47.5, 95, 270, and 475 W) and temperature (20, 25, 30, and 36 °C). The effects on the crystallization behavior of PO were evaluated by ultraviolet/visible spectrophotometry, pulsed nuclear magnetic resonance and polarized light microscopy. Results indicated that all these parameters affected crystallization behavior. HIU significantly reduced the induction time and accelerated the crystallization rate at operating temperatures above 25 °C, but there was no significant difference at 20 °C due to high supercooling. The effects of HIU were more significant at higher power level and longer irradiation time, however, the thermal effect of ultrasound also increased with longer sonication time. The optimal sonication time was approximately 120 s which accelerated the crystallization rate of PO the most. The morphology studies suggested that HIU changed the growth mechanisms of crystals and generated smaller and uniformly crystals. At 36 °C, with extremely low supercooling condition, a combined effect was observed that prevented the separation of solid phase and liquid phase of the crystallized sample, and then led to a uniform distribution of crystals.     

冷冻保存中影响胞内冰生长的因素

细胞胞内冰的形成会导致严重的细胞损伤从而导致低温贮存中的诸多问题。以蚕豆为研究对象,用细胞松弛素B溶解细胞骨架,使用低温显微系统在不同的冷却速率下进行冷冻实验。实验结果表明,使用细胞松弛素B处理过的细胞在冷冻过程中结晶温度更高,结晶时间更短,但细胞骨架对胞内冰的生长过程影响较小。外界条件起着关键作用,接种冰晶影响细胞内冰晶的形成温度及冰晶的生长速率。最后,通过光强度图对细胞的损伤程度进行了分析。     

A Direct Characterization Method of the Ice Morphology. Relationship Between Mean Crystals Size and Primary Drying Times of Freeze-Drying Processes

Abstract

In the freeze-drying process, the freezing step is one of the most important steps which determines the texture of the frozen material and, consequently, the final morphological characteristics of the freeze-dried material and its biological activity and its stability. As a matter of fact, the parameters of the freezing protocol have a direct effect on the pore size distribution and on the pore connectivity of the porous network of the freeze-dried matrix. Thus, the ice crystal morphology determines indirectly the mass and the heat transfer rates through the dry layer and, consequently, the freezing parameters have a strong influence on the total duration of the primary and secondary sublimation steps. The main objective of this study was to adapt and to develop a new optical direct microscopy method, based on the reflected flux differences, with episcopic axial lighting to characterize the structure of the different phases of a standard pharmaceutical matrix used for pharmaceutical proteins freeze-drying. First, the results obtained have been validated by another independent method, the scanning electron microscopy, carried out with freeze-dried samples. Finally, this technique has been principally used to investigate the effects of the freezing conditions on the ice crystal structure characterized by the distribution of the ice crystals mean sizes. Moreover, the influence of annealing treatment on ice crystal mean diameter and primary drying times has been also investigated.