用激光粒度仪测量燃烧合成的氮化硅粉体粒度的条件

以硅粉为主要原料,采用燃烧合成法制备微米级Si_3N_4粉体,以中粒径d50和粒径分布宽度d90-d10作为粒度评价指标,采用激光粒度仪研究了悬浊液固含量、超声分散时间、分散剂含量及沉降时间对粒度测量的影响.结果表明,激光粒度仪可用于检测燃烧合成的Si_3N_4粉的粒度分布,悬浊液固含量、超声分散时间、分散剂含量及沉降时间对粒度测量结果影响显著.水为分散介质时,最佳固含量1.25 g/kg、最佳超声时间60 min、最佳分散剂用量3 g/kg的条件下,超声分散后0.5 h内进行粒度测量最可靠.     

冷冻速率对冻干法制备BN多孔陶瓷显微结构的影响

为研究利用冻干法制备氮化硼多孔陶瓷时冻干过程中冷冻速率的影响,以分析纯六方氮化硼(h-BN)为原料,分析纯氧化硼为烧结助剂,去离子水为冷冻溶剂配制浆料,再将分散均匀、稳定的浆料通过冷冻速率分别为1.03、1.77和34.7℃·min~(-1)的三种冷冻干燥方式制备成冻干坯体,最后在1 500℃的氮气气氛下保温3 h热处理后制得BN多孔陶瓷。通过观察冻干试样的显微结构以及检测烧后多孔陶瓷试样的气孔率及体积密度,探讨了冷冻速率对冰晶生长行为以及多孔材料显微结构的影响。结果表明,通过调整浆料冷冻过程中的冷冻速率,可以实现对BN多孔陶瓷材料显微结构的控制。当冷冻速率较小(1.03℃·min~(-1))时,冰晶成核的位置随机分布,得到试样的气孔分布不均匀。随着冷冻速率的增大,试样的孔结构发生变化:当冷冻速率为1.77℃·min~(-1)时,冰晶的成核速率接近冰晶的生长速率,得到分布均匀的直通孔结构,孔径约为50μm;当冷冻速率增加至34.7℃·min~(-1)时,冰晶的成核速率超过冰晶生长速率,从而得到分布均匀的竹节状孔结构,孔径细小,约为10μm。而且材料的热导率随着多孔结构的引入会大大降低。     

团聚对亚微米5A分子筛粒度分析的影响

为了考察小晶粒分子筛团聚对亚微米级5A分子筛粒度分布结果的影响以及适宜的测定条件,实验以水为分散介质,六偏磷酸钠为分散剂,在超声波辅助作用下,用电位/粒度分析仪测定研究了试样浓度、分散剂用量和超声时间对亚微米级5A分子筛分散性的影响。结果表明,当分子筛含量大于5 g/L时,晶粒团聚现象严重,粒度分布呈现单峰(假象),导致粒度测量结果偏大;加入少量(0.005~0.195 g/L)的六偏磷酸钠,有缓解晶粒团聚作用,可将较大团聚粒子(大于2.8μm)分散为较小粒子(1.3μm左右),少量团聚粒子分散成单个晶粒,可测得较小晶粒粒径;超声作用可使较大团聚粒子(0.85~2.80μm)分布峰强度降低,相对较小粒子(0.30~0.85μm)分布峰强度增加。超声时间不宜太长,否则将加剧粒子间团聚,使得测定粒径偏大。当试样中5A分子筛含量为5 g/L,六偏磷酸钠浓度0.195 g/L、超声时间4 min条件下,测得5A分子筛试样粒径分布0.25~0.80μm,峰顶对应粒径为0.473μm,中位径(d50)为0.784μm,与电镜分析结果一致,说明是较为准确的粒度分析方法。     

用激光粒度分析仪检测细粒径SiO2基相变调湿复合材料的粒度分布

以SiO2为载体、棕榈醇-棕榈酸-月桂酸为相变材料,采用溶胶-凝胶法制备细粒径SiO2基相变调湿复合材料,以中位径(d50)为评价指标,采用激光粒度分析仪研究了溶液溶质浓度、超声时间、超声频率及分散介质对材料粒度的影响. 结果表明,激光粒度分析仪可用于检测SiO2基相变调湿复合材料的粒度分布,水对材料的分散效果比乙醇好,以水为分散介质,材料最佳溶质浓度为5~10 g/L,最佳超声时间为15 min以上,最佳超声功率大于400 W.     

第三讲 染料粉碎工艺

水不溶性染料在染色应用中,要求有足够的粒子细度和均匀的粒度分布。本讲着重叙述水不溶性染料湿粉碎过程的装置和特点与选择,影响湿粉碎的因素及粉碎操作的注意事项。     

冷冻条件对植物细胞低温保存的影响

为保证食品果蔬冷藏保存的品质,细胞层面的基础研究是必不可少的一部分,细胞内冰的形成（ⅡF）会导致严重的细胞损伤从而导致低温储存中的诸多问题。预冷过程对胞内冰晶的形成具有重要影响,因此拍摄到细胞内冰晶形成过程为优化冷冻保存方案提供必要的支持。在冷冻洋葱表皮细胞时,采用不同的降温速率,并使用低温显微系统配合高速摄像机,可视化细胞内更清晰的ⅡF过程。实验结果表明,冷却速率的增加使得在相同温度下细胞内ⅡF出现的概率降低,但细胞边缘冰晶生长速率增加,使细胞受到的损伤程度变化波动明显。     

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。     

碳酸根柱撑镁铝水滑石的水热合成研究

采用水热法合成铝镁水滑石,并通过X射线衍射(XRD)、红外光谱(FT-IR)、透射电镜(TEM)、热分析(DSC)、粒度分析等手段对合成的水滑石进行表征,研究高温水热法和低温超声辅助法合成的水滑石结构及热性能。将自制样品和进口样品进行对比测试,TEM测试表明高温水热法制备的水滑石晶型为片状,超声辅助法制备的水滑石晶型为絮状;XRD测试表明高温水热法制备的水滑石特征峰明显;粒度测试表明高温水热法制备的水滑石和进口水滑石粒径分布完全相同。研究结论为高温水热法制备的水滑石样品由于经过高温经溶解结晶过程,其晶型结晶度高。     

超声辅助提取山楂总酚的因素分析

介绍以乙醇为溶剂,超声辅助提取山楂总酚的工艺;介绍了山楂多酚在其深加工利用中的作用及对食品性质的影响。并以Folin－Ciocalteu法对超声辅助提取山楂总酚工艺的因素进行分析,如超声功率、温度、料液比、提取时间等。从单因素影响考虑,分别在超声功率160 W、提取时间20 min、提取温度55℃、料液比1：40,乙醇体积分数50％时,提取液总酚含量最大。     

SiC多孔预制体的定向冷冻干燥制备工艺研究

通过定向冷冻、冷冻干燥和高温烧结制备了具有定向层状孔隙的SiC多孔预制体,研究了浆料球磨时间、SiC粉料粒径、定向冷冻时间和冷冻干燥过程对预制体制备工艺的影响,确定了合适的定向冷冻干燥制备工艺.结果表明:SiC粉料在液相中充分地分散、润湿和形成水化膜后可以获得性能稳定的浆料,三种SiC粉料(初始粒径1.8μm、9.0μm和17.4μm)的球磨时间应分别大于12 h、14 h和20 h;定向冷冻过程中冰晶的生长和陶瓷颗粒的重排与SiC颗粒大小有着密切的关系,采用粒径1.8μm粉料制备的预制体中层状孔隙定向性和连通性最好;固相含量较低的浆料要采用较长的冷冻时间;虽然冷冻干燥温度较高时失重速率较高,但低温更有利于保持冰晶形貌,因此可选用定向冷冻温度作为冷冻干燥温度,冷冻干燥时间应不少于24 h.     

Three-Dimensional Measurement of Ice Crystals in Frozen Materials by Near-Infrared Imaging Spectroscopy

A novel technique was developed to recognize ice crystals in biological materials and to analyze their three-dimensional morphology using a Cryogenic Micro-Slicer Spectral Imaging System with a micro-slicer unit and a near-infrared spectral imaging unit. Consecutive cross-sections of a frozen sample were exposed by the multi-slicing operations with a minimum thickness of 1 µm, and their images were taken by the imaging unit. Spectroscopic analysis using a near-infrared spectrum meter showed an absorption peak at 1460 nm for pure water. Based on the observations of the absorption band of ice crystals in the wavelength range of 1450–1570 nm and its peak at 1495 nm, a commodity-type bandpass filter with a central wavelength of 1500 nm was adopted to identify ice crystals in near-infrared images. The absorption peak of water exhibited a tendency to move toward longer wavelengths with decreasing sample temperature from 25 °C to ?15 °C. The filtered images of ice crystals in frozen samples were darker than the other components at the peak wavelength of ice crystals. The three-dimensional reconstructed morphology of ice crystals revealed that they were formed along the direction of heat transfer while freezing. The proposed method provides a novel tool to investigate the effects of freezing conditions on the size, morphology and distribution of ice crystals.     

冷冻干燥过程强化中冷冻阶段优化的研究进展

冷冻干燥产品质量高,但时间长、能耗高。本文综述了冷冻干燥过程强化中冷冻阶段的优化方法,控制冷冻速率、调节冰晶成核和退火处理可以获得大而均匀的冰晶从而提高升华干燥阶段速率,但物料内部比表面积的减小会降低解吸干燥阶段速率,这类常规的冷冻阶段优化方法对弱吸湿性的物料有一定的强化效果。有机溶剂具有较高的蒸气压,作为共溶剂时可以增加传质推动力,但较低的有机溶剂残留量要求阻碍了其进一步应用。“初始非饱和多孔介质冷冻干燥”的技术思想是将液体物料首先制备成具有一定初始孔隙的冷冻物料,然后再进行冷冻干燥。物料具有的初始孔隙为水蒸气的迁移提供了便捷的通道,而且纤薄的固体基质也有利于结合水的解吸,可以同时强化升华干燥阶段和解吸干燥阶段。该技术思想是过程低消耗和产品高质量的完美结合,为解决冷冻干燥过程速率低的问题提供了新的方案。     

Influence of Ultrasound-Assisted Osmotic Dehydration and Freezing on the Water State,Cell Structure,and Quality of Radish (Raphanus sativus L.) Cylinders

Ultrasound-assisted osmodehydrofreezing technique is a partial dehydration technique prior to freezing to diminish the tissue damage and preserve the quality by quickly removing part of the water from vegetable tissues. In this study, radish cylinders with three different water contents (85%, 80% and 75%, w/w) were dehydrated by osmotic dehydration (OD) and ultrasound-assisted osmotic dehydration (UOD). The effects of OD and UOD pretreatment on the characteristics (e.g., latent heat of fusion of ice, freezable water and microstructure) of dehydrated products and quality attributes (e.g., freezing time, firmness, drip loss and ascorbic acid content) of osmodehydrofrozen products were investigated. Ultrasound application significantly shortened the time of dehydration and subsequent freezing. Compared to OD products with equal water content, UOD products exhibited less freezable water content and better preservation on firmness and microstructure. After freezing/thawing, frozen products of UOD also displayed less drip and ascorbic acid losses and better firmness than that of OD.     

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.     

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

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.     

Gefrieren und Härten von Speiseeis

Freezing and Hardening of Ice Cream For manufacture of ice cream having good bodying and structural properties the water contained in the mixture must be frozen during freezing and hardening operations into a large number of small ice crystals below a certain size and the air which is introduced has to be finely distributed.     

Frozen slurry-based laminated object manufacturing to fabricate porous ceramic with oriented lamellar structure

The freeze drying-based additive manufacturing can be used to process porous ceramics. However, the lack of freezing direction leads to the disorderly porous structure. This paper proposes a frozen slurry-based laminated object manufacturing (FS-LOM) for processing porous ceramics. Slurry was composed of water, alumina powder, and organic binder. The water in the fresh slurry layer crystallized to obtain a good support strength. The outline of 2D pattern was cut with laser to gasify ice crystal and binder. After stacking, the ice crystal freeze dried to obtain a porous structure. The lamellar ice crystals were induced to growth vertically by layer-by-layer freezing. The uniformity and orientation of the pore structure were improved, and the compression strength of the parts were improved. Due to the support of frozen slurry, the deformation of the green part was avoided.     

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.     

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.     

Optimization of freezing parameters for freeze-drying mango (Mangifera indica L.) slices

Today, there is an increasing trend toward consuming healthful food products, and dried fruits are one option for obtaining desirable nutrition in dehydrated products. Proper drying method selection is important for minimizing quality losses. Lyophilization (freeze-drying), a technique that removes water by sublimation at low temperature, can produce excellent dried products. Fast vacuum induction during freezing has been proposed as a way to control ice nucleation and to obtain enhanced lyophilized pharmaceutical products. However, this technique has not yet been applied to food freeze-drying. The aim of this work was to evaluate the effects of the freezing rate (0.4, 0.25, and 0.1°C/min), the minimum chamber pressure during vacuum-induced surface freezing (900, 700, and 500?mTorr) and the sample temperature at which the induced vacuum was applied (0, ?2, and ?4°C) on the total process time, final moisture content, rehydration capacity, total color difference, and total polyphenol content during the lyophilization of mango (Mangifera indica L.) slices. A Box–Behnken design with three factors at three levels was used to design the experiments, to generate the polynomial equations relating the dependent and independent variables, and to determine the optimal operational conditions. The results highlight the reduction of total process time (30%) at high freezing rates and the influence of the tested operating conditions on freeze-drying optimization. It was found that the optimal conditions that satisfy commercial quality goals were pressure between 500 and 650?mTorr, temperature between ?2 and 0°C, and freezing rate values close to 0.4°C/min.