Ultrafine network

Fig. 23 SEM and TEM examination of longitudinal sections, a SEM of the ultrafine network structure of BASYC , b and c TEM pictures of the BASYC layer...

MC produced by the vinegar fermentation bacterium A. xylinum is synthesized in the form of twisting ribbons, which in stationary culture forms a thick, gelatinous membrane on the surface of a liquid medium. The membrane formed in such conditions is characterized by the 3D structure made of an ultrafine network of... 

BC is characterized by an ultrafine network structure composed of ribbon-shaped fibrils with an average diameter 100 times thinner than those of plant cellulose fibers (Figure 2.9) [4]. As a result, BC membranes are a highly porous material with substantial permeability for liquids and gases and high water uptake (water content >90%) [8]. 

Although the chemical structures of plant and bacterial cellulose are identical, the physical stmcture of bacterial cellulose is unique, being composed of ultrafine fibers that form an ultrafine network. The mechanical properties of bacterial cellulose are also unique. In this chapter, the mechanism of acetic acid bacteria cellulose biosynthesis, and its biological functions, properties, and industrial applications, are described. 

Bacterial cellulose has an ultrafine network structure of cellulose fibrils. The width of the microfibrils is approximately 1/lOOOth that of the pulp fiber. Even the thiimest fiber is a chemically synthetic fiber having a size of approximately 1 pm bacterial cellulose fibril is 1/10th that of the synthetic fiber. 

It is found that bacterial cellulose has high crystallinity, high water absorption capacity, and mechanical strength in the wet state, ultrafine network architecture, and moldability in situ [24]. In addition, bacterial cellulose is biocompatible and biodegradable, thus holding great potential for biomedical applications [26]. 

Inorganics can also be synthesized and used as templates. Thus, controlled siloxane networks were formed when dispersions of alkoxysilanes (such as (MeO)3SiMe) are mixed with the suitable template matrixes. Ultrafine particles of metal oxides can be used as starting materials for the formation of metal oxide films. For instance, a mixture of a double-chained ammonium amphiphile and an aqueous solution of aluminum oxide particles (diameter about 10 100 nm) gives a multilayered aluminum oxide film when calcinated at over 300°C. 

Birmili W, Weinhold K, Nordmann S et al (2009) Atmospheric aerosol measurements in the German ultrafine aerosol network (GUAN). Part 1. Soot and particle number size distributions. Gefahrst Reinhalt Luft 69 137-145... 

Notably, the instruments used in the European Supersites for Atmospheric Aerosol Research (EUSAAR)/Aerosols, Clouds, and Trace gases Research Infrastructure Network (ACTRIS) and German Ultrafine Aerosol Network (GUAN) measurements used in this chapter are from intercalibrated measurements, where the abilities of the instruments were determined in common intercalibration workshops [15]. Overall, the instruments agree well on particle sizes between 20 and 200 nm, with the differences above 200 nm still relatively minor for number concentrations. In smallest particles sizes the instrument deviation is large, and for this reason we only consider particles larger than 30 nm in diameter in this chapter. 

Electrocatalysts One of the positive features of the supported electrocatalyst is that stable particle sizes in PAFCs and PEMFCs of the order of 2-3 nm can be achieved. These particles are in contact with the electrolyte, and since mass transport of the reactants occurs by spherical diffusion of low concentrations of the fuel-cell reactants (hydrogen and oxygen) through the electrolyte to the ultrafine electrocatalyst particles, the problems connected with diffusional limiting currents are minimized. There has to be good contact between the electrocatalyst particles and the carbon support to minimize ohmic losses and between the supported electrocatalysts and the electrolyte for the proton transport to the electrocatalyst particles and for the subsequent oxygen reduction reaction. This electrolyte network, in contact with the supported electrocatalyst in the active layer of the electrodes, has to be continuous up to the interface of the active layer with the electrolyte layer to minimize ohmic losses. 

A sol-gel process is an important method to synthesize many materials in a variety of shapes and forms such as particles, films, and bulks. This method is especially suited for the synthesis and preparation of ultrafine rare earths oxide particles at relatively low temperatures. A sol is a stable colloidal dispersion of small particles suspended in a liquid. The particles are amorphous or crystalline and particle aggregation is prevented by electrostatic repulsion. The particles in some sols interact to form a continuous network of connected particles called a gel, instead of aggregating to form larger particles (precipitates). 

They all display ultrafine fiber network structure of microfibrils of below 0.01 pm. The average water content of the BC and the modified BC pellicles is 98%-99%. After the pellicles are air-dried at room temperature (30°C), the pellicles become transparent thin films. The SEM image of the air-dried films and the re-swollen films are shown in Figures 6.6 and 6.7, respectively. 

Through ultrafine fiber and high fiber density, porosity of fibrous stmctures could be decreased and inhibit cellular ingrowth within the substitute fibrous stmcture. The dense fiber network of electrospun membrane presents a barrier for cell entry. It has been used for islets, which are used as an immiinoisolation strategy for disease... 

Figure 17-5. Ultrafine net of microbial cellulose (a) has a very smooth network of micro- and nanofibrils and (b) the cross-section of MC membrane with multilayer architecture...

Ultrafine fibers, ealled nanofibers are a unique nanomaterial because of the nanosealed dimensions in the cross-sectional direction and the macroscopic length of the fiber axis (see Figure 6.1). Therefore, nanofibers have both the advantages of fimetionality due to their nanosealed structure and the ease of manipulation due to their macroseopie length. In addition, three-dimensional nanofiber network assemblies (nanofibrous membranes or fabrics) provide good meehanieal properties and good handling characteristics [1]. 

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