Precipitated silica powders

Cuboid and cycloid niobium monocarbide (NbC) whiskers, 0.1-2.0 pm in diameter and 5-100 pm in length, and having a square-shaped tip, were recently synthesized by heating mixtures of niobium oxide (Nb203) and carbon black at temperatures over 1100°C [38]. Silicon carbide nano-whiskers, 20-50 nm in diameter and 2-5 pm in length, were carbothermally synthesized by reducing ultrafine precipitated silica powders with ultrafine carbon black by microwave heating [29]. These processes proceed without addition of metal particle catalysts, and therefore by a VS phase transformation [14] [29] [38]. 

Deposition of Additional Silica. If a gel is broken up after it is formed so that particles are in suspension, it can be strengthened by the deposition of more silica from supersaturated solution. Alexander, Her, and Wolter (271) describe a process whereby active silica or silicic acid of low molecular weight can be released into the suspension, which is maintained at pH 9-10.5 and at 95 C, at such a rate that the added silica is deposited uniformly within the gel structure. It is obvious that this is not applicable to macroscopic gels. The process is described further in connection with the manufacture of precipitated silica powders. 

The adsorption of DDA on Silicagel R and precipitated silica powder (Fig. 3) was investigated at concentrations lower than 1 mmol/dm, i.e., well below its CMC (14.8 mmol/dm [38]). The adsorption isotherms show constant adsorption above the equilibrium concentration of 0.6 mmol/dm. The amount of DDA adsorbed is lower than that of HDP by one order of magnitude for both substrates. DDA is dissolved at low pH, and therefore the acidity of the systems was adjusted to pH 2 with HCl solution. In such an acidic medium, the surface of the adsorbents is weakly positively charged [15]. In spite of this, the adsorption of cationic surfactants occurs to a small degree. The occurrence of the second step of the isotherm for the precipitated silica can be explained in terms of the second layer or surface hemimicelle formation. 

Microstructure and size analyses of the precipitated silica powders were carried out using TEM and dynamic light scattering. The avera particle size, size distribution, and yield of silica powders were affected by reaction time, temperature, and concentrations of surfactant and sodium silicate solutions (Table 2.1). The particle size of silica powders increased with reaction temperature and concentration of sodium silicate, and the yield of silica powders increased with increasing reaction time. The size distribution of silica powders was affected by concentration of surfactant PEG. The optimal preparation conditions were experimentally determined for obtaining the silica powders with nanometer size, narrow size distribution, spherical shape, and high purity without sodium carbonate and surfactant... 

Fig. 3.26 Comparison plots for compacts of silica and magnesia. In each case the adsorption of nitrogen at 78 K on the compact is plotted against that on the uncompacted powder, (a) and (b), comparison plot and adsorption isotherm for silica powder compacted at 130 ton in (c) and (d), comparison plot and adsorption isotherm for precipitated magnesia compacted at 10 ton in. Note that the upward sweep of the comparison plot commences at a relative pressure below the inception of the loop.

Various additives and fillers may be employed. Calcium carbonate, talc, carbon black, titanium dioxide, and wollastonite are commonly used as fillers. Plasticizers are often utilized also. Plasticizers may reduce viscosity and may help adhesion to certain substrates. Thixotropes such as fumed silica, structured clays, precipitated silica, PVC powder, etc. can be added. Adhesion promoters, such as silane coupling agents, may also be used in the formulation [69]. 

Physical Form. Fine white powder with particle sizes generally below Ipm. This is not the same as the commercial products fumes silica, silica gel, precipitated silica, or fused silica. It is formed during the electric arc production of elemental silicon from... 

Two more recent applications for amorphous silicas are expected to grow to large volumes. Precipitated silicas are used in the manufacture of separator sheets placed between cells in automotive batteries. Their function is to provide a controlled path for the migration of conductive ions as a result of the porosity of the silica particles. Additionally, both precipitated silicas and aerogels are being developed for use in low temperature insulation, where the low thermal conductivity of the dry silica powders makes them useful in consumer products such as refrigerators (83). 

The residue from the treatment with acetic arid is again incinerated together with the filter, the ash being mixed with precipitated silica or calcium silicate or even finely powdered sand and introduced into a platinum crucible the mass is then moistened with a little concentrated sulphuric arid and the crucible immediately covered with a glass plate having a drop of water on its lower side. In case the ash contained fluorine compounds, there will appear, after a few moments and without heating, a deposit of silica on the edges of the water-drop. 

Mix 0.5 gram of the precipitated silica with 1 gram of powdered calcium fluoride. Place the mixture in a test tube, moisten it with 36 N H2S04, and warm it gently under the hood. Dip a stirring rod in water and lower it, with a drop adhering, into the gas in the test tube. Note the precipitate that forms in the drop of water. Write equations for all the reactions, and state what rather unusual properties are shown by this experiment to be possessed by hydrofluoric acid and by silicon tetrafluoride. 

Titania, goethite and silica were used as adsorbents. Titanium dioxide (anatase) and silica powders were obtained commercially. Goethite was prepared from an FefNC solution by the precipitation of ferrihydrite.8 The suspension was held in a closed flask at 70°C for 60 hours. During this period the red brown suspension of ferrihydrite was converted to a yellow brown goethite. These oxides were washed with double distilled water to remove impurities, until the supernatant conductivity was below 2pS x cnT1. 

Smith and coworkers (6-10) have recently conducted a series of Investigations on the extraction and precipitation of a variety of organic polymers and inorganic materials including ceramic and preceramic powders. Silica powders were precipitated by rapid expansion following their extraction with supercritical water at 580-590 bar and pre-expansion temperatures higher than 723 K. The particle size and agglomeration characteristics of these powders were found to be sensitive to the concentration of silica before the expansion as... 

Typical fillers firmed silica, calcium carbonate, carbon black, silver, glass beads, metal powders, precipitated silica, aluminum oxide, montmorillonite. mica, zinc oxide... 

Fine powders being composed of particle aggregates such as precipitated silicas or white carbon are made by the decomposition of sodium silicate by sulfuric acid in an aqueous phase at high temperature. This material has microporosity as well as macroporosity due to a rapid growth and aggregation of primary particles as will be discussed later. These are used as rubber filler, paper sizing and etc. 

The purification of fine powder precipitated silica is difficult by this method. In some applications of white carbon such as a rubber filler, a certain amount of Na cation remains in order to adjust the acidity of the surface to achieve a better dispersion of particles. The silica products are fairly pure when SiCU and Si(OC2Hs)4 purified by distillation are used. 

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