Zinc oxide particles

Scanning electron microscopy shows the cement to consist of zinc oxide particles embedded in an amorphous matrix (Smith, 1982a). As with the zinc phosphate cement, a separate globular water phase exists since the cement becomes uniformly porous on dehydration. Porosity diminishes as the water content is decreased. Wilson, Paddon Crisp (1979) distinguish between two types of water in dental cements non-evaporable (tightly bound) and evaporable (loosely bound). They found, in the example they examined, that the ratio of tightly bound to loosely bound water was 0-22 1-0, the lowest for all dental cements. They considered that loosely bound water acted as a plasticizer and weakened the cement. 

Increase in concentration of aluminium and phosphoric acid in the liquid serves to slow the reaction. This observation is in line with the above reaction scheme. Increase in the aluminium content will serve to increase the thickness of the coating formed around zinc oxide particles. Increase in phosphoric acid content implies a decrease in water content and an impairment of the hydration reaction. 

The presence of water on the oxide surface can enhance the sintering of zinc oxide particles (Dollimore Spooner, 1971). The amount of water reversibly absorbed on zinc oxide surfaces is affected by heat treatment... 

Figure 9.4 The effect of sintering temperature on the morphology of zinc oxide particles. Zinc oxide from zinc oxalate (a) 400 °C, (b) 800 °C. Carmox zinc oxide (c) 400 °C, (d) 800 °C (Prosser Wilson, 1982). 

The set cement consists of zinc oxide particles bonded together by a loose matrix of zinc eugenolate (Wilson, Clinton Miller, 1973). Electron-micrographs show that the zinc oxide particles are covered by zinc eugenolate (Figure 9.5a). 

Figure 9.5b Electronmicrograph of a ZOE cement matrix after aqueous attack. The zinc oxide particles are washed clean of zinc eugenolate and the matrix is degraded to zinc hydroxide (Wilson, Clinton Miller, 1973).

The most important approach was to use poly(methyl methacrylate), PMMA, in formulations, either as a particulate filler or as a coating on zinc oxide particles (Jendresen Phillips, 1969 Jendresen et al., 1969 Civjan et al., 1972). It is claimed that such materials can be used for permanent as well as temporary cementation. 

The researches of ZnCFO compatibility with the matrix of isoprene rubber in plasticorder "Brabender" PLE 6511 have shown, that the disperse process of composite is accompanied by lower power consumption and its best compatibility in comparison with zinc oxide (fig. 4). The absence of ZnCFO particles as extraneous impurities in rubber mix also was visually observed, while the zinc oxide particles were well appreciable [7],... 

The critical factor in the development of the syndrome is the size of the ultrafine zinc oxide particles produced when zinc is heated to temperatures approaching its boiling point in an oxidizing atmosphere." The particles must be small enough (zinc oxide powder is either inhaled or taken orally. Only freshly formed fume causes the illness, presumably because flocculation occurs in the air with formation of larger particles that are deposited in the upper respiratory tract and do not penetrate deeply into the lungs. ... 

The roasting (or oxidation) of sulfide ores to yield the metal oxides. For example, in the preparation of zinc oxide the sulfide ore is mined, crushed, separated from the gangue by flotation, and then roasted in a reactor to form hard white zinc oxide particles according to the reaction... 

As noted earlier, use of TDF by Smurfit is currently limited to 1 percent of the boiler fuel (by weight) by their air permit. Smurfit hopes to increase the percent TDF burned to 5 percent when an ESP is brought on-line to control their larger boiler. Smurfit personnel believe that the use of the ESP may increase the zinc content of the ash, thereby affecting its quality. This increase in zinc is expected because the ESP will pick up the fine zinc oxide particles with much more efficiency than the scrubber. In addition, an increase in TDF burned will increase zinc levels.15... 

In a later study, Pfeifer et al. [30] prepared Pd/Zn catalysts by both pre- and postimpregnation of wash-coated zinc oxide particles with palladium and compared their performance in methanol steam reforming. The catalytic performance of the samples was tested at a 250 °C reaction temperature, 3 bar pressure, a S/C ratio of two and 250 ms residence time. The WHSV amounted as 0.3 Ndm3 (min gcat) 1. The thickness of the coatings was calculated to 20 pm. The formation of the PdZn alloy was proven to occur at temperatures exceeding 200 °C by XRD measurements. 

Spanhel, L., H. Weller and A. Henglein (1987). Photochemistry of semiconductor colloids. 22. Electron ejection from illuminated cadmium sulfide into attached titanium and zinc oxide particles. Journal of the American Chemical Society, 109, 6632-6635. 

The aluminum in the zinc phosphate cements was considered very important, van Dalen [21] recognized its importance first. The reaction of zinc oxide and phosphoric acid was greatly moderated by aluminum. This effect was attributed to formation of an aluminum phosphate gelatinous coating on zinc oxide particles. In fact, Wilson and Nicholson believe that the gelatinous substance may even be zinc aluminophosphate phase [3], which subsequently crystallizes into hopeite and aluminophosphate amorphous gel (AlP04-nH20). 

In practice a large part of the carbon produced by the reaction (2) remains in the container. A part of the metallic zinc is vaporized at 907 0 and the reaction (3) occurs with a small flame. The vapours of the zinc chloride and carbon monoxide mix together uniformly and jet out into the air through the hole where they are cooled. In this case the zinc chloride condenses as A-substance and the carbon monoxide prevents the zinc chloride particles from cohesion acting as the B-substance. The zinc oxide particles are not so visible, but they easily absorb the moisture in the air and become small liquid particles which are visible as a dense white smoke. 

In the manufacture of zinc white by the French process, metallic zinc is melted, evaporated and oxidized in the vapor state to zinc oxide with air. If the starting material is insufficiently pure, the zinc has to be purified by evaporation and condensation prior to the oxidation step. The size and shape of the zinc oxide crystals can be controlled by the oxidation conditions. After combustion the ZnO is precipitated from the ZnO/air mixture in settling chambers, in which classification of the zinc oxide particles takes place according to their size. 

You could stop at your local zinc station, have the spent electrolyte and zinc oxide particles quickly suctioned from the cells and replaced with a slurry of fresh particles and electrolyte, and be ready to go another 200-300 miles. The zinc oxide would be taken to a central plant where it would be converted back to zinc metal and returned to the stations. 

Photoredox reactions can be carried out with semiconductors. Irradiation of nitrobenzene in the presence of zinc oxide particles in alcohol produced phenylhydroxylamine in 73% yield.443 Irradiation of 4-cyanophenylazide in ethanol containing titanium dioxide gave 4-cyanoaniline in 98% yield. Photooxidation of cyclohexane in the presence of titanium dioxide gave 85.4% cyclohexanone, 2.6% cy-clohexanol, and 12% carbon dioxide.444 To be of use commercially as part of a route to nylon 6,6, the amount of carbon dioxide must be reduced. This method has also been studied for the detoxification of wastewater containing organic contaminants, such as trichloroethylene.445... 

Hydrodynamic Sizes Relative to Airborne Sizes of Metal Oxide Particles Particles in dry form were found to be smaller than 1000 nm for fine zinc oxide dust and 50 to 70 nm for ultrafine zinc oxide dust (7). Particles were sonicated for 30 minutes. When suspended in distilled water, 20% of the ultrafine (nano) zinc oxide particles were suspended, with an average hydrodynamic diameter of 170 to 250 nm. When suspended in tissue culture media 45% had an average hydro-dynamic diameter of 300 to 400 nm. Tissue culture media suspended or extracted two to three times more particle mass than did dilute buffer or deionized distilled water, because of hydrodynamic action of media minerals. Hydrodynamic diameters were only modestly larger in tissue culture media than in distilled water (7). 

If those particles had been of moderate solubility, like the calcium phosphate, iron oxide, or zinc oxide particles, they would have been suspended sufficiendy long to be dissolved. Alternatively, if they had been nearly insoluble, as were the titanium oxide, cerium oxide, and zirconium oxide particles, we would expect them to persist. Thus, the particles could either dissolve direcdy or persist as insoluble particles. Previous work with titanium dioxide has shown it to be virtually insoluble in vitro and to persist in vivo. 

Fine and nanosized zinc oxide particles caused in vivo releases of LDH, indicating cell killing cell numbers, indicating inflammation and increased polymorphonuclear leukocytes (PMN), indicating acute inflammation in rat BALF this response reduced quickly for LDH and BALF cell numbers BALF PMN persisted to 1 week after initial exposure (7). These particle responses resembled those of particles that did not persist (carbonyl iron and amorphous silica). However, crystalline silica while having similar intensities of response, persisted to 3 months after initial exposure, suggesting that the inflammation continued (7). Responses of factory workers to continued zinc fumes more closely resembled that of crystalline sihea chronic flbrosis (16, 17), or healing by secondary intent (fibroplasia and fibrosis) (31, 32, 35-37). 

Bakand S, Winder C, Khalil C et al (2006) An experimental in vitro model fm dynamic direct exposure of human cells to airborne contaminants. Toxicol Lett 165 1-10 Bartoli CR, Wellenius GA, Diaz EA et al (2009) Mechanisms of inhaled fine particulate air pollution-induced arterial blood pressure changes. Environ Health Perspect 117 361-366 Becker S, Soukup JM, Sioutas C et al (2003) Response of human alveolar macrophages to ultrafine, fine, and coarse urban air pollution particles. Exp Lung Res 29 29-44 Beckett WS, Chalupa DF, Pauly-Brown A et al (2005) Comparing inhaled ultrafine versus fine zinc oxide particles in healthy adults a human inhalation study. Am J Respir Crit Care Med 171 1129-1135... 

---