Small Amount Of SiO2 Research Articles

Orthorhombic Modifications of Tricalcium Aluminate in Portland Cement Clinker

An orthorhombic aluminate named “granular phase” from its morphology has been found in the crystallization product of the melt with the composition approximate to the interstitial material of Portland cement clinker. The chemical composition of this phase is very similar to that of the prismaticphase, yet the granular phase can be readily distinguished from the prismatic phase under the microscope by its form, birefringence and the mode of twinning.Both granular and prismatic phases are pseudo-cubic. The granular phase has an orthorhombic unit cell with the dimensions of 15.36×15.40×15.17A3, and the space group is Pbam or Pba2. The axes, a and b, of the granular phase lie diagonally to those of the prismatic phase when both structures are in corresponding orientation.The prismatic and granular phases have been shown to be isomorphous with the lowand high-Na2O forms of the orthorhombic C3A in the C3A-Na2O system respectively. These isomorphous relations indicate that the existence of Na2O is of primary importance to the stabilization of the anisotropic dark interstitial material in cement clinker, and that the granular phase contains more Na2O than the prismatic phase. NC8A3, which may contain a small amount of SiO2 as an indispensable constituent, is structurally referable to the granular phase or to the high-Na2O form of orthorhombic C3A.In the granular phase as well as the prismatic phase, a topotaxial transformation into cubic C3A caused by the vaporization of Na2O at high temperatures was observed. A compositional transformation between the high- and low-Na2O forms of orthorhombic C3A also was suggested.

Alumina Ceramic-to-Metal Seals by the “Mo-Mn Process”

The so called “Mo-Mn Process” is one of the methods for forming ceramic-to-metal hermetic seals by metallizing, nickel plating and succesive hard-soldering.High alumina (94wt% Al2O3) ceramic discs and Kovar parts were sealed together by the process. Tensile strengths of sealing layers were measured. The layers were examined by opical and scanning type electron microscope and by electron probe microanalyzer.The results were as follows:1) Tensile strengh of the sealing layers was the highest when the ceramics were metallized at 1450°C for 60minutes.2) The sealing layers were formed by many layers which were arranged in the following order;Ceramic-Intermediate layer-Metallizing layer-Ni plating layer-Hard solder-Ni plating layer-Kovar.The intermediate layer was composed of Al2O3, MnO and MgO together with small amounts of SiO2 and CaO. Main crystalline phase in the layer was MnO⋅Al2O3. The thickness of the layer was proportional to that of the adjacent metallizing layer.3) Pores in the Mo-Mn metallizing layer were filled with glass phase and nickel; the former came out from ceramic and the later from nickel-plating layer. Infiltration and interdiffusion of these materials formed strong and vacuum-tight sealing layers.

Deferration Effect on Structural Ferrous-Ferric Iron Ratio and CEC of Vermiculites and Soils

AbstractDeferration by reduction of free Fe2O3 with Na2S2O4 in the presence of Na citrate and NaHCO3 caused a change in valence state of 10 to 35 per cent of the total structural iron in micaceous vermiculites, soils, nontronite, and muscovite. An increase in Fe2+ on deferration was accompanied by an equivalent decrease in Fe3+. Subsequent treatment with H2O2 reoxidized the structural Fe2+ previously formed.Sesquioxide coatings on micaceous vermiculites were examined electron microscopically. These coatings were composed predominantly of Fe2O3 with approximately 10 per cent by weight of Al2O3 and small amounts of SiO2, as determined by chemical analysis of the deferration extracts.The cation exchange capacity (CEC) increased 10–60 per cent as a result of deferration of micaceous vermiculites and soils. Treatment of the deferrated sample with H2O2 restored the Fe3+ content to approximately the original value but the CEC was not affected. Consequently, the increase in CEC on deferration was attributed to the removal of the positively charged sesquioxide coating. The reversible change in valence of structural iron without an equivalent change in CEC was attributed to deprotonation-protonation of the structure (OH− ⇄ O2−) simultaneous with the oxidation-reduction of iron (Fe2+ ⇄ Fe3+) in the phyllosilicate layer.

Effects of Crystal Habits of Magnesium Hydroxide, and Additions of Iron Oxide and Silica on the Sintering of Magnesia

As a part of the studies on the sintering behavior of the magnesia prepared by decomposition of magnesium hydroxide, effects of the crystal habits of the hydroxide, and of the additions of iron oxide and silica on the sintering process of magnesia were examined by means of measurements of dilatation and isothermal shrinkage of the specimens.The crystal habits of the hydroxide, i.e. fibrous and massive, have no effect on the sintering process of magnesia. The addition of iron oxide promotes the sintering of magnesia, while the addition of silica rather retards the sintering, except the case when the amounts as small as 0.3% SiO2 is added, which accelerates the sintering a little. The isothermal shrinkage of the specimens of Mg(OH)2 and of the specimens consisting of Mg(OH)2 and Fe2O3 or small amounts of SiO2 are all represented by the general formula (ΔL/L0)p=B″t, which means that the sintering of these specimens is the process controlled with the diffusion mechanism. The experimental activation energy of the diffusion sintering for the specimens of the magnesium hydroxide was 47-57kcal/mole, which is considerably lower than those of the processes controlled by the similar diffusion mechanism. These low activation energy might be due to the fact that the magnesia used for these measurements were prepared by the decomposition of hydroxide at temperature as low as about 500°C, so that it contains many defects in the lattice. The addition of 0.3% Fe2O3 decreases the activation energy and the frequency factors for the sintering, but, on the contrary, the further addition of it increases both of these values. The addition of 0.3% SiO2 also decreases these values. The acceleration of the sintering of MgO by the additions of Fe2O3 and a small amount of SiO2 is due to the reduction of the experimental activation energy. For the over-all sintering of the specimens, the frequency factors as well as the activation energy must be taken into consideration.

A Radioisotope Study of the Sintering Process of Magnesia Studies on Dead-burned Sea Water Magnesia, III

A radioisotope technique was used for the determination of the rate of movement of Ca2+ and Fe3+ ions in MgO added with or without a small amount of SiO2, Al2O3, etc. As the tracers RI45Ca and RI59Fe were used. The relationship between the rate of movement of these ions and the sintering behaviors of the mixtures were also studied.The results obtained were summarized as follows:(1) The movement of these ions is caused by thermal diffusion and draining. In MgO added with CaO and SiO2, the rate of movement of Ca2+ ion increased with increasing the molar ratio CaO/SiO2, reached to a maximum when CaO/SiO2≅1, and then decreared; no movement was observed when CaO/SiO2_??_2. Additions of Al2O3 and Fe2O3 to MgO together with CaO and SiO2, however, produced different results, i.e., the rate of movement of Ca2+ in this mixture increased with increasing the ratio (CaO/SiO2) without having a maximum.(2) Additions of oxides such as TiO2, Cr2O3, MnO2, B2O3, etc., which were found, by the author's previous works, to be effective in promoting sintering of MgO and the growth of periclase crystals, always increased the rate of movement of Ca2+.(3) Comparing with Ca2+ ions, Fe3+ ions in MgO had a low rate of diffusion. Draining occured only when Ca2+ coexisted in MgO.(4) X-ray analysis showed that the compounds, CaO⋅MgO⋅SiO2, 2CaO⋅Fe2O3, 4CaO⋅Al2O3⋅Fe2O3, etc., were produced in MgO by the movement of Ca2+ and Fe3+ ions.(5) It was suggested that when the above compounds melt at high temperatures producing the liquid phase, the loard-bearing capacity of MgO is lowered.

SOME RELATIONS OF COMPOSITION TO SOLUBILITY OF ENAMELS IN ACIDES1

Review of literature.—The literature on acid resisting enamels and glasses is reviewed. Acid resisting powers produced by different oxides.—When substituted one for another, in each case in equal percentage amounts, various oxides and minerals increased the resistance of various enamels to the action of boiling 20 per cent hydrochloric acid in the following order of effectiveness: Al2O3, cryolite, Na2O, PbO, BaO, Li2O, MgO, CaF2, ZnO, SrO, CaO, B2O3. The first five mentioned were especially effective. The remainder are undesirable materials from the standpoint of acid resistance. Effect of the enamel base.—The relative effects of the materials were the same for various base enamel compositions. Effect of ZrO2 and TiO2.—ZrO2 and TiO2, when substituted for small amounts of SiO2, increased the resistance of various enamels to attack by acids, ZrO2 being most effective in this respect. The favorable action of ZrO2, is offset by a tendency to produce excessive chipping. Rutile gives less chipping than either zirconia or silica and greater acid resistance than silica. Best compositions.—Enamels 2,17,19,26,27, and 33 of table 1 showed resistance to action of the acid equivalent to that of the original competitions based on the analysis of a French acid-resisting enamel. By use of the data developed in this investigation other highly acid-resisting compositions can readily be formulated.