The effect of iron oxides on the physicochemical properties of primer enamels that regulate capillary movement of gas bubbles was investigated. The rates of capillary movement of bubbles in an enamel melt were calculated based on two mechanisms: thermodynamic and Marangoni film flow. The duration of migration of gas bubbles from the enamel layer under the effect of capillary force was determined in approximating the predominant role of the thermodynamic mechanism.The macrostructure of enamels determines their performance properties, so that it is necessary to know the mechanisms of distribution, concentration, and size of gas bubbles in the bulk. The presence of large bubbles causes rejection of enameled items, in particular, overfiring and burnout. The possibility of affecting the behavior of the bubbles allows controlling the quality of enamel coatings.One feature of glassy coatings on metals is the inhomogeneity of the composition of the oxide-silicate film caused by exchange reactions on the metal -enamel interface, accompanied by migration of iron oxides into the enamel melt. When the primer is fired, the thin contact layer of the oxide melt becomes rich in iron oxides, and their content can reach 10 -15% and more. The gas bubbles formed on the metalprimer melt interface are in the zone of a variable concentration of iron oxides.We investigated the effect of iron oxides on the physicochemical properties of primer enamels that regulate capillary movement of gas bubbles. The surface tension gradient over the height of the enamel layer that appears in firing ds/dh causes movement of the bubbles, which is prevented by the viscous resistance of the melt. Using the experimental values of the viscosity h and surface tension s obtained, the rates of capillary movement of gas bubbles in an enamel melt were calculated. Enamels from frits GK-321, GK-326, GK-331 and composition C of these three frits used in concrete enamel production were selected as the initial melts. The studies were conducted on each initial melt with a different Fe 2 O 3 content: 0, 1, 2, 4, 6, 9, 12, and 15%. 2 The surface tension of the enamel melts was investigated by the sessile drop method at 800 -850°C. The dependence s = s (Fe 2 O 3 ) of all enamels investigated has a minimum at a 8 -10% Fe 2 O 3 content (Fig. 1). This is probably due to rearrangement of the structure of the melt, and the subsequent height of the curves is caused by crystallization. The gas bubbles in the bulk of the melt thus have an interface with the enamel when the surface tension changes.The viscosity of the enamels was measured by the cylindrical indenter impression method. The effect of the temperature on the viscosity of the outer layer of primer at different concentrations of iron oxide is shown in Fig. 2. Similar dependences were obtained for all enamels studied. The position of the curves changes nonproportionally when the concentration of Fe 2 O 3 increases, probably due to crystallization.The activation energy of viscous flow E h was determined with the exper...
The IR absorption spectra of boron-containing enamel prime coatings with different content of lithium oxide Li 2 O are studied and the coordination state of the cations is analyzed taking account of the content of the main components.The lithium ion has a relatively small radius, and for this reason it tightly binds the oxygen anion. This feature of lithium cations influences the formation of the oxygen environment of boron cation B 3+ and the aluminum cation Al 3+ . In a study of the properties of silicate glasses 16Me 2 O × xB 2 O 3 × (84 -x )SiO 2 , where Me 2 O = Na 2 O + K 2 O + Li 2 O, x = 8 -14, it was concluded that in lithium glasses the transition of boron into quaternary coordination proceeds with much greater difficulty than in sodium and potassium glasses [1].The number of [BO 4 ] tetrahedra in these glasses is much smaller for the same content of alkali oxide. For this reason the oxygen fraction capable of interacting with Al 3+ is much larger. Therefore the construction of [AlO 4 ] tetrahedra in lithium glasses can proceed without oxygen being taken from the [BO 4 ] tetrahedra and destroying the latter.The oxygen bound with silicon and lithium is sufficient for the formation of [AlO 4 ] tetrahedra. Thus, the difficulties of the transition of boron into quaternary coordination explain the almost complete absence of an aluminum-boron anomaly in the properties of lithium-containing glasses [1].In the present work we studied the ground-coat frits GK-321 and GK-326 (GOST 24405-80) with partial replacement of alkali oxides (Na 2 O + K 2 O) by different weight percentage content of lithium oxide (0, 3, 6, and 12%) as well as the addition of lithium oxide (above 100%) -0.5, 1,5, 2, 4, and 6% -to the GK-321 frit.The IR spectra were recorded with a Tensor Fourier Spectrometer (Bruker Co.), operating in the frequency range 4000 -400 cm -1 .First, the structural states of Al 2 O 3 and B 2 O 3 were determined by Appen's method [2]. For this calculations of the weight content y B of alkali and alkaline-earth metal oxides were performed for all experimental samples using the relationThe computational results are presented in Table 1. An analysis of the IR spectra performed for seven samples of GK-321 frit with different content and method of adding lithium oxide gives interesting information. The IR spectra are presented in Figs. 1 and 2.According to [3] the region 1500 -1150 cm -1 where the fundamental asymmetric stretching vibrations of the atoms in the [BO 3 ] groups appear can be singled out in the IR spectra of the samples. The stretching vibrations of the atoms in the groups [BO 4 ] and [AlO 4 ] appear mainly in the region 1150 -800 cm -1 , the latter appear in the frequency interval 870 -650 cm -1 while the stretching vibrations of the atoms in the [AlO 6 ] groups appear in the interval 650 -400 cm -1 .The bands in the region 500 -470 cm -1 could be due to, aside from these vibrations, the deformational vibrations of
The possibility of calculating the CLTE, viscosity, and surface tension of frit melt for undercoat enamel using different methods with variation of the Fe 2 O 3 content and taking into consideration the experimental data is examined. Knowing the percentage ratio of three initial frits in the undercoat enamel and their experimentally determined properties, additive methods are used to determine the properties of the undercoat enamel.Many computational methods for determining the properties of glasses and enamels are known. In using these methods, their specific features are taken into account: lower accuracy as compared with the experimental data and application limited to certain definite composition ranges. Nonetheless, only the combined application of the computational and experimental methods permits arriving at the correct results.In the present work, together with experimental determination, the possibility of calculating the CLTE, viscosity, and surface tension of the frit melt for the undercoat enamel by different methods with variation of the Fe 2 O 3 content is examined. Knowing the percentage ratio of three initial frits in the under coat enamel and their experimentally determined properties, additive methods have been used to determine the properties of the undercoat enamel. Appen's method was used to calculate the CLTE for frits theoretically. A characteristic feature of this method is that instead of mass units, characterizing the oxide content in the enamel, it employs molar units -fractions or percentages. The presence in the enamel of components with variable partial properties not only complicates the calculations but it also decreases the accuracy of the results. The average CLTE can be calculated by Appen's method to within ± 2.2´10 -7 K -1 [1]. Figure 1 compares the experimentally determined CLTE data and the computed values obtained by Appen's method [1].The values of the CLTE of samples of the initial frits GK-321, GK-326, and GK-331 depend directly on the amount of alkali oxides in the melt (it decreases negligibly). The CLTE of these samples lies in the range (90 -129)1 0 -7 K -1 , which is explained by the closeness of their compositions.
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