The aim of this work was to investigate the gloss surface tension effect on bubble evolution during burning, and the relationship between gloss surface tension, melt viscosity and bubble evolution. The study showed that lowering viscous drag is more important in influencing bubble evolution than reducing surface tension.
Surface Tension
Young-Laplace Equation
In general, it is necessary to call two radii of curvature to describe a curved surface, as shown in figure 2.2.15. As shown in Figure 2.2, R1 then oscillates in the plane of the paper, i.e., it is the curvature of the profile at the point in question.
Measurement Methods Based on the Shape of Static Drops
The sessile drop technique is often used to measure the surface tension of glasses at high temperatures.16-19 This technique was originally developed by Bashforth and Adams.20 The interfacial tension between liquid and vapor is derived from the shape of a liquid sessile drop on a non -seated drop. -wetting substrate, where the droplet shape is expressed in x-y coordinate points, as shown in Figure 2.3. The surface tension is calculated based on input of the dimensional parameters measured from the image analysis and the density.
Bubble Behavior
Nucleation and Formation
The surface tension is calculated based on inputs of the dimensional parameters measured from the image analysis and the density. where the PB is the internal pressure and PL is the external pressure. The equation of motion is the statement of Newton's second law as applied to an element of fluid that moves with the mass average velocity of the fluid:23.
Growth or Dissolution
The quasi-stationary approximation requires that we neglect the motion of the bubble boundary and the convective transport term in the differential equation. Further simplification of the diffusion equation by neglecting the time derivative results in a solution commonly referred to as the quasi-steady state approximation.
Rising and Bursting
It also predicts that the rate of bubble removal will be proportional to the square of the bubble radius. However, due to the complexity of the system, no general analytical approach is available to study this bubble collapse phenomenon.
Surface Tension Effect on Bubble Behavior
Nucleation and Formation
By introducing the effects of surface tension and viscosity into Equation 2.35, Marique and Houghton solved a modified form of Rayleigh's equation for a bubble growing under different conditions:24. Their work also suggests that during the early stage of growth, the effects of surface tension and viscosity tend to "stabilize" the nuclei and prevent rapid growth for a short period of time.
Growth or Dissolution
Further, they also found that the viscosity effect was somewhat reduced at high surface tension. Weinberg's study of the effect of surface tension with the moving boundary showed that the surface tension corrections for bubble breakup are.
Bubble Bursting
Bubbles in Glasses
Viscous Sintering
Fining Agents
Cable and co-workers published the first detailed measurements of both the number and size distribution of bubbles during the clarification process.55-57 He reported fairly extensive measurements in laboratory melts of soda lime and silica. Their results also showed that the clarifying agents had a significant effect on the seediness of the melt early in the process as well as the bubble removal rate.
Oxides Effects on Surface Tension
It has been assumed that the surface tension is related to the ionic potentials (Z/r) of the glass components (Z=cation valence; r=cation radius).19 The ionic potential correlates with the binding force between the cations and oxygen in the glass. Then they can create a deformed and parallel (to surface) oriented structure on the surface and thereby lower the surface tension.68.
Conclusions
P= B− L= In heterogeneous nucleation, the pressure differential, the liquid surface tension and solid particle geometry determine the growth or dissolution of small bubble nuclei. The growth or dissolution of a bubble in a liquid is controlled by the rate of transfer of material across the bubble surface: the rate of bubble growth is reduced with the increase in surface tension.
Glaze Preparation and Application
After holding at the peak temperature for 30 minutes, the sample was rapidly cooled to below 100°C at a rate of 25°C/min.
Bubble Observation
When the desired temperature was reached, real-time images were collected with a CCD camera, recorded by an S-Video recorder and imported into a computer. The bubble number and size were determined by visual counting and measurement, and all the bubbles captured in the images were counted.
High Temperature Density and Surface Tension
A polished graphite substrate on a refractory plate was placed in the center of the tube furnace using a charging cart. The sample was kept in the cold zone during heating and then pushed into the hot zone at the measurement temperature. A sessile droplet was then formed and the temperature was lowered gradually to obtain the droplet shape at different temperatures.
The high-temperature density and surface tension were derived based on the droplet shape images of the glaze at 1200oC. Since the density and surface tension of a glassy melt change only slightly with temperature in the glassy state,69 the values are measured at 1200oC. An integrated volume method for a finite cylindrical unit was used to calculate the droplet volume as shown in Figure 3.4.
A macro written for the NIH image was used to find the coordinates of the droplet, as shown in Figure 3.5.
Characteristic Temperatures and Viscosity of Melts
The temperature of the hemisphere, T1/2, is where the contact angle is 90o, and the height of the sample is half the width of the base. Reference viscosities and their identified temperatures were inserted into equation 3.1 in order to solve for three unknown constants.
Density at Room Temperature
Introduction
Experimental
MoO 3 Effect on Bubble Evolution
- Bubble Formation
- Bubble In-situ Growth
- Bubble Rising and Bursting
The relationship between the viscous flow and the surface tension is shown in the Frenkel's energy balance and the Mackenzie and Shuttleworth equation in Section 2.4.1.50. A higher surface tension will lead to the production of larger bubbles that detach from nucleation sites. Therefore, the lowering of surface tension causes bubbles to become smaller as they detach and move to the surface.
However, there was a deviation at a surface tension of 256 mN/m, possibly because gas in the bubbles is not only oxygen. The largest bubble volume occurred when the surface tension was the highest and the viscosity was the lowest. This is because fewer bubbles nucleated and less gas dissolved in the melt at low surface tension.
An increase in surface tension from 256 mN/m to 272 mN/m significantly increased the number of large bubbles present in the glazes.
The photographs of glazes with varying levels of MoO3 and Sb2O3 in Figures 4.4 and 4.5 clearly showed that MoO3 additions resulted in little if any bubbles. Glazes with no MoO3 and the highest level of Sb2O3 produced the highest bubble volume in all cases. Increased levels of MoO3 appeared to negate any effect of Sb2O3 increasing the number and size of bubbles due to the release of oxygen at high temperature.
The surface tension value previously measured for bubble-free lead glaze additives was 252 mN/m. The effect of MoO3 and Sb2O3 on bubbles in glazes fired on a porcelain base was similar to that on an alumina base, as shown in Figure 4.25. Effect of additives on the volume of glaze bubbles in glazes fired on an alumina substrate (1) after reaching 1150oC and (2) after holding for 30 minutes.
Additives effect on bubble volume in glazes fired on a cone 10 porcelain substrate when 1150oC is reached.
Conclusions
Although Sb2O3 is used to remove bubbles in industrial glass melts, it tended to increase the number of large bubbles in glazes studied.
Introduction
Experimental
Sample Preparation
Chemical Analysis
TGA and FTIR
Results
System with Varying PbO:Na 2 O
The experimental error for surface tension is ±2.4 mN/m, and experimental errors for density at 1200oC and at room temperature are ±0.025g/cm3 and ±0.009g/cm3, respectively. As expected, a higher PbO and lower Na2O ratio significantly reduced the thermal expansion of the lead glazes. Experimental errors for Tg and Td are ±3oC and ±6oC respectively, and experimental errors for the temperature where in Pa•s) are ±8oC.
The viscosities of A-I and A-II glazes are much lower than the typical melt viscosities of industrial glazes at the maximum firing temperature.
System with Varying CaO:Na 2 O
The experimental errors for Tg and Td are ±3oC and ±6oC respectively and the experimental errors for temperature where is = 103.55(in Pa•s).
TGA and FTIR
Discussion
- Composition Variations
- RO:R 2 O Ratio Effect on Glaze Properties
- RO:R 2 O Ratio Effect on Bubble Evolution
- Statistical Analysis
- Gas for Bubble Formation
The percentage of PbO was calculated assuming that PbO is the only remaining oxide in the frits. As the RO:R2O ratio was increased in lead-containing glazes (higher PbO, lower Na2O), the surface tension was lowered from 264 to 252 mN/m, as shown in Figure 5.6. When CaO replaced Na2O in the lead-free glazes, the surface tension increased from 266mN/m to 284mN/m, as shown in Figure 5.8.
Although the glaze surface tension decreases from 264mN/m to 252mN/m with the replacement of Na2O by PbO, the glaze viscosity increases, resulting in the increase of bubble volume at 1150oC with the ratio. Data from the RO:R2O experiments and the previous study in Chapter 4 with varying MoO3 in a lead-free glaze were combined to quantify the overall surface tension-viscosity-bubble volume relationships. Viscosity and temperature are of course linked, and the gas pressure in the bubble is also a function of temperature.
The inclusion of density and temperature as independent variables in the model provided a much better fit.
Conclusions
The models predict "ideal" conditions for lowering bubble volume are surface tension values at the low end of the tested range (below 260 mN/m) and a viscosity range of approx. Passport.
Introduction
Experimental
Apparent Porosity
Optical Microscopy and EDS
Results
Bubble Evolution in Glazes on Three Different Substrates
Microscopic Evaluation of Cross-sections
Discussion
Conclusion
Summary
Additives Effect on Bubble Evolution
Most bubbles were eliminated when the surface tension was below approx. 265 mN/m, which is close to the value found for the bubble-free lead glaze. Large bubbles, visible without magnification (> 100 µm diameter), were not stable at 1150oC when the surface tension was below approx. 267 mN/m.
Glaze Composition Effect on Bubble Evolution
Crandall, "Effect of Addition of Certain Oxides on the Surface Tension of Glass, Enamel Frit and Glaze as Measured by the Falling Weight Method";. Surface Tension of Silicon, Iron, and Nickel," J. Burnet, "A Nonlinear Regression Method for the Calculation of Surface Tension and Contact Angle from the Shape of a Sessile Drop," Surf. III, Dissolution and Growth of a Rising Bubble Containing a Single Gas," J. German, “Bubble behavior in glass melts.
Growth of Vapor Bubbles in Superheated Liquids", J. Weinberg, "Surface Tension Effects in Gas Bubble Dissolution and Growth", Chem. Pennington, "Effects of Surface Tension and Viscosity on Taylor Instability", Q. Rahaman, Ceramic Processing and Sintering. Yamanaka , "Surface tension measurement of glass melt using the maximum bubble pressure method", Glass Sci.
Scholze, "The influence of viscosity and surface tension on the hot-stage microscope measurements of glasses", Ber.
