4 Fundamentals
6.7 Aluminosilicate Glasses
Table 44: Chemical main components of tested aluminosilicate glass fibers given in technical data sheets of the manufacturer
Components [ma-%]
Aluminosilicate 0812 (Schott 8252) [150], [177]
Aluminosilicate 0813 (Schott 8253) [178]
Borosilicate C5 [151]
SiO2 60.0 61.0 72.0
Na2O 0.02 0.02 6.7
Al2O3 14.0 16.5 6.8
B2O3 4.5 0.5 11.4
BaO 9.0 8.0
CaO 10.0 13.0
MgO 2.5
K2O 2.4
Table 45: Characteristic test results of hollow aluminosilicate fibers with different dimensions, the fibers were made of two types of aluminosilicate glass with different composition, borosilicate C5 fibers conduce as reference
Material
Dimension (do; di; s)
[µm]
Min. burst pressure
pmin
[MPa]
Max. burst pressure pmax [MPa]
Form parameter
b
Characteristic pressure P
[MPa]
Alumino
0812 340; 300; 20 20.7 44.9 7.1 38.0
Alumino
0813 300; 240; 30 31.2 101.1 5.8 64.4
Boro C5 320; 260; 30 41.2 78.3 8.5 55.1
Alumino
0812 400; 300; 50 37.5 61.6 7.9 53.5
Alumino
0813 400; 320; 40 50.7 102.3 7.1 79.5
Boro C5 420; 340; 40 30.2 57.3 8.8 45.9
Test results with hollow fibers made of aluminosilicate 0812 glass with dimensions of do = 340 µm, di = 300 µm and s = 20 µm showed high resistance against inner pressure in comparison to hollow fibers made of other glasses like borosilicate 3.3 or quartz glass (chapter 6.1). Comparing these results to measured data of fibers from aluminosilicate 0813 and borosilicate C5, the lowest pressure resistance is detectable for hollow fibers made of aluminosilicate 0812. Samples from aluminosilicate 0813 with do = 300 µm, di = 240 µm and s = 30 µm exhibit 55% improvement in minimum burst pressure. The maximum burst pressure is more than double compared to the other aluminosilicate and the characteristic pressure is nearly double as well. The form parameter b is smaller than the value of aluminosilicate 0812 due to the higher spread as well as the increase of improvement, which is nearly 55% at minimum and about 120% at maximum burst pressure value.
Also the data of borosilicate C5 features higher value than aluminosilicate 0812. The minimum burst pressure is twice as high as the according value of aluminosilicate 0812 and even 10 MPa higher than the value of 0813. Nevertheless, the characteristic pressure and the maximum burst pressure of borosilicate C5 are higher than those of aluminosilicate 0812, but remain below the values of according data of samples from aluminosilicate 0813.
With regard to the larger dimensions a similar behavior can be detected. Again test samples made of aluminosilicate 0813 exhibit the highest measured values of minimum and maximum burst pressure. An improvement of 35% at minimum and 40% at maximum burst pressure compared to aluminosilicate 0812 is discernible. The characteristic pressure P actually increased by the factor 1.5.
Nevertheless, aluminosilicate 0812 fibers with large dimensions of do = 400 µm, di =300 µm and s = 50 µm features higher pressure resistance than borosilicate C5 which is indicated by higher reached pressure values. The maximum burst pressure reaches a comparable range but still remains below the value of aluminosilicate 0812.
Indeed, the form parameter b is the highest in comparison which stands for a narrow distribution of measured values. However, the minimum burst pressure as well as the characteristic pressure of borosilicate C5 have smaller values than those of aluminosilicate 0812 and therefore indicate a lower resistance against inner pressure load.
The calculated failure probabilities of the three tested glasses with small dimensions are plotted against the single burst pressure values and shown in Figure 54. Again, the
visualization gives a proper way of comparison of the different behaviors and pressure resistances of the test series.
Figure 54: Comparison of the failure probability of hollow aluminosilicate glass fibers with similar dimensions in the range of do ≈ 320 µm, di≈ 250 µm and s ≈ 30 µm but different glass mixture, as comparison value the test results of borosilicate C5 fibers were plotted
The graph of aluminosilicate 0812 test samples represents an S-curve with clear and sharp development. Due to the high slope a convergence to an ideal step function is detectable. The graph of aluminosilicate 0813 does not exhibit such a high slope but slides to the right significantly, as illustrated in the diagram. This is a distinct indicator for higher pressure resistance due to higher burst pressure reached by the test samples.
Indeed, the minimum burst pressures as initial points of the curves are close together.
Nevertheless, an improvement of almost 50% is detectable at the initial point. The maximum values of both series feature a significant distinction which leads to the lower slope of aluminosilicate 0813 but also leads to an improvement of 120% in maximum.
The form and development of the graph of borosilicate C5 are comparable to aluminosilicate 0812. Similar to aluminosilicate 0813, the graph slides to the right and indicates a higher pressure resistance.
Calculations of failure probabilities of test series with larger dimensions were done and visualized as well. The diagram is given in Figure 55 whereby again the failure probability is plotted against the burst pressure of single test samples.
Figure 55: Comparison of the failure probability of hollow aluminosilicate glass fibers with similar dimensions in the range of do ≈ 400 µm, di≈ 300 µm and s ≈ 50 µm but different glass mixture, as reference value the results of borosilicate C5 fibers was taken
In comparison to the test samples with smaller dimensions, the graph of aluminosilicate 0812 represents a clear and sharp S-curve with high slope. Again the curve of aluminosilicate 0813 shows a lower slope which indicates a wider spread of measured pressure values. However, the graph slides to the right. As a result, the pressure resistance of aluminosilicate 0813 is higher than aluminosilicate 0812 of similar dimensions.
The graph of borosilicate C5 again shows similar development and form than the graph of aluminosilicate 0812. But the graph of C5 slides to the left compared to 0812.
Therefore the resistance against inner pressure of borosilicate glass in tested dimensions is lower than aluminosilicate glass independent of composition.
The comparison between the two different aluminosilicate glasses repeatedly demonstrated the influence of chemical composition on the mechanical properties.
During previous investigations of borosilicate glass fibers the impact of boron and alumina oxide on the pressure resistance was examined (chapter 6.5). An increase of alumina oxide at the same time as a decrease of the percentage of boron oxide leads to an improvement of pressure resistance. The addition of alkaline and alkaline earth oxides in general leads a weakening of the silica network due to the breaking of bridge oxygen [3]. The formation of disconnecting points leads to decreasing temperatures which are important in the manufacturing and processing of glass [3], [4]. Al2O3 and B2O3 areclassified as stabilizers and can effectuate a reduction of disconnecting points dependent on their concentration. The physical properties of glass thereby are dependent, most notably on the percentage of boron oxide, which has an open structure.
In high concentrations boron oxide has a destabilizing effect [4], [2] whereby the limiting concentration depends on the types of additives and their percentage.
The two examined glass types from the aluminosilicate group exhibited different chemical compositions. Independent of the dimensions, fibers made of aluminosilicate 0813 featured the highest resistance against inner pressure load. The measured burst pressures and the resulting characteristic pressures of fibers of small, as well as of large dimensions, reached values that are nearly twice as much as in the comparison with aluminosilicate 0812.
The chemical composition of the two glass types might be the reason for that behavior.
The percentage of silica as the main component and network former in both glasses exhibited nearly the same value. Nevertheless, aluminosilicate 0812 had 14.0% of alumina oxide and 4.5% of boron oxide. In contrast, aluminosilicate 0813 featured a percentage of Al2O3 of 16.5% and an almost negligible amount of 0.5% boron oxide.
Compared to the reference glass borosilicate C5 the amount of B2O3 is low. However, attention has to be paid to the amount of silica which is significantly higher for borosilicate. Therefore it can be assumed that the effect of boron and alumina oxide in the composition of borosilicate is different to the effect in aluminosilicate. The concentration of 4.5% of B2O3 in 0812 can lead to a decrease of resistance due to the small percentage of 60% silica. Therefore it can be assumed that the concentration of
4.5% B2O3 exceeded the limiting concentration of stabilizing effect and boron oxide became a network modifier. An exact value of the limiting concentration of B2O3 is not given in the literature due to its dependency on the percentage of silica and any other component [3], [4]. Therefore for each chemical composition it is varying.
Aluminosilicate 0813 featured only 0.5% of boron oxide but a simultaneously higher percentage of alumina oxide. Al2O3 led to a stiffening of the network structure due to the effect of a network builder. The very low percentage of 0.5% B2O3 led to an increasing impact on the mechanical resistance as well. In that low concentration boron oxides acts as a network builder and a reduction of disconnecting points is effectuated by Al2O3 and B2O3.
The used reference material borosilicate C5 had a significantly higher percentage of 72% of SiO2 wherefore a higher amount of network builder is available. The limiting concentrations of Al2O3 and B2O3 changed in comparison to aluminosilicate. Al2O3 did not have to act as a network builder in that dimension as in the aluminosilicate glasses with only 60% of silica. Nevertheless, fibers of C5 showed contrasting results in pressure resistance. Samples of small dimensions reached burst pressures between both aluminosilicate glasses, while fibers of large dimensions reached the smallest pressure resistance compared to aluminosilicate glasses.
At this juncture the deviation in dimensions had to be considered. Focusing on the small dimensions fibers made of aluminosilicate 0812 exhibited the smallest wall thickness which could be an advantage. However, the test samples had the largest diameter as well. Hence, the tested samples exhibited different inner surfaces and volumes which acted on those surfaces. Smaller wall thickness led to higher pressure resistances in earlier investigations only in union with smaller dimensions.
The comparison of test samples of large dimensions led, however, to a completely opposite result. In comparison with borosilicate C5 and aluminosilicate 0813, fibers made of aluminosilicate 0812 had the highest wall thicknesses. In addition, samples of 0813 and C5 exhibited higher inner diameters. Nevertheless, aluminosilicate 0813 showed the highest resistance against inner pressure load independent of dimension.
The investigation of hollow glass fibers made of different aluminosilicate glasses showed the positive effect of high percentages of Al2O3 on the pressure resistance. Indeed, the deviations in dimensions had to be considered. However, the higher percentage of alumina oxide led to higher reached burst pressures by having both smaller and larger wall thicknesses. Therefore the effect of dimensions could be exceeded. In comparison
to borosilicate fibers with similar dimensions higher pressure resistance can be reached as well.
As result of this investigation it can be summarized that:
- Fibers of two different aluminosilicate glasses in two different dimensions were investigated. Aluminosilicate 0813 featured higher percentage of Al2O3 and significant lower amount of boron oxide compared to aluminosilicate 0812.
- Aluminosilicate 0813 showed an improvement in pressure resistance by pressurized fibers. Based on the characteristic pressure of test series the increased aluminum oxide amount led to an improvement between 50% and 70% compared to aluminosilicate 0812 with lower Al2O3 amount.
- Compared to borosilicate C5 glass as well an improvement was detected of 15% and 70%, respectively. Consequently, the increase of Al2O3 with simultaneous decrease of B2O3 led to improved pressure resistance as seen for borosilicate glass as well.
- Aluminosilicate 0813 exhibited constantly the highest burst pressure resistance in this study.
Table 46: Development of pressure resistance of aluminosilicate fibers compared to a borosilicate C5 fiber
Aluminosilicate 0812
Aluminosilicate 0813 Small dimension Decreased Increased Large dimension Increased Increased