Study on the Effect of Different Fe2O3/ZrO2 Ratio on the Properties of Silicate Glass Fibers

A series of silicate glass fibers with different ratios of Fe2O3/ZrO2 were prepared, and their corrosion resistance, mass loss, and strength loss were characterized. The crystallization and melting properties of the fibers were analyzed by differential scanning calorimetry (DSC), high temperature viscometer, and high temperature microscope. The results show that the deformation temperature, sphere temperature, hemisphere temperature, and crystallization temperature of the fiber initially decrease and then increase with the increase of Fe2O3/ZrO2 ratio, while the molding temperature decreases with the increase of the ratio of Fe2O3/ZrO2. When the ratio is close to 1 : 1, its alkali resistance is almost same as that of AR-glass fiber, and the drawing process performance is better. However, with the increase of the ratio, its alkali resistance continues to decline and the poor wire drawing performance is not conducive to the drawing operation.


Introduction
Basalt fiber is a new kind of inorganic nonmetallic silicate fiber prepared by melt drawing process with natural basalt ores as raw materials [1,2]. Compared with ordinary glass fiber, basalt fiber has excellent high tensile strength, elastic modulus, creep resistance, fatigue resistance, high temperature resistance, and other properties [3,4] and is generally considered as a material with excellent alkali resistance [5][6][7][8], which could be widely used in many fields such as civil engineering, transportation, and marine engineering [9,10].
Most silicate glass fiber is easily eroded by alkali in alkaline environment, such as ordinary alkali-free glass fiber and high strength glass fiber. In the 1970s the British Pilkington company invented alkali resistant glass fiber and improved the alkali resistance of fiber [11], which solved the problems of strength reduction, cracking, and leakage of the ordinary glass fiber due to the alkaline reaction with cement. However, the raw materials for alkali resistant glass fiber are very expensive, which limits its wide application.
By comparison of the components of basalt fiber and alkali resistant glass fiber, it can be concluded that the characteristic component of basalt fiber is Fe 2 O 3 and the characteristic component of alkali resistant glass fiber is ZrO 2 . Referring to the solubility product constant table, the solubility product constants of Fe(OH) 3 and ZrO(OH) 2 are 4.0 × 10 −38 and 6.3 × 10 −49 , respectively, which are both very small. Therefore, it can be also understood for the mechanism of alkali erosion that Fe 2 O 3 on the fiber surface will be converted to Fe(OH) 3 jelly under the action of alkali solution and form a dense film on glass surface through dehydration polymerization, which can prevent the erosion to fiber from the alkali solution and hydrate. Similarly, when the alkali resistant glass fiber is in alkaline solution, ZrO 2 on the fiber surface will be converted to ZrO(OH) 2 jelly and form a dense film on glass surface to prevent the erosion [12].
The alkali resistant glass fiber contains ZrO 2 with absence of Fe 2 O 3 , while basalt fiber contains Fe 2 O 3 without ZrO 2 . In recent years, Lipatov et al. studied the influence of ZrO 2 on the alkali resistance, mechanical properties, and crystallization properties of basalt fiber [13,14].
In this paper, based on the small solubility product constant of Fe(OH) 3 and ZrO(OH) 2 and the formation of a protective film on the glass fiber surface, silicate glass fibers 2 Advances in Materials Science and Engineering

Preparation
Process. The raw materials were weighed according to Table 1, mixed uniformly, and melted in the home-made platinum crucible. Melting was kept at 1510 ± 10 ∘ C for 24 hours with continuous stirring to obtain a clarified and homogenized glass melt, which was then poured on to a heat resistant steel plate and cooled down to obtain a glass block as the samples. The samples were placed in a selfdesigned drawing crucible for wire drawing, and the sample fibers with required diameters were obtained by adjusting the temperature of the magma, the height of the liquid surface, and the speed of the wire drawing machine.

The Testing
The crystallization temperature of the glass samples was measured by a differential scanning calorimetry (DSC, DSC200F3Maia). The high temperature viscosity and melting properties of the glass samples were measured by high temperature rotational viscometer (Rheoteonic II) and high temperature microscope (OCA20LHT), respectively.

Effect of Fe 2 O 3 /ZrO 2 on Crystalline Phase of Samples.
Silicate glass fiber is a product of high temperature melting and drawing. In order to investigate whether the process contains undamaged mineral phase or crystalline phase, the fiber was analyzed by X-ray diffraction, as shown in Figure 1.  Therefore, the fiber was analyzed by X-ray diffraction and was shown in Figure 1. It can be seen from Figure 1 that the X-ray diffraction pattern of the fiber has only steamed bread peaks similar to glass. This shows that silicate glass fibers do not have any mineral phases and crystalline phases and exist in the form of amorphous.

Effect of Fe 2 O 3 /ZrO 2 on Mass Loss Rate of Samples.
The melted glass pieces were crushed and the granules were sieved through a 20-40 mesh sieve to obtain powder samples. Samples with known mass ( 1 ) were placed in a conical flask of 2 mol/L NaOH solution and then placed in a water bath at a constant temperature of 99 ± 0.5 ∘ C. The conical flasks were connected to a reflux condenser and sealed with plastic wrap. After 24 h, the powder was weighed again to obtain its mass ( 2 ), and the mass loss rate was calculated. The measured data were presented in Table 2, and the mass loss rate of each sample was shown in Figure 2.
The weight loss rate of 1# sample is the smallest, the weight loss rates of 1#∼4# samples increase with the increase of Fe 2 O 3 /ZrO 2 ratio, and the weight loss rate of 3# is close to that of AR-glass indicating that their alkali resistances are almost equal. The weight loss rate of the E-glass without ZrO 2 and Fe 2 O 3 is the highest, which indicates that the E-glass is most resistant to alkali attack.

Effect of Fe 2 O 3 /ZrO 2 on Strength Retention of Samples.
Prepared fibers were immersed in Ca(OH) 2 saturated solution and heated at 99 ± 0.2 ∘ C for 4 h. The tensile strength of the fibers before and after etching was measured according to   ASTMD3379-75. A single fiber was bonded to the board, as shown in Figure 3. The experiment was carried out by using the 100 cN sensor tensile tester and the testing speed was 2 mm/min. During the test, once the fiber was fixed by the instrument, both sides of the cardboard were cut off with the scissors. For each fiber sample, 25 effective strength values were recorded and then were averaged. The tensile strength retention rates are calculated and presented in Table 3. The trend of strength retention rate is shown in Figure 4.
It can be seen that the tensile strength of the 1# sample is the highest before the alkali etching, and the retention rate of the strength after the alkali erosion is the largest. The strength retention rates of 1#∼4# samples decrease with the increase of the ratio of Fe 2 O 3 /ZrO 2 , and the strength retention rate of 3# is most close to that of AR-glass fibers, indicating that the alkali resistance of the two samples is the same. E-glass fibers do not contain ZrO 2 and Fe 2 O 3 , so its strength retention rate is the lowest indicating the worst alkali resistance. Reference [13] mentions that the addition of less than 3.1 wt% ZrO 2 increases the alkali resistance of the basalt fibers by 37%. The addition of more than 3.1 wt% ZrO 2 to the glass batch reduces the alkali resistance and tensile strength of the basalt fibers. Compared with this paper, there are some differences in data. The reason may be that the chemical composition of the sample is different.
In summary, the addition of ZrO 2 in the glass could significantly improve the alkali resistance of the glass system, because the ZrO 2 on the glass surface will be converted into ZrO(OH) 2 jelly and form a layer of dense membrane on the glass surface by dehydration polymerization to make it more resistant to the corrosion of the alkali solution and other hydrides [12][13][14][15][16][17]. The mass loss rate and strength retention rate of the glass samples decrease with the increase of the ratio of Fe 2 O 3 /ZrO 2 . It is worth noting that when the ratio of Fe 2 O 3 /ZrO 2 is close to 1 : 1 (3# sample), the mass loss rate and strength retention rate are almost the same as those of AR-glass. The results show that 3# and AR-glass have the similar ability to resist alkali corrosion. In glass system, Fe 3+ exists in two states: [FeO 4 ] and [FeO 6 ] [18,19]; when a monovalent alkali metal ions or divalent alkali metal ions exist, Fe 3+ will form [FeO 4 ] tetrahedron and get into the -Si-O-Si-network as a network connector, so that the structure of the glass becomes tighter. On the contrary when there are no monovalent alkali metal ions and divalent alkali metal ions or the ion concentration is low, Fe 3+ would be in the glass network gap to form [FeO 6 ] octahedron. At the same time, Fe 3+ ions also have a certain accumulation effect on the oxygen ions in the [SiO 4 ] tetrahedron, which makes the structure denser [20]. Both of them could prevent further corrosion of the alkali solution on the fiber surface and the stability of the vitreous structure will not be broken. The relevant research has been carried out by Hwang and Lim [21], and it is considered that the oxidation state of iron has the following dynamic equilibrium relation.
Low temperature, strong alkalinity, oxidizability → ← High temperature, strong acidity, reducibility Because the pure FeO does not exist in nature, when the temperature reaches 1475 ∘ C, part of Fe 2 O 3 would be broken down and release oxygen to form Fe 2+ , so Fe 2+ and Fe 3+ will always coexist in the silicate glass and the ratio varies with the melting conditions.  Temperature ( ∘ C) An important structure of ferrous ferrite (Fe 3+ -O-Fe 2+ ) forms, when Fe 3+ and Fe 2+ coexist in a silicate glass system. When Fe 3+ ions in Fe 3+ -O-Fe 2+ structure are in tetrahedral coordination, they act as the network formers, while when Fe 2+ ions are in octahedral coordination they are the network modifiers. When there are excess Fe 3+ ions, the excess Fe 3+ could only be in octahedral coordination network modifier position, and the formation of Si 4+ -O-Fe 3+ structure could make the fiber has tighter structure, better chemical stability, and better mechanical performance. On the other hand, when there are excess Fe 2+ ions, the excess Fe 2+ are also in octahedral coordination network modifier position, and the formation of Si 4+ -O-Fe 2+ structure could also prevent further erosion of the alkali solution on the fiber surface.

Effect of Fe 2 O 3 /ZrO 2 on Crystallization Behavior of
Samples. The glass transition temperature ( ) and the crystallization exothermic peak temperature ( ) [22] of the glass samples were measured at the heating rate of 10 ∘ C/min in the high purity nitrogen atmosphere, which are shown in Table 4 and in Figure 5.
It could be seen that the glass transition temperature initially decreases from 711.35 ∘ C to 637.75 ∘ C and then rises to 719.25 ∘ C, and the crystallization exothermic peak temperature also decreases from 1062.15 ∘ C to 1036.05 ∘ C and then rises to 1059.35 ∘ C. These results indicate that and have a parabola-like trend with the increase of Fe 2 O 3 /ZrO 2 . When the ratio of Fe 2 O 3 /ZrO 2 is close to 1 : 3, and reach the minimum values of 637.75 ∘ C and 1036.05 ∘ C respectively, and then and begin to rise again. When the ratio of Fe 2 O 3 /ZrO 2 is less than 1 : 3, the crystallization temperature of the glass samples is reduced, and the crystallization ability  is improved. Furthermore, when the ratio of Fe 2 O 3 /ZrO 2 is more than 1 : 3, the crystallization temperature of glass samples begins to increase, and the crystallization ability is reduced. This is because Fe 3+ is mainly present in the form of [FeO 4 ] when Fe 2 O 3 is initially added. Fe 3+ has an internal polarization effect on the network frame of silicate glass, which makes the network connected tightly, the viscosity of silicate glass increased, and the crystallization ability enhanced. With the increase of the content of Fe 3+ , Fe 2 O 3 would be mainly [FeO 6 ], manifesting as the increase of the external polarization, which results in gradually loosed network structure of silicate glass, the reduced viscosity, and the reduced crystallization ability. At the same time, of 3# and AR-glass are not significantly different, while of 3# is significantly lower than that of the AR-glass. Therefore, liquid phase is easier to form in the 3# sample during the melting process which can reduce energy consumption. Reference [13] mentions that when the content of ZrO 2 increases from 0 to 6.9 wt%, increases from 940 ∘ C to 1005 ∘ C. The results show that and increase with the increase of ZrO 2 content. The results of this study are consistent with our study.

Effect of Fe 2 O 3 /ZrO 2 on High Temperature Viscosity of
Samples. The high temperature viscosity of glass samples was tested. The viscosity-temperature curve is shown in Figure 6, and the corresponding temperature for the samples at log = 2.5 is shown in Table 5.
Considering the inclinations of the curves in Figure 4, the viscosity of 1# has the largest changing trend as the temperature decreases, followed by 2#, 3#, and 4# successively. The viscosity-temperature curve of E-glass is smooth and the viscosity is small. The trends of viscosity curves of AR-glass and 1# samples are basically the same, which might be due to the similar ZrO 2 content of 1# and AR-glass. ZrO 2 is octahedral coordination ( = 8), and the radius of Zr 4+ is large. Under the same electric charge condition, with the increase of the coordination number ( ) of the cation in the glass melt, the accumulation of silicon oxygen groups is enhanced and then the viscosity increases. Therefore, the viscosity of 1# and AR-glass is relatively larger. The viscosity of 3# is significantly better than that of AR-glass, which could meet the test results of crystallization properties of glass samples. As shown in Table 5, with the increase of the ratio of Fe 2 O 3 /ZrO 2 in the glass samples, the corresponding temperature of log = 2.5 decreases, which is the drawing molding temperature. At this time, the drawing molding temperature of 3# is obviously lower than that of AR-glass, which could be beneficial to energy saving and production cost reduction.  Figure 8.

Effect of Fe
From Figure 8, it could be seen that the DT, ST, and HT of the glass samples initially decrease and then increase with the increase of Fe 2 O 3 /ZrO 2 , and the change trend is presented in Figure 5. When the ratio of Fe 2 O 3 /ZrO 2 is close to 1 : 1, the DT, ST, and HT decline to the lowest temperature. Therefore, the melting temperature of 3# is the lowest. Compared with AR-glass, 3# has lower melting temperature, which is accordant with the results of high temperature viscosity test.
When the Fe 2 O 3 content in the glass samples increases, the ZrO 2 content will decrease, which results in the change trend of the melting temperature of 1#∼4# samples. Zr 4+ is present in the form of [ZrO 6 ] octahedral in the zirconium silicate melts. There are at least two silicon oxygen tetrahedrons between Zr 4+ ions, and the bond strength of Zr-O-Si bond is high and is not readily destroyed at higher temperature. As a result, the melting temperatures of 1# and AR-glass are higher, while the melting temperatures of 2#, 3#, and 4# decrease with the decrease of ZrO 2 content (increasing

Conclusions
(1) With the increase of Fe 2 O 3 /ZrO 2 ratio, the alkali resistance of silicate glass fiber decreases. When the ratio of Fe 2 O 3 /ZrO 2 is close to 1 : 1, the alkali resistances of 3# and AR-glass are almost same and are much higher than that of E-glass.
(2) With the increase of Fe 2 O 3 /ZrO 2 ratio, the crystallization temperature of silicate glass shows a trend of decreasing first and then increasing. When the ratio of Fe 2 O 3 /ZrO 2 is close to 1 : 1, of 3# and AR-glass are almost same, while is significantly lower than that of AR-glass. Therefore, the 3# has better crystallization properties compared with AR-glass.
(3) With the increase of Fe 2 O 3 /ZrO 2 ratio, the viscosities of 1# and AR-glass are the highest, and the viscosities of 2#, 3#, 4#, and E-glass decrease in sequence. When the ratio of Fe 2 O 3 /ZrO 2 is close to 1 : 1, 3# has lower viscosity and drawing molding temperature compared with AR-glass, and the wire drawing is easier in the process of fiber preparation.
(4) With the increase of Fe 2 O 3 /ZrO 2 ratio, the DT, ST, and HT of silicate glass initially decrease and then increase. When the ratio of Fe 2 O 3 /ZrO 2 is close to 1 : 1, the DT, ST, and HT of 3# have the lowest values, and 3# sample has a lower melting temperature compared with AR-glass, which could save energy consumption and reduce the production cost.
(5) If Fe 2 O 3 is used to replace a part of ZrO 2 , the ARglass fiber still has good melting characteristics, chemical corrosion resistance, and mechanical properties, which could greatly reduce the production cost of AR-glass fiber.

Conflicts of Interest
The authors declare that they have no conflicts of interest.