Silica Sol as a Filler in Process of Materializing Mullite Fiber (SiO2.Al2O3)

Research has been carried out on the effect of adding silica sol as a filler in making mullite fiber (SiO2.Al2O3). The process of making mullite fiber is carried out using the cast molding method. Sample preparation of (I)silica sol and alumina fiber. (II) silica sol with alumina fiber and kaolin. (III) alumina fiber with kaolin and distilled water. Then the samples were mixed until homogeneous in their respective containers. Then printed in a PVC pipe measuring 2.81 cm in diameter and 1.5 cm high. Mullite fiber samples were dried at a temperature of 110°C for 1 hour, then sintered at a temperature of 1200°C, 1300°C, 1400°C, 1500°C. The density of mullite fiber increases linearly, the porosity decreases linearly, and the compressive strength of mullite fiber increases linearly as the sintering temperature increases. XRD characterization shows that in the three mullite fiber samples the same phase appears, namely mullite, sillimanite has an orthorhombic structure, and cristobalite has a tetragonal structure.


Introduction
Mullite Fiber has been developed for various advanced applications due to its superior mechanical flexibility, high temperature resistance, and excellent chemical stability which is often applied in high temperature furnaces [1].Mullite is an aluminum silicate compound widely used in traditional refractory applications.Most traditional ceramic products have mullite as part of the final phase composition because they usually contain clay and silicon as starting materials [2].The microstructure of materials contains silica which has a relatively low melting point [5].Mullite is a ceramic made from silicon oxide in the Al2O3.SiO2 system, consisting of two constituent compounds: aluminum oxide and silicon oxide [1].Mullite is widely used in various fields such as electronics, optics and high temperature structural parts.Mullite has low thermal conductivity, low thermal expansion, low dielectric constant and high mechanical strength [3,4], and its high resistivity allows it to be used as a high-temperature insulating material and a high-voltage electrical insulating material [6].Silica sol is one of the most widely used silica-based materials.It is used in the pharmaceutical industry, ceramics, paints, and specialty chemical applications.Silica sol can usually be used as an adsorbent; usually used to desiccate polar compounds, fill chromatography columns, and as an insulator [7].With the principle of ion exchange, silica sol can also absorb metal ions, but its capacity to absorb metals is very limited.Silica sol is not only useful for absorbing moisture, but also has the ability to reduce levels of heavy metal ions [8].The metal ion Cd 2+ , for example, is a heavy metal ion that can be adsorbed by silica sol and efforts must be made to reduce its levels [9].A form of silica, silica sol (SiO2), is produced from the agglomeration of sodium silicate sol (Na2SiO3).This gelatinous sol can be dehydrated, producing an inelastic glass-like solid or granules [10].Heating and stirring are used to dissolve sodium silicate in water with the aim of increasing molecular collisions in the solution [11].The heated mixture is then filtered while hot, producing a sodium silicate solution as the filtrate and a mixture of metal oxides and other impurities as the residue.

Preparation of Mullite Fiber (I)
Alumina fiber weighed 44.4 grams and kaolin 5.6 grams.Then put it in a container containing 80 ml of distilled water and stir manually.The mixed material is molded with a PVC pipe with a diameter of 2.81 cm.Then the samples were dried in an oven at 110 °C for 1 hour.Next, the samples were sintered at temperatures of 1200 °C, 1300 °C, 1400 °C and 1500 °C.

Silika Sol and Alumina Fiber (II)
50 grams of alumina fiber was weighed, then put into a container containing 80 ml of silica sol and stirred manually.The mixed material is molded into a PVC pipe with a diameter of 2.81 cm.Then the samples were dried in an oven at 110 °C for 1 hour.Next, the samples were sintered at temperatures of 1200 °C, 1300 °C, 1400 °C and 1500 °C.

Silika Sol, Alumina Fiber, and Kaolin (III)
Weighed 44.4 grams of alumina fiber and 5.6 grams of kaolin, then put it in a container containing 80 ml of silica sol and stirred manually.The mixed material is molded into a PVC pipe with a diameter of 2.81 cm.Then the samples were dried in an oven at 110 °C for 1 hour.Next, the samples were sintered at temperatures of 1200 °C, 1300 °C, 1400 °C and 1500 °C.Based on Figure 1 From the silica sol diffraction pattern (a), silica sol samples have been obtained showing the presence of several crystal phases, including cristobalite, SiO2 (in the form of amorphous silica), ellestadite, and coesite.Coesite is a crystalline form of silica (SiO2) that typically forms under high pressure and high temperature.Ellestadite is a mineral that contains calcium, silicon, oxygen, and sulfur.Ellestadite can be found in various geological environments, especially in contact zones between intrusive igneous rocks and metamorphic rocks.In the silica sol phase, SiO2 (silicon dioxide) is usually in amorphous or gel form.Silica sol is a colloidal suspension of very small SiO2 particles in a liquid, such as water or an organic solvent.[17] The kaolin diffraction pattern (b) shows that the relative intensities of the peaks obtained by the kaolinite and quartz phases were identified as the main mineral components.Although no quantification was carried out, the relative height of the diffraction peaks suggests that the quartz phase content is much lower than kaolinite.And it can also be seen that there is aluminum silicate content in aluminum and aluminum oxide in alumina.There is more visible aluminum oxide than aluminum silicate as indicated by JCPDS.[22] Then in mullite fiber (c) it shows that the crystal phases formed in the three variations of mullite fiber above are mullite and sillimanite as the dominant phase and cristobalite as the secondary phase.[23]  AFK(III) mullite fiber has a percentage of mullite, sillimanite and cristobalite of 60%, 39% and 1% respectively.Mullite fiber SF(I) has a percentage of mullite, sillimanite and cristobalite of 63%, 33% and 4% respectively.For mullite fiber SFK(II) has a percentage of mullite, sillimanite and cristobalite of 65%, 31% and 5% respectively.Silimanite is an aluminosilicate mineral that belongs to the clay mineral group and is often found in metamorphic rocks.This mineral has the chemical formula Al2SiO5 and its crystal structure forms long fibers or prisms.Mullite is a ceramic with an orthorhombic crystal structure with lattice parameters a = 7.54 Å, b = 7.69 Å and c = 2.88 Å. Kristobalite is a crystalline form of silica (SiO2).It is one of various polymorphisms of SiO2, meaning it has a different crystal structure than other forms such as quartz, tridymite, and lechatelierite.This phase is formed because the silicon contained in the sample does not react with other elements to form the mullite phase.

Relationship between mullite fiber density and sintering temperature
Mullite Fiber density testing by measuring mass per unit volume.We can see Figure 2 [21,24].

Figure 2 Relationship between Mullite Fiber Density and Sintering Temperature
Figure 2 explains the differences in density values for each sample composition.In fiber mullite AFK(III) and mullite SF(I), it can be seen that mullite SF(I) shows a dominantly high density value compared to the density of AFK(III).This is because the silica and water contents are different materials, the mixture is easier to compact, and the bulk density of the mixture increases while the cavity of the mixture decreases.As a result, silica makes the aggregate voids denser, pushing the aggregate grains apart, while water does the opposite [13].If we compare SF(I) mullite and SFK(II) mullite, it can be seen that the density value of SF(II) mullite is predominantly higher compared to SFK(II) mullite.This is because the melting point of kaolin is lower than silica, so the kaolin granules shrink faster when burned, and the silica fills the ceramic cavity [14].The increase in density is due to the compaction process (densification) which causes changes in structure and microstructure, where the particles bind together so that the grain size is larger and covers the entire surface.The relationship between sintering temperature and sample density is linear, as shown in the graphic figure 2. This is due to the compaction process of the sample during the sintering process, which means that the sample becomes denser as the sintering temperature increases, so its density increases [15].

Relationship between mullite fiber porosity and sintering temperature
The influence of the presence of porosity can be modeled to make precise predictions on material properties.Regarding sintered materials, the basic idea of considering two phases (pores + full solid material) and the mixture rule may be an acceptable starting point.Porosity is calculated through density measurements, through measuring the weight of specimens in air and in pure water, to calculate the amount of porosity of a material.Porosity is one way to determine the quality of bricks [24].

Figure 3 Relationship between Mullite Fiber Porosity and sintering temperature
The porosity test can also be intended as an initial indication of the possibility of water diffusion into the cavities in the body of a ceramic composite product [16,17].It can be seen in AFK(III) mullite and SF(I) mullite that the porosity value in AFK(III) mullite is predominantly higher than SF(I) mullite.This is because silica covers the pores in the mixture, so that the percentage of pores or air cavities in the mixture becomes smaller [13].Meanwhile, it can be seen from SF mullite and SFK(II) mullite that SFK(II) mullite has a predominantly higher porosity value than SF(II) mullite.This is due to the shrinkage of kaolin which is at its melting point so that silica also plays a role in covering the cavities or pores that are empty due to the shrinkage of kaolin [14].The graphic image above shows the sample porosity value.From the graph it can be seen that the higher the sintering temperature, the porosity value will tend to decrease [15].Based on the results shown in the graphic image above, it was found that the porosity value tends to decrease with increasing sintering temperature, this is in accordance with the theorem of the ceramic sintering process, namely the relationship between density and porosity is inverse to the function of sintering temperature [18].

Relationship between mullite fiber compressive strength and sintering temperature
Compressive strength testing is carried out to obtain the compressive strength value of the test material that has been made.The compressive strength value obtained is the result of the maximum load received by the test material divided by the cross-sectional area of the test object [19,25].In mullite SF(I) and mullite SFK(II) it can be seen that the compressive strength value of mullite SF(I) is predominantly higher than mullite SFK(II).This is because SFK(II) mullite which contains kaolin shrinks at its melting point, creating empty cavities, silica plays a role in filling the existing cavities.However, with the same silica content, the silica added to SFK(II) mullite is not sufficient to close all the pores.So the compressive strength of SF(I) mullite is predominantly higher than SFK(II) mullite [20].The relationship between sintering temperature is also directly proportional to the compressive strength value.The higher the sintering temperature and the greater the SiO2 additive, the compressive strength value tends to increase [18].The longer the heating time and the higher the heating temperature, the larger the neck size so that the bonds between particles are stronger.When the material composed of these particles is burned, the contact surface area of the particles grows [20].

Conclusion
In mullite fiber research, several conclusions can be obtained Silica is able to fill empty pores or cavities in the process of binding the composition in making mullite.So mullite is formed which has a very good density.Silica plays a role as a binder in uniting all the compositions in the process of forming mullite.Characterization of physical and mechanical properties shows that the density and compressive strength experience a linear increase with the SF(I) mullite sample at a sintering temperature of 1500°C having the highest density value of 0.765 gr/cm3 and the highest compressive strength of SF(I) mullite at a sintering temperature of 1500° C has a compressive strength of 5.544 MPa, while mullite fiber porosity decreases linearly with the SF(I) sample with a sintering temperature of 1500°C having the lowest porosity of 9.532% at each temperature increase.In all samples characterized using X-Ray Diffraction there were Mullite, Sillimanite and Cristobalite phases.In this study, the AFK(III) sample had a mullite phase of 60%, sillimanite 39%, Cristoballite 1%; The SF(I) sample has a mullite phase of 63%, sillimanite 33%, cristobalite 4%; and the SFK(II) sample has a mullite phase of 65%, sillimanite 31%, cristobalite 4%.The crystal structure characteristics obtained in mullite and sillimanite have an orthorhombic structure and cristobalite has a tetragonal structure.

3 .Figure 1
Result and Discussion 3.1 Characterization of Silica Sol, Kaolin and Mullite Fiber using XRD We can see the characteristics of the silica sol, kaolin and mullite fiber materials in the picture below.X-Ray Diffraction (a): Silika sol, (b): Kaolin, dan (c): Mullite Fiber

Figure 4
Figure 4 Relationship between mullite fiber compressive strength test and sintering temperature

Table 2
Percentage structure of mullite fiber