Sintering aids (SA) have great potential in enhancing the thermodynamic properties of refractory materials at medium temperatures (6001 200°C). Boron-based compounds are suitable because the phase transformations (i.e., decomposition, oxidation, etc.) they undergo at relatively low temperatures can lead to the generation of a boron-rich liquid phase and transient liquid phase sintering.
5 Sintering Aids
The effects of 5 sintering aids: boron trioxide (B2O3), boric acid (H3BO3), sodium borosilicate (BS), magnesium borate (BM) and boron carbide (B4C) in high alumina castable compositions combined with hydrated Al2O3 at 0.5%, 1.0% and 2.0%. To confirm whether transient liquid phase sintering has advantages in refractory materials and in which temperature range these compositions may show yield state and have better performance than non-additive castables. For this purpose, fluidity, apparent porosity, XRD, thermoelastic modulus, thermal shock resistance and mechanical properties were tested. Based on the above results, it is not recommended to introduce B2O3 into high alumina castables because it is a hygroscopic material that reduces the fluidity of the composition and makes the sample difficult to prepare. Other boron-containing compounds can be added directly to the high alumina mixture without causing major changes in its processing steps.
The importance of selecting a suitable sintering aid source and optimizing its content in the castable composition is important because these parameters may affect the overall rheological and thermodynamic properties of the castable. The ratio contains 0.5% H3BO3 (or B4C), 1% BS and 2% BM, which can enhance the wear resistance, thermal shock resistance and thermodynamic strength at 1000℃ and 1200℃, making them potential for use in refractory castables.
Improving the Availability of Monolithic Refractories
In order to meet the needs of some users, such as environmental protection issues and optimization of various industrial production processes, refractory products are constantly being improved. Therefore, the development of advanced materials (especially monolithic materials) is encouraged. For example, the petrochemical and aluminum industries (most of the equipment operates at maximum operating temperatures of about 800℃ and 1250℃) usually still use refractory products originally designed for the steelmaking section. However, these materials can only show microstructural sintering or densification and better thermodynamic properties when subjected to higher temperatures (>1300℃). Therefore, it is necessary to design novel refractory compositions and enhance their availability to match their operating temperatures.
One possible option is to add sintering aids (SA) to refractory castables to increase the densification rate at low temperatures (i.e. 600~1000℃) to reduce energy consumption or to develop products that can be used in a wider temperature range (800~1400℃). Sintering aids are easy to handle in powder form and can be introduced directly into the castable dry mix. Due to their interaction with other raw materials and the generation of gases that affect the environment, these additives produce a liquid phase in the designed microstructure during the first heat treatment of the refractory, followed by reactions and the appearance of new refractory phases (transient liquid phases). Braulio et al. evaluated the effects of various sintering aids (TiO2, ZrO2, MgF2, BM and magnesium borosilicate) on the properties of Al2O3-MgO-CaO-SiO2 castables. The selected sintering aids acted as mineralizers or densifiers for spinel (Mg Al2O4) and calcium hexaaluminate (CA6). The possibility of generating CA6 lamellae with a high aspect ratio and the presence of a small amount of liquid phase in the refractory matrix increased, resulting in excellent creep resistance, for example when boron-containing additives were used.
Addition of Boron-Containing Sintering Aids to Pure High-Alumina Castable Compositions
Addition of boron-containing sintering aids to pure high-alumina castable compositions (considering the use of hydrated Al2O3 sources as the main binder) is a method for designing advanced refractories for the petrochemical sector. As described in the literature, H3BO3 was introduced into the fine Al2O3 platelet mix with the aim of designing a nacre-shaped biomimetic microstructure. This allowed the specimen to sinter more efficiently and enhance the bond strength between the platelets through the in-situ formation of 9Al2O3·2B2O3. 9Al2O3·2B2O3 has a low thermal expansion coefficient, an elastic modulus of 350GPa and a mullite-type structure. The prepared high-alumina specimens containing H3BO3 had flexural strengths of 672MPa and 280MPa at room temperature and 1 200℃, respectively, and fracture toughnesses of 11MPa·m1/2 and 6MPa·m1/2, respectively. In addition to H3BO3, other compounds (i.e. BS, BM, etc.) may also be used in the presence of fine and reactive Al2O3, as well as in coarse aggregates of this oxide. Based on these aspects, what is the effect of 5 different boron-containing sintering aids, B2O3, H3BO3, BS, BM and B4C, added at 0.5%, 1.0% and 2.0% to hydrated Al2O3-bound high-alumina self-flowing castable compositions? In order to determine two questions: 1) Is transient liquid phase sintering beneficial in these refractories? 2) In what temperature range in use are these compositions likely to exhibit yielding and improved properties compared to castables without additions? Ambient and hot mechanical tests were performed.
Suitable Sintering Aids for High-Alumina Castables
Different boron sources (B2O3, H3BO3, BS, BM and B4C) were added to hydrated Al2O3 bonded castables to better understand their role as sintering aids and to develop reinforced refractory materials that need to be applied at intermediate working temperatures (600-1200°C).
Introduction of B2O3 in castables is not recommended because this hygroscopic material reduces the fluidity of the composition and makes sample preparation difficult. Other boron-containing compounds can be added directly to the high-alumina mix without having a major impact on subsequent processing steps. However, it is very important to choose a suitable sintering aid and optimize its content in the castable composition in order to sinter faster without affecting its thermodynamic properties.
The addition of only 0.5% H3BO3 or B4C to the material caused the formation of B2O3 (l) and further deposition of aluminum borates (2Al2O3·B2O3 and 9Al2O3·2B2O3) when the samples were calcined above 600℃. These transformations accelerated the sintering or densification process, resulting in enhanced high-temperature flexural strength wear and thermal shock resistance of the refractory. However, if too much 9Al2O3·2B2O3 is generated in the evaluated system (i.e., 1% B4C is added to the mix), this phase may not be very adaptable in the generated microstructure (thermal expansion mismatch), which will lead to defects and reduce the stiffness of the castable. The absence of aluminum borates in the compositions containing 0.5%~2.0% BS indicates that this sintering aid effectively promotes the sintering process (about 800℃). However, the residual glass phase (from the sintering aid) leads to a stronger softening of the samples (even when evaluating the pre-fired samples) above 1000 °C. On the other hand, 9Al2O3·2B2O3 and Mg Al2O4 were found in the fired samples containing BM. Despite the precipitation of these phases in the microstructure of the castables, all evaluated compositions do not show complete transient liquid phase sintering. As the samples still show a greater softening (due to the presence of the liquid phase) at high temperatures compared to the material without sintering aids.
It can be pointed out that hydrated Al2O3 in combination with the addition of H3BO3, BS, BM and B4C to the castables are suitable sintering aids. As a result, faster sintering and improved thermomechanical properties of the evaluated refractories at medium temperatures are achieved. However, the selection of the appropriate boron source and its content must be done because they may affect the overall rheological and thermomechanical properties of the castable composition.
