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Studying the Effect of Nanoadditives on Phase Transformations, Microstructure, and Properties of Magnesia-Doloma Refractories

Shahraki, Aziz | 2025

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 58299 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Nemati, Ali; Khachatourian, Adrine Malek
  7. Abstract:
  8. Magnesia-doloma refractories offer several advantages, such as stability in alkaline environments, the production of clean steel, abundant domestic sources of high-purity raw materials, and low production costs. Recently, research and industrial efforts have been initiated to develop effective and practical magnesia-doloma refractories due to the vast dolomite resources available in the country. However, these refractories suffer from a major drawback—rapid hydration caused by the presence of free CaO phase. Therefore, to enhance their performance, the present study was designed in several stages. In the first part, the study included two stages. In the first stage, the effect of adding 0, 5, and 10 wt% Fe (introduced via iron(III) nitrate nonahydrate) on the modification of phenolic resin was investigated at temperatures of 600, 800, 1000, and 1200 °C. The results showed that increasing the temperature from 600 to 1200 °C raised the degree of graphitization for the sample containing 5 wt% Fe from 10.46% to 73.25%, and for the sample with 10 wt% Fe from 32.56% to 82.54%. In the second stage, dead-burned magnesia and doloma powders were used as the main raw materials for the refractory matrix, and the effects of 4 wt% unmodified and Fe-modified phenolic resins on the properties of the refractories were studied. The results indicated that the modified resin containing 10 wt% Fe exhibited superior properties compared to the others, primarily due to its higher graphitization level. For example, the cold crushing strengths of composites containing 0, 5, and 10 wt% Fe-modified phenolic resins were 22.2, 22.7, and 23.5 MPa, respectively. The second part comprised four stages. In the first stage, carbon nanofibers (CNFs) were synthesized via carbonization of electrospun polyacrylonitrile (PAN) precursor at various temperatures up to 1200 °C. Based on results such as graphitization degree, length, average diameter, and morphology, CNFs carbonized at 1000 and 1200 °C exhibited better graphitization (69.77% and 74.42%, respectively) than the other samples. In the second stage, 3 wt% of CNFs carbonized at different temperatures were added to the composites to determine the optimal carbonization temperature. CNFs carbonized at 600 and 800 °C exhibited cracks and defects in the composite samples due to sample preparation. Ultimately, CNFs carbonized at 1000 °C were selected due to their appropriate mechanical strength. In the third stage, CNFs synthesized at 1000 °C were added to MgO–CaO composites in varying amounts (0, 0.5, 1, 1.5, 2, and 2.5 wt%). The results indicated that increasing the CNF content up to 1.5 wt% improved physical and mechanical properties due to pore filling, better densification, and activation of reinforcement mechanisms. However, further addition up to 2.5 wt% led to deterioration in properties due to CNF agglomeration. Therefore, the composite containing 1.5 wt% CNF was selected as the optimum formulation. In the fourth stage, both modified phenolic resin (4 wt%) and CNF additives (0, 1, 1.5, and 2 wt%) were simultaneously incorporated into the MgO–CaO composite. The results demonstrated that, in the presence of the modified resin, improved properties were achieved at lower CNF content compared to the previous stage. The third part of the study also consisted of three stages. In the first stage, alumina nanofibers (AlNFs) were synthesized via electrospinning using a solution containing polyvinyl pyrrolidone (PVP) and aluminum nitrate nonahydrate. The electrospun nanofibers were then calcined at different temperatures up to 1400 °C. Based on the evaluation of AlNF properties, the sample calcined at 1200 °C -exhibiting a dense crystalline α-alumina structure- was selected as optimal. In the second stage, α-AlNFs were incorporated into the MgO–CaO ceramic matrix in varying amounts up to 2.5 wt%. The composite containing 2 wt% AlNF was identified as optimal, as the formation of intermediate phases such as CaAl4O7 and Ca3Al2O6- through solid-state sintering and molten phase mechanisms, respectively -enhanced densification and improved composite properties. However, further increase to 2.5 wt% led to a reduced slope in mechanical property improvement due to excessive grain growth. In the third stage, both modified phenolic resin containing 10 wt% Fe (4 wt% addition) and varying AlNF contents (0, 1, 1.5, and 2 wt%) were incorporated into the MgO–CaO system, and the key physical and mechanical properties were evaluated. Results indicated that the presence of modified resin improved the properties at lower AlNF content compared to the previous stage.
  9. Keywords:
  10. Refractory ; Magnesia-Doloma Refractories ; Carbon Nanofiber ; Hydration Resistance ; Mechanical Properties ; Physical Properties ; Alumina Nanofibers

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