The Influence of Al2O3-Cr2O3 Solid Solution Particles on the Performance of High-Chromium Bricks

High-chromium bricks are shaped refractory products made primarily from industrial-grade chromium trioxide and alumina, with the addition of small amounts of zirconium oxide, etc., and fired at high temperatures. The chromium trioxide content is not less than 75%, and the combined content of chromium trioxide, alumina, and zirconium oxide is not less than 98%. Some even have a chromium trioxide content as high as approximately 90%. This high chromium trioxide content endows high-chromium bricks with excellent refractory properties and high-temperature stability.

Advantages and Characteristics of High-Chromium Bricks

High-chromium bricks possess the following characteristics:

• (1) High Refractoriness: High-chrome bricks have a refractoriness far exceeding that of ordinary refractory products, maintaining structural stability at high temperatures and resisting softening and deformation. Therefore, they can be used in thermal equipment such as industrial furnaces and kilns with extremely high temperature requirements.
• (2) High High-Temperature Strength: Under high-temperature conditions, high-chromium bricks maintain high strength and possess excellent wear and impact resistance. They can resist the erosion and friction of materials inside the furnace, extending the service life of the furnace lining.
• (3) Strong Corrosion Resistance: They have good resistance to some acidic and alkaline chemicals and are not easily corroded by molten slag and gases inside the furnace, thus ensuring the integrity and stability of the furnace lining.
• (4) High Thermal Conductivity: They have high thermal conductivity, enabling rapid heat transfer within the furnace, improving energy utilization efficiency, and reducing energy consumption.

Due to their excellent refractoriness and high-temperature stability, high-chrome bricks can be widely used in various high-temperature environments. In steelmaking, high-chromium bricks are widely used in the linings of high-temperature furnaces such as blast furnaces, converters, and electric furnaces, including the belly and waist of blast furnaces. High-chromium bricks effectively resist the erosion of high-temperature gas and slag, improving the service life and production efficiency of the blast furnace. In glass melting furnaces, high-chromium bricks can be used in the pool walls and furnace bottom, resisting the erosion and scouring of molten glass while possessing good heat insulation properties, thus contributing to improved glass melting quality and production efficiency. High-chrome bricks also have important applications in the smelting of non-ferrous metals such as copper, aluminum, and zinc. In areas like the tuyeres and slag line of copper smelting furnaces, high-chromium bricks effectively resist the erosion of high-temperature melt and slag, extending the service life of the furnace lining.

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The Influence of Al2O3-Cr2O3 Solid Solution Particles on the Performance of High-Cr Bricks

By optimizing particle size distribution, increasing the sintering temperature, and adding novel accelerators and binders, the strength of high-chromium bricks can be improved.

Specific experimental schemes for adding different fused Al2O3-Cr2O3 solid solutions to high-chromium bricks in granular form are presented. K1, K2, K3, K4, K5, and K6 represent the aggregate portions of the samples, which are 100%, 95%, 90%, 75%, 10%, and 0% aluminum-chromium, respectively.

The changes in bulk density and apparent porosity of various samples with different Al2O3-Cr2O3 solid solutions added to high-chromium bricks in granular form are also shown. It can be seen that as the Al2O3 content in the added Al2O3-Cr2O3 solid solution increases, the bulk density of the product initially decreases. Then, in scheme K6, i.e., with 0% Aluminum-Cr2O3, the apparent porosity increases, and correspondingly, it first increases, then decreases again with 0% Aluminum-Cr2O3. In this part of the experiment, because the bulk density and apparent porosity of the product are affected not only by the degree of sintering densification but also by the properties of the particles themselves and the theoretical density of different compounds, they do not have a significant effect on predicting local variation trends.

The cold strength of each sample with different Al2O3-Cr2O3 solid solutions added to high-chrome bricks in granular form was measured. It can be seen that the room-temperature flexural strength and room-temperature compressive strength have the same trend, i.e., first decreasing and then increasing. At scheme K4, i.e., with 75% Aluminum-Cr2O3 particles, the minimum values ​​of 23.8 MPa and 126.3 MPa were reached, respectively. The strengths of schemes K6 and K1, with 0% and 100% aluminum-chromium particles, respectively, were the highest and second highest among all samples, at 42.8 MPa and 192.9 MPa, and 30.4 MPa and 172.1 MPa, respectively. The differences in the physical properties of the products were very significant.

The high-temperature flexural strength variation trends of the high-chromium brick samples with different Al2O3-Cr2O3 solid solutions in granular form were clearly shown. Scheme K4, with 75% aluminum-chromium particles, had the lowest high-temperature flexural strength at 15.5 MPa. This was significantly lower than Scheme K1 (24.5 MPa) with 100% aluminum-chromium particles and Scheme K6 (24.2 MPa) with 0% aluminum-chromium particles. The high-temperature flexural strength showed a trend of first decreasing and then increasing.

The residual flexural strength of high-chrome bricks with different Al2O3-Cr2O3 solid solutions added in granular form after thermal shock initially decreased and then increased, reaching a minimum at scheme K4 with 75% aluminum chromium material particles added.

The effect of Al2O3-Cr2O3 solid solution particles on the static slag resistance of high-chromium bricks. After comparison, it is clear that the thickness of the residual slag layer gradually decreases and disappears from scheme K1 to scheme K5, while a significant slag layer reappears in scheme K6, with an average thickness of approximately 1 mm.

The effect of Al2O3-Cr2O3 solid solution particles on the resistance to coal slag erosion of high-chromium bricks. Macroscopically, the resistance to coal slag erosion increases from 10.1% to 19.4% in scheme K1 to scheme K5, then decreases to 13.3% in scheme K6.

The distribution of Si, Ca, and Fe element contents at different distances from the slag surface after static slag resistance tests for schemes K1, K3, K5, and K6, based on the effect of Al2O3-Cr2O3 solid solution particles on the resistance to coal slag erosion of high-chrome bricks. Comparing schemes K1, K3, and K5, it can be seen that the penetration depth and amount of SiO2, CaO, and Fe2O3 in coal slag gradually increase from K1 to K5. Taking the penetration depth of SiO2, CaO, and Fe2O3 as an example, the penetration depths in scheme K1 are 2mm, 3mm, and 1.5mm, while in scheme K5 they are 7mm, 7mm, and 2.5mm, showing a significant increase in penetration depth. Comparing schemes K5 and K6, it can be seen that the penetration depth and amount of SiO2, CaO, and Fe2O3 in scheme K6 are relatively small. Again, taking penetration depth as an example, the values ​​in scheme K6 are 5mm, 5mm, and 2mm, significantly smaller than the corresponding values ​​in scheme K5. Considering the influence trend of different Al2O3-Cr2O3 solid solution particles on the apparent porosity of high-chromium bricks, it can be concluded that the apparent porosity of high-chromium bricks is one of the key factors determining the penetration depth and amount of SiO2, CaO, and Fe2O3 in coal slag.

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The decrease in chromium oxide content is not the cause of the decline in the physical properties of high-chromium bricks; the changing trends in these indicators are due to differences in densification caused by varying degrees of sintering.

Except for slightly different trends in bulk density and apparent porosity among the sample groups due to interference from other factors, the cold strength, high-temperature flexural strength, and residual flexural strength after thermal shock of high-chromium bricks with different Al2O3-Cr2O3 solid solutions added in granular form all showed the same trend: first decreasing and then increasing, reaching a minimum at scheme K4 using 75% aluminum chromium particles. These analyses show that the changing trend is consistent with theoretical predictions. That is, the increase in mixing entropy results in a smaller decrease in Gibbs free energy, which is not conducive to effective sintering.

From the perspective of differentiated high-chrome bricks based on location configuration, compared to K1 and K6, the samples from other schemes all showed a decline in sintering performance (expressed as apparent porosity and bulk density), thermal shock stability, and slag erosion resistance, and therefore cannot be applied. However, compared to K1, while K6 exhibits reduced resistance to slag erosion, it demonstrates improved strengths such as room-temperature flexural strength, room-temperature compressive strength, and thermal shock resistance. Therefore, this design can be used at the cone bottom of a Texaco gasifier, or at the upper part of the cylinder and the cone bottom of a four-nozzle opposed gasifier.

The fire-facing bricks produced using the K6 design, when used at the cone bottom of a Texaco gasifier, initially had a service life of approximately 6500 hours. After trials, the service life increased by nearly 1000 hours, reaching 7564 hours. Similar results were achieved in industrial applications at the cone bottom of a four-nozzle opposed gasifier. Therefore, the use of high-chromium bricks with 0% aluminum-chromium particles at the cone bottom of a gasifier is effective in improving service life.

Improving the Sintering Densification of the Matrix

Improving the sintering densification of the matrix can significantly improve various physical properties of the product, such as room temperature flexural strength and room temperature compressive strength. From the perspective of the structural composition and performance of high-chrome bricks, the entire system can be simply divided into two parts: an aggregate part with a particle size greater than 1 mm and a matrix part with a particle size less than 1 mm. Therefore, the overall mechanical properties of the material will likely be a combination of the following three parts:

• (1) Mechanical properties of the aggregate part.
• (2) Mechanical properties of the matrix part.
• (3) Mechanical properties of the aggregate-matrix bond.

The sintering conditions of the aggregate particles and matrix bond at the same particle size in schemes K1 and K6 show that, compared to K1, the product in scheme K6 has a tighter bond between the aggregate particles and the matrix, resulting in higher sintering density. Through the comparison of schemes K1 and K6, it can be found that improving the sintering densification between the aggregate and the matrix can improve various physical properties of the product. The photographs of the fracture surfaces of K1 and K6 products show that the fracture does not occur entirely in the matrix or along the edges of the aggregate particles, but rather spans both the matrix and aggregate portions.

Based on the above analysis, it can be concluded that improving the performance of the aggregate particles, improving the sintering of the matrix, or promoting the densification of the sintering between the aggregate particles and the matrix will all contribute to improving the various physical properties of high-chromium brick products.

Rongsheng Refractory Materials Manufacturer

Refractory materials are indispensable for ensuring the stable operation of kilns. As the “heart” of the kiln, the configuration and quality of refractory materials determine the service life and production efficiency of the kiln. With the comprehensive development of the refractory industry, higher requirements are being placed on refractory materials, which in turn promotes the development and technological progress of refractory materials. Rongsheng Refractory Materials Manufacturer will continue to provide reliable refractory lining materials for high-temperature industrial furnaces. Contact Rongsheng for free samples and quotations.

Performance Advantages of Silicon Carbide Lithium Battery Saggers

Against the backdrop of the rapid development of new energy vehicles and energy storage industries, the demand for lithium batteries has exploded. As the core container in the sintering process of lithium battery cathode materials (such as ternary materials and lithium cobalt oxide), the performance of the sagger directly affects sintering efficiency, material quality, and production costs. Traditional alumina saggers are gradually becoming insufficient to meet the demands due to problems such as rapid high-temperature wear and short lifespan. Silicon carbide (SiC) lithium battery saggers, with their unique material properties, are becoming a key direction for industry upgrading. This article will analyze silicon carbide lithium battery saggers from two dimensions: performance advantages and market prospects.

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Performance Advantages of Silicon Carbide Lithium-ion Battery Saggers: High-Efficiency Sintering

Silicon carbide is a ceramic material composed of silicon (Si) and carbon (C), whose crystal structure endows it with excellent physicochemical properties. It exhibits significant advantages in the field of lithium-ion battery saggers:

1. Ultra-High Temperature Resistance and Thermal Stability

The sintering temperature of lithium-ion battery cathode materials typically needs to reach 800-1000℃ (some high-nickel ternary materials even exceed 1200℃). Traditional alumina saggers are prone to crystal phase transformation at this temperature, leading to volume shrinkage and cracking. Silicon carbide, however, has a melting point as high as 2700℃ and almost no crystal structure change below 1400℃. Its coefficient of thermal expansion is only 4.5×10⁻⁶/℃ (compared to 8×10⁻⁶/℃), enabling it to withstand extreme high temperatures for extended periods without deformation or cracking, significantly extending its service life.

2. Extreme Corrosion Resistance: Resistant to Molten Salt and Atmosphere Erosion

During sintering, the sagger needs to come into contact with molten materials such as lithium salts (e.g., lithium carbonate) and transition metal oxides, while being exposed to air or an inert atmosphere. Alumina saggers are prone to reacting with alkaline substances, gradually corroding their surface, leading to rough inner walls and material contamination. Silicon carbide has extremely strong chemical stability, resisting acid and alkali erosion from room temperature to high temperatures (except for hydrofluoric acid), and does not react with lithium salts, maintaining a smooth inner wall for a long time, preventing impurities from contaminating the cathode material and improving product purity.

3. High Thermal Conductivity and Low Energy Consumption: Accelerated Sintering, Cost Reduction and Efficiency Improvement

Silicon carbide has a thermal conductivity as high as 120-150 W/(m·K) (compared to only 20-30 W/(m·K) for alumina), enabling rapid heat transfer, resulting in a more uniform temperature distribution within the sintering furnace, shortening heating time, and improving production efficiency. Meanwhile, its low heat capacity reduces heat accumulation and lowers sintering energy consumption (actual energy savings of approximately 15%-20%), meeting the cost reduction and efficiency improvement needs of the lithium battery industry.

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4. Ultra-long lifespan: Overall cost reduction of over 60%

Traditional alumina crucibles, due to high-temperature wear and corrosion during continuous use, have an average lifespan of only 30-50 cycles. Silicon carbide crucibles, with their high-temperature and corrosion resistance, can achieve a lifespan of 200-300 cycles (some high-end products exceed 500 cycles). Although the cost per crucible is 30%-50% higher than alumina, the overall cost of use is reduced by over 60%, resulting in significant economic benefits.

Application Areas of Silicon Carbide Saggers

1. Explosive Downstream Demand: A “Must-Have” for New Energy Vehicles and Energy Storage

According to data from GGII (Gaogong Lithium Battery Research Institute), global lithium battery shipments are growing, with power lithium batteries accounting for over 70%. As the core cost unit of lithium batteries (accounting for approximately 40%), the cathode material requires a huge amount of saggers during its sintering process. Estimating the demand of tens of millions of saggers per GWh of ternary cathode material, the annual demand in the power lithium battery sector alone is enormous. With the rapid growth of global energy storage installations, the sagger market will further expand.

2. Dual Catalysts of Policy and Technology: Accelerated Replacement Process

High-nickel content and single-crystal material are becoming the upgrade directions for cathode materials. High-nickel materials require higher sintering temperatures (above 1200℃), placing more stringent demands on sagger performance. Traditional alumina can no longer meet these requirements, making silicon carbide saggers the inevitable choice. Furthermore, leading battery manufacturers (such as CATL and BYD) and cathode material manufacturers (such as Ronbay Technology and Dangsheng Technology) have accelerated the adoption of silicon carbide crucibles, driving the industry’s substitution process.

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3. Technological Iteration: Breakthroughs in Cost Reduction and Scale

Early silicon carbide crucibles were limited in large-scale application due to their complex manufacturing process (requiring high-temperature sintering and surface coating treatment) and high cost. In recent years, companies have reduced the cost per unit by optimizing sintering processes (such as pressureless sintering + reactive infiltration) and developing low-cost raw materials (recycled silicon carbide micropowder). Simultaneously, some manufacturers have achieved higher production capacity, and large-scale production further reduces costs.

Core Carrier for Industrial Upgrading

Silicon carbide lithium-ion battery crucibles, with their advantages of high temperature resistance, corrosion resistance, and long lifespan, have solved the pain points of traditional crucibles, becoming the “upgraded standard” in the sintering process of lithium-ion battery cathode materials. With the rapid growth of the downstream lithium-ion battery industry, policy support for high-end materials, and continuous breakthroughs in manufacturing technology, the silicon carbide crucible market is poised for explosive growth. According to industry forecasts, the global lithium-ion battery crucible market size will increase, with silicon carbide crucibles also accounting for a larger share, becoming a key link in the new energy industry chain that combines technological barriers and market potential.

What Types of Special Kiln Furniture are Used to Support Ceramic Blanks?

Sintering vessels are specialized kiln furniture used to support the fired ceramic blanks or to hold powders, such as cathode materials, magnetic powders, and high-purity ceramic materials, which are calcined and synthesized before undergoing heat treatment in roller kilns, pusher kilns, or tunnel kilns. Depending on the firing process of the user, this kiln furniture will be subjected to different heating conditions, and the material of these products depends on the type of sintered body and the heat treatment process.

Saggers/Crucibles

Saggers are used to hold powders (lithium-ion battery cathode materials, magnetic powders, high-purity ceramic powders) for heat treatment in roller kilns, pusher kilns, and tunnel kilns. They are generally formed using extrusion, machine pressing, casting, and isostatic pressing processes, with the appropriate forming process selected based on the composition and structure of the product. Widely used materials include cordierite-mullite, corundum-mullite, silicon carbide, and graphite. Their application is most prevalent in the synthesis of lithium-ion cathode materials.

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Cordierite-mullite saggers are widely used in the field of lithium-ion battery cathode materials due to their excellent thermal shock resistance and economic efficiency.

Aluminum-silicon saggers generally have a short lifespan due to the strong alkalinity and low melting point of lithium carbonate/lithium hydroxide, which are highly corrosive to acidic refractory materials.

Corundum saggers are mainly used for calcining some high-purity powders in environments with less severe thermal shock conditions and high operating temperatures. For example, the calcination of high-purity alumina powder requires the sagger to be fired at 1800℃, using alumina active powder with an Al2O3 content of 99.9wt% and low-sodium white corundum. The binder uses low-ash content (Ash ≤ 0.01wt%), ensuring effective impurity control throughout the raw material process, and achieving a low coefficient of thermal expansion through sufficient high-temperature firing.

Graphite and silicon carbide saggers possess high thermal conductivity, high-temperature resistance, and excellent thermal shock resistance. While their oxidation resistance is poor, they exhibit excellent resistance to alkaline corrosion under reducing atmospheres. Graphite saggers are commonly used as containers for loading materials during high-temperature sintering in reducing atmospheres, applied in lithium iron phosphate sintering and electromagnetic material sintering. Traditional graphite saggers are produced through machining, which is inefficient and costly. Silicon carbide saggers are also widely used in pharmaceuticals, fine chemicals, engineering metallurgy, and pickling industries.

Firing Plates

Firing plates must withstand the thrust of movement and the friction of loading and unloading products during service, and must not crack under thermal cycling. While meeting the thermal shock resistance requirements, improving the bending and crack resistance of the firing plate is crucial. The firing plate material must have excellent chemical inertness and not react with the products it supports. Firing plate materials include alumina, zirconium oxide, and composite materials, mainly used in electronic ceramics and special ceramics.

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Corundum firing plates refer to high-end kiln furniture with α-Al₂O₃ as the main crystalline phase. They possess excellent properties such as high strength, corrosion resistance, high temperature resistance, and wear resistance. They exhibit minimal deformation at high temperatures (>1650℃), but have high sintering temperatures and poor thermal shock stability. During the firing process of lead zirconate titanate piezoelectric ceramics, corundum firing plates face phenomena such as central warping, surface layered powdering, and peeling.

Zirconia is a corrosion-resistant, high-temperature refractory oxide. Zirconia sintering plates are frequently used as sintering pads in dielectric ceramics, powder metallurgy, chip capacitors, and ferrite magnetic materials to prevent parts from sticking together during sintering and to prevent the loss of electromagnetic properties of electronic components.

The firing temperature of conductive ceramics and device ceramics is generally between 1400 and 1650℃, and they are mostly corundum-mullite, corundum, or composite materials.

Barium titanate ceramics are a star product in the electronic ceramics industry. Sintering is a crucial step in its preparation process, determining the densification and microstructure of barium titanate; therefore, the quality of the sintering plate is particularly important. Because barium titanate has a low melting point (1625℃), high density, and is alkaline, traditional aluminosilicate kiln furniture easily reacts with it, leading to product contamination. Foreign kiln furniture manufacturers have applied plasma spraying technology to the sintering plate preparation process. The firing plate features a corundum-mullite interlayer and a zirconium oxide cladding coating, offering high thermal shock resistance and preventing reaction or adhesion with the fired chip-type multilayer ceramic capacitors.

With industrial development, the variety of products fired has increased, expanding the application areas of kiln furniture beyond ceramics to include fine chemicals and lithium-ion batteries. The performance requirements for kiln furniture materials, considering the characteristics of the fired products, are no longer limited to mechanical strength and thermal shock stability. Some applications also require excellent corrosion resistance and non-contamination of the fired products. Different corrosive media necessitate different requirements for kiln furniture materials. For example, in the production of lithium-ion battery cathode materials, kiln furniture materials must possess excellent thermal shock stability and resistance to alkaline lithium compounds. In the production of piezoelectric ceramics, kiln furniture materials must resist lead and its compounds.

Rongsheng High-Performance Corundum-Mullite Kiln Furniture

With the rapid development of science and technology and the economy, magnetic materials, functional ceramics, and electronic ceramics have been widely used in information, electronics, machinery, and chemical industries. This has also driven the rapid growth in demand for high-performance corundum-mullite kiln furniture materials.

Kiln furniture is a special type of refractory material, primarily serving to support and protect fired products. Product forms mainly include pushers, firing supports, and saggers. During service, kiln furniture is subjected to high temperatures, compression, friction, and thermal cycling, and its damage often manifests as fracture or deformation. Therefore, the key performance requirements for kiln furniture materials lie primarily in their high-temperature mechanical properties, which determine their performance, such as high-temperature flexural strength, high-temperature creep resistance, and thermal shock resistance. Improving these performance characteristics involves the design of the material’s microstructure and its resistance to damage under complex stress conditions under high-temperature loads. This requires in-depth research and development in areas such as raw material selection, matrix material treatment, process control, and thermal stress distribution.

Through rational selection of raw materials, optimized microstructure design, scientific product firing process, and stress distribution calculation, high-performance corundum-mullite kiln furniture has been developed. It can be widely used in the sintering of structural ceramics, electronic ceramics, and powder metallurgy parts.

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Technical Advantages of High-Performance Corundum-Mullite Kiln Furniture

1. Operating temperature up to 1750℃.
2. Strong creep resistance.
3. Meets the requirements of environments with strong thermal shock.

Applications of Corundum-Mullite Kiln Furniture

Corundum-mullite kiln furniture is an essential consumable refractory material in the ceramics industry, characterized by high demand and wide application. It is mainly used in the sintering of high-tech ceramics and powder metallurgy parts, such as electronic ceramics (MLCCs, varistors, magnetic materials, filters), Al2O3 structural ceramics (substrates, ceramic films, spark plugs, grinding media), ZrO2 structural ceramics (zirconia knives, mobile phone backplates, bearings, fiber optic ferrules), stainless steel, titanium alloys, etc.

High-performance corundum-mullite kiln furniture is used in pusher kilns to fire zirconia knives. High-performance corundum-mullite kiln furniture is used in pusher kilns to fire alumina ceramics.