Ceramic vs Graphite Crucible – A complete comparison

Crucibles are of vital importance in metallurgy, chemical engineering, scientific research and other fields. The development of industrial technology and scientific research has set higher requirements for their performance. Ceramic and graphite crucibles are widely used due to their unique properties. Exploring the differences between them can assist practitioners in making selections and improving work efficiency.

Basic Introduction

Ceramic Crucible

Ceramic crucibles are made by firing oxides such as alumina and magnesia, as well as non-oxide materials like silicon carbide and silicon nitride. Among them, alumina crucibles are the most commonly used. They are classified by the content of alumina, such as 95% and 99%. The higher the content, the stronger the heat resistance and chemical stability. A 99% alumina crucible can withstand a temperature of 1800℃. In addition, silicon carbide ceramic crucibles play an important role in scenarios with high requirements for heat conduction due to their high hardness and high thermal conductivity.

Graphite Crucible

Graphite crucibles are made from natural or artificial graphite. Natural graphite has a stable crystal structure, few impurities, and good electrical conductivity, thermal conductivity, and lubricity. Artificial graphite is made from petroleum coke and other high-temperature treatments, with high purity and high density. In actual production, graphite is often mixed with additives such as clay to improve molding and mechanical properties. Graphite crucibles have excellent heat resistance, thermal conductivity, and chemical stability due to their graphite structure.

Comparison of Physical Properties

Density

The density of ceramic crucibles varies depending on the material. The density of alumina ceramic crucibles ranges from 3.6 to 4.0 g/cm³, while that of silicon carbide ceramic crucibles is approximately 3.2 g/cm³. In contrast, the density of graphite crucibles is in the range of 1.6 to 1.8 g/cm³, which is significantly lower. This low density characteristic makes graphite crucibles lighter during transportation and use, significantly reducing labor intensity.

Hardness

Ceramic crucibles have high hardness, such as alumina ceramic crucibles with Mohs hardness reaching 9, only slightly lower than diamond. They are less prone to wear during use and can maintain shape and performance. Graphite crucibles have Mohs hardness ranging from 1 to 2, and their texture is soft. However, graphite crucibles have good self-lubrication properties, which can play a unique role in scenarios requiring reduced friction such as mechanical processing and sliding parts.

Comparison of Chemical Properties

Chemical Stability

Ceramic crucibles exhibit excellent stability in most chemical environments. For example, alumina crucibles can resist various acid and alkali attacks and do not react easily at high temperatures. However, their stability is affected in strong corrosive media such as hydrofluoric acid. Graphite crucibles have good stability at room temperature. But in high-temperature oxygen-containing environments, graphite reacts with oxygen to form carbon dioxide, causing the crucible to deteriorate.

Corrosion Resistance

Ceramic crucibles have strong resistance to acid and alkali corrosion, so they show significant advantages in acidic environments. However, some ceramic crucibles may slowly corrode in strong alkaline environments. Graphite crucibles can be corroded by aqua regia, and when in contact with metal melts for a long time, they will carbonize. These will change the structure of the crucible and affect its service life.

Comparison of Thermal Properties

Resistance to High Temperature

Graphite crucible is made of graphite, which has an extremely high melting point, reaching to 3652℃. In practical use, graphite crucible can withstand the high temperature from 1200℃ to 1600℃. And it has good chemical stability, so it does not react with common metals and compounds at high temperatures. The main components of ceramic crucible are various ceramic materials, such as alumina, zirconia and so on. Different ceramic materials’ high temperature resistance is different, which usually can tolerate the high temperature from 1000℃ to 1600℃.

Thermal Conductivity

Graphite crucible has good thermal conductivity, so when heating or cooling, the temperature distribution in it is more uniform. It is beneficial to improve melting efficiency, reduce local overheating or undercooling, shorten heating time, and reduce energy consumption. The thermal conductivity of ceramic crucible is relatively poor, so when heating, the heat transfer is slow. And it is easy to appear local high or low temperature areas, affecting the melting or reaction effect.

Comparison of Mechanical Properties

Brittleness and Toughness

Graphite crucible has a certain impact resistance, and it can withstand a certain degree of mechanical stress. It is not easy to break, even when subjected to slight collisions or sudden temperature changes during use. Ceramic crucible’s brittleness is large, toughness is poor, thus it is easy to break and damage. You should be careful to avoid collision and sudden temperature changes when using and handling with it.

Compressive Strength

Graphite crucible has high compressive strength, and it can withstand a certain pressure. In the melting process loaded with liquid metal and other heavy objects, it is not easy to deformation or damage due to pressure. The compressive strength of ceramic crucible is also high. But compared with graphite crucible, it is more likely to break or damage when subjected to greater pressure.

Comparison of Preparation Process and Cost

Preparation Process

Graphite crucible: The preparation process includes raw material selection, mixing, molding, roasting and other steps. You need to select high purity graphite raw material, mix it to make the composition uniform. Next, make the shape of the crucible by extrusion, molding and other methods. And then improve the strength and density by high temperature roasting.

Ceramic crucible: Its preparation process includes raw material grinding, batching, molding, sintering, etc. Firstly, you should crush the ceramic raw materials into fine powder, mix them according to the proportion. Then through grouting, pressing and other forming methods to make the crucible body. And finally densifying the body by the high temperature sintering densifies to form a ceramic crucible with certain strength and performance.

Cost Analysis

The raw material cost of graphite is relatively high. And its preparation process is relatively complex, which require the process of high temperature roasting. So these result in the high cost of graphite crucible. However, because of its long service life and good performance, the comprehensive use cost may be low. Ceramic crucible’s overall cost is lower than that of the graphite crucible because of the low cost of ceramic raw materials. But due to the brittleness and easy damage, it may need to be replaced frequently, increasing the use cost.

Comparison of Application Domains

Metallurgical Industry

Because of graphite crucible’s high temperature resistance, corrosion resistance, good thermal conductivity, you can often use it in the melting of non-ferrous metals, such as copper, aluminum, zinc, etc.. It improve the melting efficiency, and does not react with the metal to ensure the purity of the metal. Ceramic crucible is mainly used for some special metal or alloy smelting. Such as the condition which requires very high purity and the ceramic crucible does not react with the metal.

Chemical Industry

Graphite crucible is suitable for some high-temperature and highly corrosive chemical reactions, such as some inorganic salt melting reaction. It can resist the corrosion of various chemical substances, and has applications in chemical synthesis, material preparation and other fields. You can use ceramic crucible for some chemical reactions, such as chemical experiments and synthesis in some acid-base environments that are not too strong.

Scientific Research and Experimentation

You can use graphite crucible for high temperature sintering, melting experiments. It can meet a variety of experimental requirements, and has no pollution to the experimental material. Ceramic crucible is often used in scientific research experiments for temperature, chemical environment which requirements are not particularly harsh experiments. Such as some simple chemical analysis, sample processing, etc.

Conclusion

Ceramic crucible and graphite crucible have their advantages and disadvantages. The former is more suitable for corrosive or conventional heating scenarios, and the latter is suitable for ultra-high temperature requirements. When choosing, it is necessary to consider the temperature, atmosphere, chemical environment and cost.