The heat transfer performance of ceramic material is crucial for ceramic component manufacturing. It is the overall heat dissipation capacity of the ceramic materials.
The heat transfer performance of ceramic materials depends on and changes with several criteria such as thermal conductivity, specific heat capacity, temperature, and thermal contact resistances.
In this article, we will explain the factors affecting the heat transfer performance of ceramic materials. Let’s begin.
What is the Heat Transfer Performance of Ceramic Materials?
The heat transfer performance of ceramic materials measures how effectively they can transfer heat from one point to another. It is determined by and closely related to the material’s thermal conductivity and influenced by some environmental elements and conditions. It helps to identify how well the material transfers heat in practical use in a given condition.
Different Factors Affecting The Heat Transfer Performance of Ceramic Materials:
Ceramics are not highly conductive like metals because they have no free electrons. The phonon(lattice vibrations) is the main heat transfer mechanism of ceramics. The heat transfer performance of ceramic materials depends on several intrinsic materialistic factors and some environmental extrinsic factors.
The following are the major factors impacting the heat transfer performance of ceramic materials.
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Thermal conductivity: Thermal conductivity is the key factor contributing to the heat transfer performance of ceramic materials. It is the heat-conducting ability of a material denoted by ‘k’ and measured in Watts per meter-Kelvin(W/mK). Higher thermal conductivity of the ceramic materials ensures better heat transfer performance at some specific conditions.
However, the thermal conductivity of the ceramic materials is influenced by some materialistic properties such as:
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Chemical Structure: The chemical structure of a ceramic material is the most powerful factor affecting its thermal conductivity. It means the material’s atomic arrangement, bond strength, and molecular symmetry. Chemical structure determines the heat transfer efficiency between the molecules of the material.
Most ceramic materials consist of two or more chemical elements. They are the compounds of metallic and non-metallic material.
Ceramic materials have two types of atomic structures crystalline and noncrystalline. Crystal ceramics have strong covalent bonds within the crystal lattice, which have an ordered structure. This arrangement allows free phonon movements to transfer heat efficiently.
The noncrystalline ceramics have disordered structures that increase the phonon scattering and reduce the thermal conductivity of ceramic.
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Material Particle Size: The raw materials particle size is another significantly influencing factor of ceramic heat conductivity. The particle size affects the phonon scattering, porosity, and grain boundary.
If the particle size of the ceramic materials becomes smaller, more grain boundaries are in them. The higher grain boundary increases the phonon scattering and reduces the thermal conductivity.
On the contrary, larger particles have fewer grain boundaries, which ensure smooth phonon travel over longer distances and result in higher thermal conductivity.
The smaller particle size ceramic has increased porosity, creating air pockets. Those air pockets work as thermal insulators to reduce the ceramic material’s conductivity.
The sintering process and densification are also affected by the particle size which in turn impacts ceramic heat conductivity.
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Porosity: The porosity is the volume of blank spaces.
Porosity (%) = (Volume of Voids / Total Volume) x 100.
The blank spaces inside the ceramic due to the porosity are full of air. As air and gaseous elements have low thermal conductivity, the presence of air in the ceramic materials breaks up the heat transfer channels. These broken heat transfer channels reduce the thermal conductivity.
Hence, the higher porosity in ceramic materials reduces the ceramic thermal conductivity. A proper sintering process can reduce the porosity and improve heat conduction in ceramic materials.
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Density: The density of ceramic materials has a notable impact on thermal conductivity. In general, higher-density materials have high thermal conductivity.
The denser materials have strong atomic structures to ensure efficient heat transfer through the lattice vibrations. The grains of the denser ceramics are tightly packed together, which reduces the thermal resistance at grain boundaries.
High-density ceramic materials also have less pore and void space which improves their heat transfer capacity. The phonon scattering of dense ceramics is reduced due to the fewer pores and defects improving conductivity. Hence, ceramics with higher density have high thermal conductivity.
For example, High-density alumina(Al2O3) ceramic has a thermal conductivity of around 30 W/m-k, but the porous alumina only has 2-5 W/m-k thermal conductivity.
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Impurities: The impurities of ceramics influence their thermal conductivity. It reduces the heat transfer mechanism of the ceramic materials. Impurities such as foreign atoms increase the phonon scattering.
Increased phonon scattering reduces the mean free path and decreases the thermal conductivity.
Some other impurities create lattice distortion to hinder the phonon movement. Additionally, impurities increase the porosity in ceramics and create air pockets that work as thermal insulators.
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Sintering Method: The sintering process is a vital step in ceramics manufacturing. Ceramics undergo several physical and internal changes in sintering, which change the mineral compositions and microstructure of final products.
The sintering process is completed at a definite temperature, time, heating, and cooling. These sintering parameters impact the thermal conductivity of the ceramic materials. The higher sintering temperature increases the thermal conductivity by reducing the porosity and improving the grain-to-grain contact.
2. Specific Heat Capacity: The ceramic specific heat capacity is the energy required to increase the temperature of 1 kg of ceramic materials by 1℃. The formula for specific heat capacity is Cv = Q / (ΔT × m).
Where, Q= heat energy,
m = mass
C = specific heat capacity
ΔT= change in temperature
The specific heat capacity of ceramic materials impacts their heat absorbing, storing, and releasing capacity which then influences the heat transfer performance. Higher specific heat capacity materials can store more heat energy. It creates a buffer effect to prevent rapid temperature changes and protect ceramic components from thermal shock.
On the contrary, lower specific heat capacity increases the heat response speed of the materials. These materials with lower Cv heat up and cool down quickly.
However, lower Cv means less heat dissipation and less heat transfer performance.
3. Ambient Temperature:
In general, the heat transfer performance of ceramics increases with the increasing ambient temperature. There are some exceptions. For some crystalline ceramic materials (such as alumina, silicon carbide, and aluminum nitride) heat transfer performance reduces with the increase in ambient temperature.
For some other amorphous ceramics(e.g. silica glass, porcelain), heat transfer performance remains almost constant at varying temperatures.
4. Surface Characteristics: The surface characteristics of ceramic materials affect heat transfer performance by impacting conduction, convection, and radiation. Smooth surfaces enhance thermal conductivity and heat transfer efficiency by increasing the contact area. The rough surface decreases the conductivity and heat transfer performance due to more air gaps.
5. Thermal Contact Resistances: Thermal contact resistance(TCR) is the resistance to heat flow at the interface between two attached solid materials. At the point of touching the interface, there creates microscopic surface roughness, air gaps, and imperfect contact which are considered together as TCR. The TCR reduces heat transfer efficiency due to the insulating air pockets at the contact point of the materials.
6. Environmental Conditions: Different environmental conditions impact the heat transfer performance of ceramic materials by influencing their thermal conductivity and stability.
Ambient temperature, humidity, moisture, atmospheric composition, thermal cycling, radiation, and mechanical stresses are the main environmental elements impacting the heat transfer performance of ceramics. They influence the overall heat management efficiency.
7. Geometry and size of the ceramic components: The geometry and size of the ceramic components influence conduction, convection, and radiation.
The high surface area-to-volume ratio of thin and porous ceramic components enhances heat dissipation by increasing surface contact with the surrounding environment. On the other hand, the bulkier components with a lower surface area-to-volume ratio reduce heat transfer efficiency due to increased thermal resistance.
The heat transfer performance of ceramic materials is also influenced by the thickness of the components made. Usually, thinner ceramic components can transfer heat faster than thick components to increase heat transfer performance.
The shape of ceramic products further has effects on the ceramic materials. The flat surfaces of products ensure uniform heat distribution and the cylindrical and spherical shapes improve heat retention.
These geometrical condition and their effect on the heat transfer performance are crucial for manufacturing different high-temperature structural ceramic components.
FAQs
Q. What are the factors that affect heat transfer?
Answer: Factors affecting the heat transfer of the materials are their mass, specific heat capacity, and temperature variation.
Q. What is the heat transfer of ceramics?
Answer: The heat transfer of ceramics is the process through which heat moves inside the ceramic materials. The ceramic heat transfer occurs by conduction, convection, and radiation. Conduction is the main heat transfer mechanism in ceramics, which is accomplished by the phonon vibration.
Q. What are the three major effects of heat transfer?
Answer:
The major heat transfer effects are conduction for solid materials, convection for liquids and gases, and electromagnetic waves or radiation. Heat is transferred using these three methods randomly based on the material properties.
Q. Which is the most thermally conductive material?
Answer: The most thermally conductive material is diamond(2000 – 2200 W/m•K) which has 5 times more conductivity than copper.
Q. What is the copper thermal conductivity?
Answer: The copper thermal conductivity is 398 W/m•K.
Conclusion: Heat transfer performance is one of the crucial considerations for manufacturing different ceramic components. The thermal conductivity of ceramics is the most important factor that greatly changes their overall heat transfer performance.
After all, most of the influential factors affecting heat transfer performance are covered in this article. Furthermore, if you have any queries related to the heat transfer and thermal conductivity of ceramic materials, ask in the comment section. Our experts will reply to you asap.