흑연의 비열 이해

Instruction

Graphite is a carbon allotrope with a unique crystal structure that plays a key role in many scientific and industrial fields. In these applications, the thermal properties of graphite, especially the specific heat, are critical. In-depth study of graphite specific heat is of great significance for optimizing material design. It improves energy utilization efficiency and ensures stable operation of engineering systems.

Theoretical basis of specific heat

What is Specific heat?

Specific heat, usually denoted by the symbol c, refers to the amount of heat absorbed (or emitted) by a unit mass of a substance by increasing (or decreasing) the unit temperature. In the International System of Units, usually measure it in joules per kilogram Kelvin (J/kg·k)). From a macroscopic point of view, specific heat is a measure of the heat capacity of a substance. It reflects the ability of a substance to store heat energy. At the microscopic level, the specific heat is closely related to the movement and interaction of atoms and molecules inside a substance. And the different structure and composition of a substance lead to its specific heat showing unique values and changing rules.

The physical significance of specific heat of graphite

Specific heat has far-reaching physical significance. It intuitively reflects the difficulty of heat exchange between a substance and the outside world in the process of temperature change. For example, a substance with a high specific heat has a relatively small temperature change when absorbing or releasing the same heat. This means that the substance is able to buffer temperature fluctuations more effectively. And it can be used as an ideal heat storage and regulation medium in thermal management systems. In the earth’s climate system, the high specific heat characteristics of water enable it to absorb and store a large amount of solar radiation energy. Regulate the temperature changes on the earth’s surface, and maintain a relatively stable climate environment. Similarly, the specific heat characteristics of graphite play a similar key role in its various application systems. It affects the heat transfer, distribution and conversion process.

Measurement method of the specific heat of graphite

Principle and technical classification of calorimetry

Calorimetry is based on the law of conservation of energy and is a common method for measuring the specific heat of graphite. Differential scanning calorimetry (DSC) is a typical method. When measuring, the instrument heats or cools the sample and the reference material at the same speed. And the heat flow difference will be generated due to the difference in the specific heat of the two. According to the specific heat of the reference material, it can calculate the specific heat of the sample. DSC has high precision, strong sensitivity, wide temperature range (low temperature to several hundred degrees Celsius), and small sample dosage. It is suitable for laboratory research on fine thermal properties of graphite. But it requires high uniformity and purity of the sample and is susceptible to environmental interference.

Other measurement techniques

In addition to calorimetry, there are other techniques for measuring the specific heat of graphite. For example, the laser flash method is used to heat the surface of the graphite sample instantaneously with a high energy laser pulse. According to the theoretical model of heat conduction, the specific heat is calculated by measuring the temperature rise rate on the back of the sample, the geometric size of the sample and the thermal diffusion coefficient. This method can measure the specific heat at a high temperature of thousands of degrees Celsius. It is valuable for studying the high-temperature thermal properties of graphite.

However, it is sensitive to the surface quality and optical properties of the sample. And the data processing is complicated, requiring accurate theoretical models and mathematical calculations. In addition, the thermal balance method is to put the sample in a known temperature environment to reach the heat balance. According to the heat transfer principle to calculate the specific heat. But its measurement period is long, low precision, practical application is limited.

Characteristic analysis of graphite specific heat

Temperature dependence

Graphite specific heat varies significantly with temperature. In the low temperature region (<100 K), the specific heat of graphite is proportional to the third power of temperature. Because the acoustic branch of lattice vibration dominates and the mode “freezes” and the energy is quantized. As the temperature rises and enters the middle temperature region (100 K-600 K), the specific heat gradually deviates from Debye’s law. More lattice vibration modes are excited, and the contribution of electrons to the specific heat gradually appears. The change trend of the specific heat with temperature becomes more complicated, showing nonlinear growth. In the high temperature region (>600 K), the specific heat of graphite approaches a relatively stable value. It is mainly determined by the classical energy equipartition theorem. Indicating that the ability of graphite to absorb heat at high temperatures tends to be saturated.

이방성

Graphite layered structure causes specific heat anisotropy. Interlayer covalent bond, strong atomic interaction, high specific heat. The van der Waals force between layers is weak in confinement and low in specific heat. This is prominent in the heat conduction, the intra-layer heat transfer is efficient, and the inter-layer is limited, so that the thermal response and balance are different, affecting the actual thermal performance. For example, when graphite is used as a cooling material, it is necessary to consider this characteristic to optimize the orientation and structure to achieve the best heat dissipation effect.

Factors affecting the specific heat of graphite

Effects of crystal structure defects

Crystal structure defects affect the specific heat of graphite. Vacancies, dislocations, and layer faults disrupt lattice regularity, change interatomic interactions. And affect vibration and energy distribution. Vacancy increases the low frequency vibration of atoms, dislocation and stratification produce strain energy, and increase the specific heat. The experiments show that the specific heat of graphite increases with the increase of defects in low and middle temperature regions. In high temperature sensor materials, the reasonable introduction of defects can optimize the specific heat. Meet the requirements of thermal stability and response rate, and show the value of regulating thermal performance.

Impurity doping effect

Impurity doping significantly affects the specific heat of graphite. Atoms such as nitrogen and boron enter the lattice, changing the electronic structure and chemical bond properties. Nitrogen doping increases the density of electron cloud. Increases the degree of freedom of electron thermal motion, and increases the contribution of electron to specific heat. Boron doping may change the lattice vibration mode, introduce new energy levels, and increase the specific heat in a specific temperature range. Experiments show that the appropriate amount of doping can make the graphite in a specific range of higher specific heat.

Role of graphitization degree

Graphitization degree is closely related to the specific heat of graphite. When the degree of graphitization is low, there are many disordered structures and defects, low specific heat and complex changes. As the degree of 흑연화 increases, the specific heat of the high temperature region is close to the ideal graphite value. And the temperature range of the low temperature region follows Debye’s law is expanded, and the curve is smoother. Because the graphitization makes the lattice vibration and electron motion more stable and orderly. In industrial production, the graphitization process is optimized to regulate the graphitization degree. Accurately control the specific heat, balance the electrical conductivity and thermal stability. And meet the diverse needs of different fields for the thermal properties of graphite.

Comparison of specific heat of graphite with that of other substances

The specific heat of silicon is similar to that of graphite. But their thermal properties have advantages and disadvantages in different application scenarios. The high specific heat of water makes it an excellent medium for heat storage and temperature regulation. The specific thermal properties of metals combined with their good electrical conductivity are widely used in industrial heat conduction.

Application of graphite specific heat in different fields

Energy storage field

You can use graphite in lithium-ion batteries and supercapacitors. As a negative electrode material, its specific heat can absorb the heat generated by charge and discharge. It slows down the temperature rise of the 배터리, and prevents the performance attenuation caused by overheating. Combined with the thermal conductivity optimization structure, it can improve the stability and reliability of the battery at different temperatures. And promote its application and performance in many fields.

Heat dissipation of electronic devices

Graphite is a potential material for electronic device heat dissipation due to its high internal thermal conductivity and moderate specific heat. With the device miniaturization and high performance, heat dissipation becomes the key. The graphite heat sink can quickly conduct heat and buffer heat fluctuations, such as the chip in the mobile phone. It can reduce the chip temperature, improve stability and performance, and help the device miniaturization and integrated development.

Aerospace Thermal protection

흑연 and its composite materials are important in aerospace thermal protection systems. When the aircraft re-enters the atmosphere, the graphite-based material slowly heats up to resist high temperature erosion by virtue of its high melting point, high temperature stability and specific heat performance. The design of space shuttle thermal protection tiles takes specific heat into account. And the structure is optimized to achieve reliable protection, ensure space missions, and help explore the universe.

결론

As one of the key thermophysical properties of graphite, the specific heat has shown rich connotation and important value in both theoretical research and practical application. In the future, with the development of technology and interdisciplinary research, breakthroughs are expected to help graphite play a greater role in multiple fields and promote industrial upgrading.