CR2450 battery

Basic principles of thermal analysis technology for CR2450 battery

The basis of thermal analysis technology is that when the physical and chemical states of a substance change (such as sublimation, oxidation, polymerization, curing, sulfidation, dehydration, crystallization, melting, lattice change or chemical reaction), it is often accompanied by changes in thermodynamic properties (such as enthalpy, specific heat capacity, thermal conductivity, etc.). Therefore, the physical or chemical change process of a substance can be understood by measuring the changes in its thermodynamic properties. Now, the method of determining the state change based on the performance changes (such as thermal energy, mass, size, structure, etc.) caused by the temperature change of a substance is collectively referred to as thermal analysis. Traditional thermal analysis techniques include thermogravimetric analysis (TGA), differential thermal analysis (DTA) and differential scanning calorimetry (DSC). (1) Thermogravimetric analysis (TGA) Many substances often undergo mass changes during heating or cooling. The magnitude of the change and the temperature at which it occurs are closely related to the chemical composition and structure of the substance. Therefore, by using the characteristics of the mass change of a substance during heating or cooling, different substances can be distinguished and identified. This method is called thermogravimetric analysis. Thermogravimetric analysis can be used to study the thermal stability, thermal decomposition temperature, decomposition reaction temperature, etc. of a substance. If the volatile components produced by decomposition are simultaneously input into a gas chromatograph to determine the composition of the decomposition products, the thermal degradation mechanism of the substance can be studied. The mass of the sample is recorded and analyzed as a function of time or temperature, and the resulting curve is called a thermogravimetric curve. The vertical axis of the thermogravimetric curve represents the change in the mass of the sample, and the horizontal axis represents time or temperature. Thermogravimetric analysis can be used to study the thermal stability, thermal decomposition temperature, decomposition reaction temperature, etc. of the substance. If the volatile components produced by decomposition are simultaneously input into a gas chromatograph to determine the composition of the decomposition products, the thermal degradation mechanism of the substance can be studied. The first-order derivative of the TG curve with respect to temperature or time, that is, the function relationship between the rate of change of mass and temperature or time is recorded as a DTG curve, which can further obtain more information such as the rate of mass change. From the slope of each point on the weight loss curve, the weight loss rate (dW/dt) at each temperature can be calculated, and thus the decomposition rate constant (K) and the activation energy of the reaction can be calculated. (2) Differential thermal analysis Differential thermal analysis (DTA) measures the relationship between temperature and heat flow related to the internal thermal transition of the material, and studies the thermal effect of the sample under a controllable temperature program. Differential thermal analyzer can quickly and accurately analyze various characteristic temperatures of samples, such as melting point and phase transition temperature. The application range is very wide, especially in material research and development, performance testing and quality control. Under program temperature control conditions, the temperature difference between the sample and the reference material is measured as a function of the ambient temperature. When the furnace temperature rises at a constant rate, after a certain period of time, the heating of the sample and the reference material reaches a stable state, that is, the two heat up at the same rate. If the sample and the reference material have the same temperature, AT=0, then the thermoelectric potential generated by their thermocouples is also the same. Due to the reverse connection, the thermoelectric potential generated is equal in magnitude and opposite in direction, which just cancels out, and there is no signal on the recorder; if there is a thermal effect during the sample heating process, and the reference material has no thermal effect, there will be a temperature difference, AT≠0, and the signal on the recorder indicates the size of AT. If the sample melts, decomposes, removes adsorbed water and crystal water, or destroys the lattice during heating, the sample will absorb heat. At this time, the temperature of the sample will be lower than the temperature T2 of the reference, that is, T2>Tl, and a thermoelectric potential will be generated in the closed loop. With the end of the endothermic reaction of the sample, Ti and T2 tend to be equal again, forming an endothermic peak. Obviously, the more heat absorbed in the process, the larger the endothermic peak area formed on the differential thermal curve. When the sample is oxidized, the lattice is rebuilt, and new minerals are formed during heating, it is generally an exothermic reaction. The temperature of the sample rises, and the temperature of the two welding points of the thermocouple is T1>T2. The thermoelectric potential generated in the closed loop forms an exothermic peak. The basic principle of differential thermal analysis is that the temperature difference between the sample and the reference is caused by the thermal change generated by the sample during heating or cooling. This temperature difference is reflected by the thermocouple placed between the two. Its size is mainly determined by the thermal characteristics of the sample itself. Therefore, the interpretation of the differential thermal curve may achieve the purpose of phase identification. (3) Differential Scanning Calorimetry Differential scanning calorimetry is a method of recording the energy required to establish a zero temperature difference between a sample and a reference material under the same temperature conditions and over time or temperature. The record is called a differential scanning calorimetry curve. The vertical axis represents the amount of heat added per unit time. Differential thermal analysis and differential scanning calorimetry curves reflect a series of physical or chemical changes accompanied by thermal phenomena that occur in the tested sample within different temperature ranges. Regarding the determination of the transition point on the differential thermal curve, the intersection of the tangent line at the point where the curve begins to deviate from the baseline and the tangent line with the maximum slope of the curve is closest to the thermodynamic equilibrium temperature. Therefore, the extrapolation method is used to determine this point as the starting point or transition point of the reaction temperature on the differential thermal curve. The extrapolation method can determine the starting point and the end point of the reaction. The peak point is the starting point of the endothermic reaction of the sample, also known as the starting temperature point of the endothermic reaction or phase transition. It indicates that at this temperature, the reaction process begins to be measured by the differential thermal curve. In the figure, 6 corresponds to the maximum temperature difference measured by the differential thermocouple, and the temperature at point 6 is called the peak temperature. This point neither indicates the maximum reaction speed nor the end of the exothermic process. Usually the peak temperature is easier to determine, but its value is also affected by the heating rate and other factors, and varies more than the starting temperature. The endothermic process forms the abd peak at a certain temperature between point 6 and point d (completed at point c in the figure). No more heat is released after point c, and a new baseline appears on the curve. The determination of the reaction endpoint c is very necessary because the temperature at which the reaction ends can be obtained. Assuming that the natural heating (or cooling) process of the substance is carried out according to the exponential law, the curve data after point 6 can be used to obtain the lower end of the curve with lg (AT-ATa). The point on the curve that begins to deviate from the straight line (that is, it does not obey the exponential law) is point c.

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