1 Introduction
With the continuous development of building energy conservation in China, various new energy-saving materials have been widely used in building engineering. The thermal insulation performance of building materials is an important evaluation index. According to the relevant regulations, the main technical indexes such as thermal conductivity of energy-saving materials used in construction projects should be witnessed and sampled before entering the construction site. However, in actual construction projects, some inspectors are not familiar with the testing standards and methods of thermal conductivity and other indicators of energy-saving materials, resulting in the testing results can not truly reflect the performance of energy-saving materials. According to relevant codes and standards, this paper analyzes the influencing factors of thermal conductivity of materials. The main influencing factors include density, humidity, molecular structure and chemical composition, heat flow direction, temperature and so on. Several main methods for measuring thermal conductivity are discussed, including steady-state method and unsteady-state method. So as to ensure that the accuracy of the determination method is improved and the error is reduced.
2 Influencing factors of thermal conductivity
Thermal conductivity λ refers to the heat transferred through the area of 1m2 in unit time when the temperature on both sides of the material is 1K or 1℃, and the unit is W/(m.K). According to the concept of thermal conductivity, we can know that the greater the thermal conductivity of a material, the worse its thermal insulation performance. For different materials, their thermal conductivity may vary greatly, and there are many factors that affect the thermal conductivity of materials, including density, humidity, molecular structure and chemical composition, heat flow direction, temperature and so on. Therefore, in order to accurately measure the thermal conductivity of materials, it is necessary to analyze and discuss the main influencing factors of thermal conductivity of materials.
2. 1 density
Generally speaking, the greater the density of a material, the greater the thermal conductivity of the material, and conversely, the smaller the thermal conductivity of the material. For porous materials, the main factors affecting the thermal conductivity of materials are the porosity and pore characteristics of materials. Generally speaking, the greater the porosity, the lower the thermal conductivity. Under the same porosity, the material with communicating holes will have higher thermal conductivity than the material with closed holes, because the communicating holes have the function of air convection. In addition, the larger the pore size, the greater the thermal conductivity of the material.
2.2 humidity
The thermal conductivity of water is much greater than that of materials, so there will be more water vapor and water in the pores of materials after wetting, thus improving the thermal conductivity of materials. Generally speaking, the greater the water content of a material, the greater the thermal conductivity. The thermal conductivity of ice is four times that of water. If the water in the material is frozen into ice, the thermal conductivity of the material will be greatly increased than that of the material containing water. Table 1 shows the relationship between thermal conductivity and moisture content of aerated concrete.
2.3 molecular structure and chemical composition
The molecular structure and chemical composition of materials have great influence on the thermal conductivity of materials. For example, the thermal conductivity of glassy substances is much smaller than that of crystals, because glassy substances cannot form crystal lattices. For porous materials, whether the structure of the solid part is crystal or glass, it will not have a great influence on the thermal conductivity of the material. This is because the thermal conductivity of porous materials is mainly determined by the air in them, and the influence of solid parts is weakened.
2.4 direction of heat flow
For anisotropic materials, the thermal conductivity of materials in different directions will be different, so the direction of heat flow will also affect the thermal conductivity of materials. Generally speaking, if the heat flow is along the extension direction of the material fiber, the thermal resistance is small and the thermal conductivity is large. On the contrary, if the heat flow is along the vertical direction of the material fiber, the heat conduction and water absorption are small. 2.5 Temperature has a great influence on the thermal conductivity of the material, because the higher the temperature, the stronger the thermal movement of the solid molecules of the material, which will increase the thermal conductivity of the air in the pores of the material and the heat dissipation effect of the hole wall. Therefore, it can be considered that the thermal conductivity of materials increases with the increase of temperature.
3 thermal conductivity detection method
Generally speaking, the methods that can be used to measure the thermal conductivity of building materials include steady-state method and unsteady-state method, in which the steady-state method includes protective hot plate method and heat flow meter method, and the unsteady-state method includes pulse method. The detection methods of these thermal conductivity will be introduced below.
3. 1 steady state method
The steady-state method is simple in principle, convenient in calculation and high in accuracy, but its test equipment is complex and the test time is long. In addition, the steady-state method requires higher samples. If the size and flatness of the specimen are in a stable state, contact thermal resistance will occur, which will affect the accuracy of the test. 3. 1. 1 principle The basic principle of the protection hot plate method is to establish a one-dimensional constant heat flow in a uniform plate-like specimen with parallel surfaces in the central metering area of the protection hot plate device under steady-state conditions, which is similar to the heat flow existing in a large wireless plate surrounded by two parallel temperature equalizing plates. Where q represents one-dimensional constant heat flow; A represents the area of the calculation unit; δ t represents the temperature difference between the hot and cold surfaces of the sample; D stands for thickness. ⑵ Generally, protective hot plate devices can be used, including double specimen device and single specimen device. In a two-sample device, a heating unit is sandwiched between two almost identical samples, and a cooling unit is arranged outside each sample. As shown in figure 1, it is a double-specimen protection hot plate device. In a single sample device, one side of the heating unit replaces the sample cooling unit with a heat insulation material and a back protection unit. (3) Test points ① Selection and size of test pieces. When the thermal conductivity of materials is detected by using the double sample protection hot plate device, the dimensions of the two samples should be consistent, and the thickness error of the two samples should be controlled within 2% as required. The size of the sample should ensure that it completely covers the surface of the heating unit, and its thickness should not be less than the actual thickness. ② Specimen preparation. The surface of the sample shall be treated to ensure smoothness and close contact with the panel. The non-parallelism of the sample surface should be controlled within 2% of the thickness. ③ state adjustment. Before testing the thermal conductivity of the specimen, the treated specimen should be put into the dryer for state adjustment. If the test time is within the time required for the sample to absorb a large amount of water from the laboratory air, it should be put into the dryer in time before the sample is dried to ensure that the sample remains dry. If the detection time exceeds the time required for the sample to absorb a large amount of water from the laboratory air, put the sample in the standard laboratory air to adjust the state of the sample to keep it in balance with the indoor air. According to the requirements, the standard laboratory air state is 293K 1K, 50% 10% RH. ④ Transition time and measurement interval. According to the requirements, the test device and specimen should ensure enough thermal balance time to accurately measure the thermal properties of materials. When there is mass transfer phenomenon in the specimen, the determination time of the specimen should be controlled over 24 hours. 3. 1.2 principle of the heat flow meter method (1) The basic principle of the heat flow meter method is that when the hot plate and the cold plate are in a constant temperature and stable state, the heat flow meter device establishes a unidirectional stable heat flow in the central measuring part of the heat flow sensor and the central part of the specimen, which is similar to the heat flow existing in an infinite flat wall. ⑵ Device In the heat flow meter method, the device used is shown in Figure 2, and its main components include heating unit, heat flow sensor, sample and cooling unit. ⑶ Test points ① Specimen selection and size. When measuring the thermal conductivity with a double-sample heat flow meter, the thickness difference between the two samples should be controlled within 2%. The size of the sample should ensure that it completely covers the surface of the heating unit, and its thickness should not be less than the actual thickness. ② Specimen preparation. The surface of the sample shall be treated to ensure smoothness and close contact with the panel. The non-parallelism of the sample surface should be controlled within 2% of the thickness. ③ state adjustment. In the heat flow meter method, the conditions of the sample state are the same as those described in the protective hot plate method. ④ Transition time. In the absence of similar experience, the measurement interval should be the product of density, specific heat, thickness and thermal resistance or 300s, and generally five readings of thermal resistance should be taken, and the difference between each reading should be controlled within 65438 0%.
3.2 Unsteady state method
Unsteady state method mainly includes pulse method. This method is simple in equipment and convenient to operate, and can measure the thermal mass coefficient of materials under different humidity without hole temperature refrigeration device. Thermal conductivity, thermal conductivity, specific heat and other parameters can be obtained by one-time test of materials. 3.2. 1 principle The basic principle of the pulse method is to heat the specimen for a short time during the test, so that its temperature changes, and the thermal conductivity of the material can be calculated according to the changing characteristics of the material temperature. 3.2.2 Device In the pulse method, the main components of the device used include a heater, three test pieces, temperature measuring elements and thermoelectric elements. 3.2.3 Test points (1) Specimen selection includes three specimens, including one thin specimen and two thick specimens. The size of thin specimen is 200mm× 20mm× (20 ~ 30) mm, and the size of thick specimen is 200mm× 20mm× (60 ~100) mm. According to the requirements, the difference of apparent density of specimen should be controlled within 5%. The surface of the sample should be smooth and the thickness should be uniform. The contact surface between test pieces should be close. (2) state adjustment. The state of the sample should be adjusted according to the determined conditions. If it is necessary to measure the thermal conductivity of samples with different water contents, the samples should be placed in the specified humidity environment. If it is necessary to measure the thermal conductivity in dry state, the sample should be dried to constant weight.
4 conclusion
For building materials, thermal conductivity is an important index of their energy-saving performance. There are many factors that affect the thermal conductivity of materials, and there are many testing methods that can be used. Therefore, before measuring the thermal conductivity of building materials, appropriate testing methods should be selected according to the characteristics of materials to improve the testing accuracy and ensure the accuracy of data.
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