Current Status and Analysis of Domestic Carbon Fiber Thermal Insulation Material Preparation Methods

 

1. Introduction

Carbon fiber insulation material, also known as carbon fiber insulation material, carbon fiber curing felt and carbon fiber hardening felt, is made by molding carbon fiber and a certain proportion of binder, and undergoes curing molding, carbonization, graphitization, machining and other processes to make carbon fiber insulation material.

 

This type of insulation material is widely used in polysilicon ingot furnaces and single crystal silicon furnaces in the photovoltaic industry, as well as vacuum heat treatment furnaces and atmosphere high-temperature sintering furnaces in the metallurgical industry. It is used for insulation material, and has the characteristics of high temperature resistance, low heat capacity, and good insulation performance. At the same time, it plays a positive role in energy saving and emission reduction of equipment use, and improving product quality.

 

Carbon fiber insulation material, also known as carbon fiber insulation material, carbon fiber curing felt and carbon fiber hardening felt, is made by molding carbon fiber and a certain proportion of binder, and undergoes curing molding, carbonization, graphitization, machining and other processes to make carbon fiber insulation material.

 

This type of insulation material is widely used in polycrystalline silicon ingot furnaces and monocrystalline silicon furnaces in the photovoltaic industry, as well as vacuum heat treatment furnaces and atmosphere high-temperature sintering furnaces in the metallurgical industry. It is used for thermal insulation materials and has the characteristics of high temperature resistance, low heat capacity, and good thermal insulation performance. At the same time, it plays a positive role in energy conservation and emission reduction in equipment use and improving product quality.

 

The market demand for carbon fiber insulation materials is extremely large. Taking the polycrystalline ingot furnace in the photovoltaic industry as an example, the current domestic polycrystalline ingot furnace statistics are shown in Table 1. The annual market demand is about 679 tons, with a value of about 400 million yuan. Including the annual use of high-temperature equipment in the metallurgical industry, the annual domestic market demand is more than 1 billion.

 

With the development of society and industrial progress, the market’s performance requirements for carbon fiber insulation materials are becoming more and more stringent, and products with excellent performance and outstanding functional characteristics are becoming more and more valued.

 

The development trends of carbon fiber insulation materials are mainly in the following aspects:

(1) High-purity high-efficiency insulation materials;

(2) Functionalized high-efficiency insulation materials;

(3) Insulation materials with ultra-high thermal insulation effects.

 

2. Classification Of Domestic Carbon Fiber Insulation Materials

 

Carbon fiber insulation materials can be divided into three categories according to their structure: laminated structure, needle-punched integral structure and short fiber structure. Laminated structure refers to an insulation material product made by hot pressing using soft felt layers or carbon fiber layers, impregnated with glue and laid.

 

Needle-punched integral structure refers to an insulation material that is formed by connecting carbon fibers into a felt body of a certain thickness through a needle-punching process, impregnated with glue, and directly hot-pressed. Short fiber structure is an insulation material that is formed by dispersing short fiber powder and solid glue particles in a solution by wet method, forming by means of filtration, etc., and hot pressing.

 

The three types of products use different molding processes according to the product structure and have different product characteristics. The comparison is shown in the following table.

 

 

These three types of carbon fiber insulation materials are the mainstream forms currently occupying the market. They are used in different parts of high-temperature furnace equipment for different functions and have played a good role.

 

3. Influence of Thermal Insulation Performance Of Carbon Fiber Raw Materials

 

There are three main types of carbon fiber filaments in the selection of carbon fiber filaments, PAN-based carbon fiber filaments, viscose-based carbon fiber filaments and asphalt-based carbon fiber filaments. The type of carbon fiber affects the thermal insulation performance of the product. The thermal conductivity coefficient data of the carbon fiber type are shown in the following table

 

 

In terms of raw materials for carbon fiber felt, there are mainly the following forms:

 

(1) PAN-based carbon fiber felt, asphalt-based carbon fiber felt, viscose-based carbon fiber felt, chopped carbon fiber needle-punched felt, pre-oxidized carbon fiber felt, etc.

(2) Chopped carbon fiber needle-punched felt. (3) Short fiber powder with a length of 0.5-5 mm. The selection of raw materials not only determines the molding process, but also determines the characteristics of the product.

The thickness of PAN soft carbon felt is 5-10 mm. The thermal conductivity of carbon fiber insulation materials made by laminating this type of soft carbon felt is in the range of 0.1-0.15 W/(m*K) at room temperature, and in the range of 0.3-0.6 W/(m*K) at 1000C.

Zhang Zhijun et al. used PAN soft felt, viscose-based soft felt and asphalt-based soft felt to prepare three types of insulation boards with a density of 0.15g/cm³, and tested their thermal conductivity at room temperature and 1000℃ as shown in the following table. From the results, the thermal insulation performance of the laminated soft felt is closely related to the type of raw materials, and the viscose-based carbon fiber felt has the best thermal insulation performance.

 

Table The influence of different materials on the thermal conductivity of carbon fiber insulation materials:

 

 

The integral needle-punched carbon fiber insulation felt uses PAN-based carbon fiber filament raw materials. Due to the introduction of vertical fibers during the needle-punching process, its thermal conductivity is relatively high. The carbon fiber insulation tube prepared by needle-punching structure proposed by Liao Jiqiao et al. has a density of ≤0.3g/cm³ and a thermal conductivity of 0.3 W/(m*K) at 1000℃. This thermal conductivity is relatively higher than that of the laminated structure.

The carbon fiber felt with short fiber filament structure is composed of ultra-short fibers of 0.5-5mm. There is no connected heat conduction channel in the insulation board, so it has advantages in thermal insulation performance. The carbon fiber insulation material formed by short-cut fiber wet method proposed by Zhang Zhijun et al. of Hunan Southern Boyun has a density of 0.09g/cm³, a thermal conductivity of 0.05 W/(m*K) at room temperature, and a thermal conductivity of 0.18-0.25 W/(m*K) at 1000℃.

Comprehensive comparison shows that the type of carbon fiber raw materials has a great influence on the thermal insulation performance of the product. The raw materials can be optimized to improve the thermal insulation performance of the thermal insulation material. Under the same type of raw materials, the short fiber structure is better than the laminated structure in terms of single structure, and the overall needle-punched structure has the worst thermal insulation performance.

In fact, the actual application of thermal insulation materials not only looks at the thermal insulation performance, but also takes into account the corresponding mechanical properties, ash value, and functional gas erosion resistance, etc., so it is not possible to judge which one is better or worse in a single way, but should be combined to judge which one is most suitable.

 

1. The influence of process on the functionalization of thermal insulation material performance

 

The main production processes of carbon fiber thermal insulation board are as follows:

 

 

First, one of the directions of functionalization of carbon fiber insulation materials is to improve the surface’s resistance to atmospheric erosion. Based on the above basic processes, process routes with different characteristics have been derived. The most common functionalized insulation materials are those with density gradients or surface hardening that improve the surface and overall mechanical properties of the insulation board, which are soft inside and hard outside. Liang Lizhen proposed using a hard felt layer as the outer layer and a soft felt layer as the inner layer, brushing glue between the layers and stacking them, and then pasting carbon fiber cloth on the outer surface for pressing and forming, so that the insulation material has a hard surface and good strength and surface properties. Insulation felt products with carbon cloth, graphite paper and carbon-carbon sheets on the surface are often seen on the market. Chen Xiaofei et al. proposed to laminate and solidify soft felt, carbon cloth and flexible graphite paper after dipping them in glue, and formulate a double composite surface structure of graphite paper and carbon cloth on the surface to improve the surface resistance of the insulation material to atmospheric erosion.

 

Secondly, another direction of functionalized insulation materials is to improve the thermal structural function of thermal insulation performance. Wu Xiao et al. proposed a process method in which a carbon fiber vacuum filtration forming layer is composited with a viscose-based carbon fiber needle-punched felt layer, a surface treatment layer is coated by an adhesive and needle-punching, and then molded and post-processed. The excellent thermal insulation performance of short fibers and the excellent overall performance characteristics of the needle-punched structure are utilized to achieve a functional carbon fiber thermal insulation material with high strength and thermal insulation effect. Wang Guigang et al. proposed to modify carbon fiber powder with epoxy resin, and to form a spatial mesh structure with disordered arrangement of carbon fibers by adding a foaming agent, etc. The obtained thermal insulation material has good thermal insulation performance and strength value. Chen Xinhua et al. and Wang Bin proposed to obtain a rigid thermal insulation felt with developed voids and excellent thermal insulation effect by foaming carbon fiber mesh tires with phenolic resin.

 

Finally, the function of carbon fiber thermal insulation materials in terms of purity is improved and optimized. In a high-temperature atmosphere environment above 1000°C, maintaining low volatiles in carbon fiber thermal insulation materials is the basic condition for achieving high quality of furnace products. A lot of research and practice has also been done in China. Xu Chunchao compared the methods of improving carbon fiber insulation materials and found that, compared with the types of carbon fiber filaments, asphalt-based carbon fiber is purer than PAN-based carbon fiber and has a lower ash value. Compared with the process structure, the ash value of the overall needle-punched structure insulation material is lower than that of the short fiber structure insulation material, and the ash value of the laminated structure insulation material is the highest. Compared with the final high-temperature treatment temperature, the ash value at 2000℃ is lower than that at 1800℃, and the ash value at 1600℃ is the highest. A high maximum treatment temperature is conducive to reducing the ash value.

 

In summary, the common process methods for realizing functional carbon fiber insulation materials are described above, but to highlight a certain function of the product, it is often necessary to reduce the other functional characteristics of the product. For example, in general, the product of the same process has a low density, better thermal insulation performance, and weaker overall mechanical properties. The various functional processes mentioned above improve surface performance or overall performance, but also weaken thermal insulation performance. In order to improve the purity of the product, the heat treatment temperature is usually increased, but the high heat treatment temperature also sacrifices a certain degree of thermal insulation performance.

The three types of carbon fiber products have specific characteristics and are used in different parts, so the process routes of products with high purity, high surface hardness and good thermal insulation performance are constantly being developed.

 

1. Performance requirements and field applications of carbon fiber thermal insulation products

 

The preparation of materials must meet the needs of the market. As a thermal insulation material in the high temperature resistant field, it must meet the following three requirements

(1) Have good thermal insulation performance and low heat capacity.

(2) According to the functional needs of the product, it must have certain functional requirements such as strength, surface performance or purity.

(3) Finally, it must meet the environmental adaptability of the product and meet the service life requirements. In the field of polycrystalline ingot furnaces in the photovoltaic field, carbon fiber thermal insulation material products are the most widely used, which require good thermal insulation performance and surface gas erosion resistance. The composite thermal insulation hard felt prepared by Chen Xiaofei et al. has a high temperature thermal conductivity of 0.3-0.4W/(m*K) at 1600℃ and a compressive strength of ≥0.5MPa. The density of the carbon fiber insulation felt made by Chen Xinhua is about 0.20 g/cm³, and the thermal conductivity at 1000C is 0.25 W/(m*K). The density of the carbon fiber insulation felt made by Zhang Zhijun et al. is about 0.1g/cm³, and the thermal conductivity at high temperature of 1000℃ is 0.15-0.25 W/(m*K). This type of structural carbon fiber insulation products meet the requirements of polycrystalline ingot furnaces and are relatively common in the market.

 

In the field of high-temperature furnaces in the metallurgical industry, due to the high strength required, the overall needle-punched structure of carbon fiber insulation felt is more widely used. The insulation felt made by Liao Jiqiao et al. has a density of 0.39g/cm³, an ash value of less than 30ppm, a thermal conductivity of ≤0.8 W(m*K), a stable structure and can bear a certain load. The carbon fiber insulation material made by Zheng Shuyun et al. has a density of 0.1-04 g/cm³, a thermal conductivity of less than 0.4 W/(m* K), and a bending strength of 1.4-2.14MPa.

In the field of single crystal pulling, including silicon single crystal pulling furnace, sapphire single crystal pulling furnace, etc., the carbon fiber insulation materials used in this field not only meet the requirements of good insulation performance and overall strength performance, but also have very strict requirements on ash content, often requiring ash content ≤ 100ppm. At present, this type of insulation material is mainly provided by foreign manufacturers, and the main suppliers are Germany’s SGL, Japan’s Kureha, Japan’s Toyo Tanso, and the United States Morgan.

 

Conclusion

 

The three types of carbon fiber insulation materials are evaluated from the perspective of future development direction. The characteristics of raw materials, production processes, and product performance and application in three aspects. In order to achieve products that meet the requirements of the use environment, there are adjustable and optimized methods in raw materials and processes. For functional short fiber insulation materials with high requirements for thermal insulation performance, short fiber structures can be selected; for functional insulation materials with high structural requirements, needle-punched structures are preferred; for those with low requirements for thermal insulation performance but requirements for overall mechanical properties and functionality, laminated carbon fiber insulation materials can be considered, and then the functional characteristics of the product can be achieved through process design.

Common functionalization includes: (1) surface performance improvement of graphite paper, carbon cloth, resin, thin carbon-carbon plate, etc. achieved from the process; (2) high purity and ultra-high purity functional direction from the selection of raw materials and high temperature treatment; (3) high thermal insulation performance direction of structural design from the thermal conductivity mechanism of the product, etc. The above functionalization is to achieve compliance with the requirements of the product use environment and improve the service life of the product.

The domestic preparation process routes of carbon fiber insulation materials are very similar. The process is improved in many aspects according to product requirements to achieve product diversity. This is the advantage of domestic products. However, compared with foreign products, there are relatively few products that can achieve the ultimate in a certain functional performance of carbon fiber thermal insulation materials. For example, ultra-high purity, ultra-high thermal insulation performance, ultra-high strength and high thermal insulation are still monopolized by foreign products. Therefore, domestic carbon fiber thermal insulation materials still need further efforts to improve and improve the preparation process to achieve the ultimate performance.