AU2020101806A4

AU2020101806A4 - Solid-state temperature control material and preparation method thereof

Info

Publication number: AU2020101806A4
Application number: AU2020101806A
Authority: AU
Inventors: Xiaobin Dong; Fangong KONG; Xia MENG; Letian QI; Shoujuan WANG; Ruhe Zhao
Original assignee: Qilu University of Technology
Current assignee: Qilu University of Technology
Priority date: 2020-03-07
Filing date: 2020-08-13
Publication date: 2020-09-17
Anticipated expiration: 2028-08-13
Also published as: CN111269699A

Abstract

The invention discloses a solid-state temperature control material and a preparation method thereof Main components of the solid-state temperature control material are agricultural and forestry waste fiber, water, agar, a crystalline hydrous salt, a phase change temperature regulator, a nucleating agent, and a filler. According to the invention, a gradient fiber network is constructed using agricultural and forestry wastes and the filler, thereby effectively improving the problems of crystalline hydrous salt phase change materials such as supercooling and phase separation. Meanwhile, solid-state materials that can be processed into any shape and have a small phase change volume change is prepared on the basis of a characteristic that agar solutions can solidify at normal temperature. The solid-state temperature control material of the invention has simple production process and good temperature control effect, can be widely used in the fields of building energy conservation, solar thermal storage or the like, and has great economic values and social benefits.

Description

SOLID-STATE TEMPERATURE CONTROL MATERIAL AND PREPARATION METHOD THEREOF

Technical Field The invention relates to the technical field of solid-state phase change temperature control

materials, and in particular to a solid-state temperature control material jointly stabilized by

agricultural and forestry waste fiber and agar and a preparation method thereof

Background Art From ancient times to today, the development of human society has not been possible without

energy. Especially after modem times, human beings are increasingly dependent on fossil energies

such as coal, petroleum and natural gas, which directly leads to severe energy shortage and

ecological environmental pollution. Therefore, how to improve the efficiency of energy utilization

and develop new clean energy has become a major issue related to social development. Countries

all over the world have paid great attention to this and conducted a lot of researches. Phase change

materials have high latent heat of phase change, and can effectively control environment

temperature by absorbing or releasing a large amount of heat during the phase change process,

achieving the purposes of heat preservation or cold accumulation. Such materials can greatly

ameliorate the incoordination and mismatch between supply and demand in energy systems,

avoiding large waste of energy. These materials have a promising application prospect in fields

such as solar thermal storage, building energy conservation, electronic device temperature control

or the like, and thus have been receiving increasing attention from researchers all over the world.

In the field of medium- and low-temperature phase change materials, hydrous salt is a widely

studied inorganic phase change material. Compared with paraffin, polyol and ester acid organic

phase change materials, inorganic hydrous salts have the advantages of high thermal conductivity,

great latent heat of phase change, non-flammablility, non-toxicity, low price and easy availability.

However, there are two common problems for such materials in practical use: first of all,

separation of liquid and crystal is likely to occur after multiple solid-liquid phase change cycles,

which will reduce heat storage of the materials dramatically; and secondly, during a cooling

process of the materials, heat cannot be released even if temperature has been dropped below a

melting point temperature, which is to say, supercooling occurs. In order to solve the above

problems, many attempts have been made by researchers. Patent Application Publication

CN109370536A discloses a composite phase change material and a preparation method thereof. In this patent, sodium carboxymethylcellulose is used as a thickening agent and accordingly the problems of the phase change material sodium acetate trihydrate, such as supercooling and phase separation, are improved. Similarly, in the Patent Application Publication CN106753256A, a nucleating agent and a thickening agent are added into an inorganic hydrous salt phase change material at the same time, in order to adjust the degree of supercooling of the material and prevent phase separation from occurring. However, the actual effect achieved by the mere use of the nucleating agent and the thickening agent depends too much upon conditions such as concentration and system pH, so that precise control is required. Hence, the process has a certain complexity. Patent Application Publication CN105131910A discloses that use of a physical phase separation inhibitor, such as asbestos, can improve the cycle stability of an inorganic hydrous salt phase change material system, and the phase change material has little thermal performance degradation after 500 cycles. However, asbestos dust easily causes pollutions to atmosphere and water and is also a carcinogen leading to various cancers. As a solid-liquid phase change material, practical use of the hydrous salt, in addition to the above two problems, is also restricted by its inherent shortcomings including large phase change expansion coefficient and volume change, great difficulty to process and form and the like.

Summary of the Invention The invention is aimed at solving the above shortcomings in the prior art, and also providing a solid-state inorganic hydrous salt phase change temperature control material jointly stabilized by agricultural and forestry waste fiber and agar, and a preparation method thereof. This material not only significantly improves the problems of the inorganic hydrous salt such as supercooling and phase separation so that its cycle stability is greatly enhanced, but can also maintain a solid state within a certain temperature range after heat-absorbing phase change, thereby enhancing the processability of the phase change material. Also in the present application, the agricultural and forestry waste fibers is used in place of harmful inorganic fibers, which contributes to recycling and reuse of waste. A solid-state temperature control material, comprises the following raw materials by mass percentage: 4% to 10% of agricultural and forestry waste fiber, 50% to 60% of water, 0.5 to 1.8% of agar, 20% to 40% of a crystalline hydrous salt, 2% to 8% of a phase change temperature regulator, 3%

to 6% of a nucleating agent, and 0.2% to 1% of a filler. A raw material for the agricultural and forestry waste fiber is one or more of bagasse, wheat straw, and corn stalk. The crystalline hydrous salt is one or more of sodium sulfate decahydrate, calcium chloride hexahydrate, sodium carbonate decahydrate, disodium hydrogen phosphate dodecahydrate, and zinc nitrate hexahydrate. The phase change temperature regulator is one or more of sodium chloride, potassium chloride, ammonium chloride, ammonium sulfate, and magnesium chloride hexahydrate. The nucleating agent is one or more of borax, urea, sodium silicate decahydrate, barium hydroxide octahydrate, and nano-silica. The filler is one or more of hydroxymethyl cellulose, bacterial cellulose, polyacrylamide, and polyacrylamide-sodium polyacrylate copolymer. A preparation method of the solid-state temperature control material, comprises the following steps of: (1) adding washed agricultural and forestry waste into hot water at 90-95°C for soaking for 30-60 min, and then transferring the agricultural and forestry waste into a pressing and defibering machine and processing the same until a defibered state is achieved; (2) transferring the defibered agricultural and forestry waste into a reaction kettle, adding an aqueous solution containing sodium hydroxide, hydrogen peroxide and a chelating agent, and heating to 75-80°C under stirring for purposes of reacting for 60-120 min; then, transferring the resultant mixture into the pressing and defibering machine for pressing and defibering, transferring the obtained agricultural and forestry waste into the reaction kettle again and processing the obtained agricultural and forestry waste in the same way; (3) adding the processed mixture into a disk mill for grinding, filtering off the mixture by suction, repeatedly washing the agricultural and forestry waste fiber obtained by suction filtration to remove excess reaction reagents, and finally dispersing the agricultural and forestry waste fiber in water to obtain a fiber suspension; (4) heating the fiber suspension to 90-95°C, and adding the agar, the crystalline hydrous salt, the phase change temperature regulator and the nucleating agent into the fiber suspension until fully dissolved; and (5) lowering the temperature of the mixed liquid to 70-75°C, adding the filler into the mixed liquid, and finally, slowly cooling the mixed liquid to room temperature under stirring, to obtain the solid-state temperature control material jointly stabilized by the agricultural and forestry waste fiber and the agar. In the aqueous solution of the step (2), the sodium hydroxide has a mass concentration of 3% to 6%, the hydrogen peroxide has a mass concentration of 4% to 7%, and the chelating agent has a mass concentration of 0.2% to 0.5%. The chelating agent in the step (2) is one or more of ethylenediaminetetraacetic acid, diethylenetriaminepentaacetic acid, and diethylenetriamine pentamethylene phosphonic acid.

The fiber suspension in the step (3) has a mass percentage concentration of 8% to 15%. According to the invention, fibers prepared from agricultural and forestry waste are used to form a physical fiber network by replacing inorganic fibers that are harmful to environment and human health. Gaps of the network are further filled up using fillers like macromolecules or polymers, and meanwhile a nucleating agent is employed to lower the activation energy of crystallization reaction between inorganic salts and water. Therefore, the common issues of the inorganic hydrous salt phase change material such as supercooling and phase separation can be effectively overcome, and cyclic stability of the material in use is guaranteed. In addition, in consideration of the shortcomings in traditional inorganic hydrous salts such as large solid-liquid phase change volume change and great difficulty to process and form, the invention, on the basis of a characteristic of agar that it forms gel after being dissolved in water and cooled, results in a phase change material that can still maintain a solid state within a certain temperature range after heat-absorbing phase change, which improves the practical application potential of this phase change material. The sodium hydroxide and hydrogen peroxide serve to swell and soften the agricultural and forestry waste, and loosen fiber tissue structures in order to facilitate subsequent pressing and defibering processes. The working mechanism of the sodium hydroxide is to dissolve out short-chain hemicellulose and a small amount of lignin by reacting with hemicellulose in the agricultural and forestry waste through swelling, so that fibers are separated from primary wall layer and secondary wall layer. The working mechanism of the hydrogen peroxide is to decompose into water and a hydrogen peroxide ion under alkaline conditions. The hydrogen peroxide ion can break up macromolecule side chains of the lignin and turn it into small molecule lignin for dissolution. And also, the hydrogen peroxide ion introduces carboxyl groups into lignin molecules to increase the hydrophilicity of the lignin and soften the lignin. The chelating agent acts to prevent ineffective decomposition of the hydrogen peroxide. This is because some heavy metal ions contained in the agricultural and forestry waste cause decomposition of the hydrogen peroxide, and the chelating agent complexes with these metal ions to reduce their impact on the hydrogen peroxide. Compared with existing hydrous salt inorganic phase change materials, the invention has the following advantages. (1) With a hybrid fiber network formed by agricultural and forestry waste fiber and macromolecules or polymers, coupled with use of the nucleating agent, the degree of supercooling of the inorganic hydrous salt can be effectively controlled, the occurrence of phase separation can be prevented, and the recycling performance of phase change materials can be promoted. (2) Based upon a characteristic that gel is formed subsequent to cooling of an agar solution, the inorganic hydrous salt can still maintain a solid state within a certain temperature range after heat-absorbing phase change. Moreover, the gel has good strength and toughness, facilitating practical processing and application of the material. (3) The fiber raw materials used in the invention have wide sources and are low in price. This can not only lower production costs, but also promote recycling and reuse of agricultural and forestry wastes. (4) The solid-state phase change material provided in the invention has a suitable phase change temperature, simple preparation method, and broad application prospects in the fields of solar thermal storage, building energy conservation, electronic equipment temperature control or the like. (5) The aqueous solution containing sodium hydroxide, hydrogen peroxide and the chelating agent is added into the agricultural and forestry waste and then heated and stirred to prepare the agricultural and forestry waste fiber, and the agricultural and forestry waste fiber prepared by such a method has high yield and less pollution during the preparation process. This is due to the fact that, use of the method can retain most of the lignin in the agricultural and forestry waste, and thus results in a higher yield of the final fiber. In addition, the chemical raw materials added in the method do not contain sulfites, and wastewater does not contain sulfur compounds. Therefore, treatment for the wastewater is relatively easy, and pollution load of the wastewater is reduced.

Brief Description of the Drawings FIG1 is a scanning electron micrograph of the solid-state phase change temperature control material of the invention; and FIG 2 is a graph showing temperature changes of the solid-state phase change temperature control material of the invention as the environment temperature rises or drops. In FIG. 2, 1-environmental temperature rise; 2-sample temperature rise; 3-environmental temperature down; 4- sample temperature down. Detailed Description of the Invention For better clarification of the objects, technical solutions and advantages of the invention, the technical solutions of the invention will be described clearly and completely below. Apparently, the examples described herein are some but not all of examples. All other examples obtained on the basis of the examples in the invention by a person of ordinary skill in the art without any creative efforts shall fall into the protection scope of the invention. Example 1 2 Washed corn stalks were cut into small pieces of about 4 cm2, added into hot water at 90°C and soaked for 60 min, and then transferred into a pressing and defibering machine and processed until a defibered state is achieved. Then, the defibered corn stalks were transferred into a reaction kettle where an aqueous solution containing 6% sodium hydroxide, 6% hydrogen peroxide and 0.2% ethylenediaminetetraacetic acid was added, and the resultant mixture was heated to 80°C under stirring for reacting for 90 min. After that, the resultant mixture was transferred into the pressing and defibering machine for pressing and defibering, and the obtained corn stalks were transferred into the reaction kettle again and processed in the same way. Upon completion of the processing, the mixture was added into a disk mill for grinding, the corn stalk fiber obtained by filtration was repeatedly washed with water, and the washed corn stalk fiber was dispersed in water at a concentration of 10% to form a suspension. 100 g of the above corn stalk fiber suspension was heated to 90°C under stirring, and 1.5 g of agar, 40 g of sodium sulfate decahydrate, 5 g of ammonium chloride and 5 g of borax were added into the suspension until fully dissolved. Subsequently, the temperature of the mixed liquid was lowered to 70 °C, 1 g of polyacrylamide was added under stirring. After the polyacrylamide was fully dissolved, the mixed liquid was slowly cooled down to room temperature to obtain the solid-state temperature control material. The scanning electron micrograph of the sample is shown in FIG. 1. It can be clearly seen from FIG.1 that the fibers and the hydrous salt and agar filled therein were closely combined with each other, demonstrating that the material was synthesized successfully. The material prepared in this example had a phase change temperature of 31.2 °C and an enthalpy of phase change of 225.6 J/g. There is no significant degradation in performance after 500 phase change cycles. Example 2 2 Washed bagasse were cut into small pieces of about 4 cm2, added into hot water at 90°C and soaked for 30 min, and then transferred into a pressing and defibering machine and processed until a defibered state is achieved. Then, the defibered bagasse were transferred into a reaction kettle where an aqueous solution containing 3% sodium hydroxide, 4% hydrogen peroxide and 0.5% diethylenetriaminepentaacetic acid were added, and the resultant mixture was heated to 80°C under stirring for reacting for 90 min. After that, the resultant mixture was transferred into the pressing and defibering machine for pressing and defibering, and the obtained bagasse were transferred into the reaction kettle again and processed in the same way. Upon completion of the processing, the mixture was added into a disk mill for grinding, the bagasse fiber obtained by filtration was repeatedly washed with water, and the washed bagasse fiber was dispersed in water at a concentration of 8% to form a suspension. 100 g of the above bagasse fiber suspension was heated to 90°C under stirring, and 2 g of agar, 45 g of calcium chloride hexahydrate, 8 g of potassium chloride and 6 g of barium hydroxide octahydrate were added into the suspension until fully dissolved. Subsequently, the temperature of the mixed liquid was lowered to 70 °C, 0.6 g of polyacrylamide-sodium polyacrylate copolymer was added under stirring. After it was fully dissolved, the mixed liquid was slowly cooled down to room temperature to obtain the solid-state temperature control material. The temperature change of the sample with the increase or decrease of ambient temperature is shown in Fig. 2. It can be seen from the figure that with the increase or decrease of ambient temperature, the change of material temperature is small, and there is obvious hysteresis phenomenon, which indicates that the material can control the temperature change through phase change absorption and release heat. The material prepared in this example had a phase change temperature of 28.0 °C and an enthalpy of phase change of 159.4 J/g. There is no significant degradation in performance after 500 phase change cycles. Example 3 2 Washed wheat stalks were cut into small pieces of about 4 cm2, added into hot water at 95°C and soaked for 60 min, and then transferred into a pressing and defibering machine and processed until a defibered state is achieved. Then, the defibered wheat stalks were transferred into a reaction kettle where an aqueous solution containing 4% sodium hydroxide, 6% hydrogen peroxide and 0.4% diethylenetriamine pentamethylene phosphonic acid were added, and the resultant mixture was heated to 75°C under stirring for reacting for 120 min. After that, the resultant mixture was transferred into the pressing and defibering machine for pressing and defibering, and the obtained wheat stalks were transferred into the reaction kettle again and processed in the same way. Upon completion of the processing, the mixture was added into a disk mill for grinding, the wheat stalk fiber obtained by filtration was repeatedly washed with water, and the washed wheat stalk fiber was dispersed in water at a concentration of 12% to form a suspension. 100 g of the above corn stalk fiber suspension was heated to 90°C under stirring, and 1 g of agar, 55 g of sodium carbonate decahydrate, 10 g of sodium chloride and 8 g of sodium silicate decahydrate were added into the suspension until fully dissolved. Subsequently, the temperature of the mixed liquid was lowered to 75 °C, 1.5 g of bacterial cellulose was added under stirring. After it was fully dissolved, the mixed liquid was slowly cooled down to room temperature to obtain the solid-state temperature control material. The material prepared in this example had a phase change temperature of 30.5 °C and an enthalpy of phase change of 234.8 J/g. There is no significant degradation in performance after 500 phase change cycles. Example 4 2 Washed corn stalks and wheat stalks were cut into small pieces of about 4 cm2, added into hot water at 90°C and soaked for 60 min, and then transferred into a pressing and defibering machine and processed until a defibered state is achieved. Then, the defibered corn stalks and wheat stalks were transferred into a reaction kettle where an aqueous solution containing 5% sodium hydroxide, 6% hydrogen peroxide and 0.5% ethylenediaminetetraacetic acid was added, and the resultant mixture was heated to 80°C under stirring for reacting for 120 min. After that, the resultant mixture was transferred into the pressing and defibering machine for pressing and defibering, and the obtained corn stalks and wheat stalks were transferred into the reaction kettle again and processed in the same way. Upon completion of the processing, the mixture was added into a disk mill for grinding, the corn stalk and wheat stalk fiber obtained by filtration was repeatedly washed with water, and the washed corn stalk and wheat stalk fiber were dispersed in water at a concentration of 15% to form a suspension. 100 g of the above corn stalk and wheat stalk fiber suspension were heated to 95°C under stirring, and 2 g of agar, 20 g of sodium carbonate decahydrate, 20 g of sodium sulfate decahydrate,4 g of ammonium sulfate and 5 g nano-silica were added into the suspension until fully dissolved. Subsequently, the temperature of the mixed liquid was lowered to 75 °C, 1 g of hydroxymethyl cellulose was added under stirring. After it was fully dissolved, the mixed liquid was slowly cooled down to room temperature to obtain the solid-state temperature control material. The material prepared in this example had a phase change temperature of 32.2 °C and an enthalpy of phase change of 230.5 J/g. There is no significant degradation in performance after 500 phase change cycles. Example 5 2 Washed corn stalks were cut into small pieces of about 4 cm2, and washed bagasse were added into hot water at 90°C and soaked for 60 min, and then transferred into a pressing and defibering machine and processed until a defibered state is achieved. Then, the defibered corn stalks and bagasse were transferred into a reaction kettle where an aqueous solution containing 5% sodium hydroxide, 6% hydrogen peroxide and 0.5% ethylenediaminetetraacetic acid was added, and the resultant mixture was heated to 80°C under stirring for reacting for 120 min. After that, the resultant mixture was transferred into the pressing and defibering machine for pressing and defibering, and the obtained corn stalks and bagasse were transferred into the reaction kettle again and processed in the same way. Upon completion of the processing, the mixture was added into a disk mill for grinding, the corn stalk and bagasse fiber obtained by filtration were repeatedly washed with water, and the washed corn stalk and bagasse fiber were dispersed in water at a concentration of 10% to form a suspension. 100 g of the above corn stalk and bagasse fiber suspension were heated to 95°C under stirring, and 1 g of agar, 30 g of disodium hydrogen phosphate dodecahydrate, 30 g of zinc nitrate hexahydrate,6 g of magnesium chloride hexahydrate, 2 g of borax and 2 g of urea were added into the suspension until fully dissolved. Subsequently, the temperature of the mixed liquid was lowered to 70 °C, 0.8 g of polyacrylamide was added under stirring. After it was fully dissolved, the mixed liquid was slowly cooled down to room temperature to obtain the solid-state temperature control material. The material prepared in this example had a phase change temperature of 34.0 °C and an enthalpy of phase change of 180.7 J/g. There is no significant degradation in performance after 500 phase change cycles. Example 6

Washed corn stalks and wheat stalks were cut into small pieces of about 4 cm2, added into hot water at 95°C and soaked for 60 min, and then transferred into a pressing and defibering machine and processed until a defibered state is achieved. Then, the defibered corn stalks and wheat stalks were transferred into a reaction kettle where an aqueous solution containing 6% sodium hydroxide, 7% hydrogen peroxide and 0.2% ethylenediaminetetraacetic acid, and 0.2%

diethylenetriaminepentaacetic acid were added, and the resultant mixture was heated to 80°C under stirring for reacting for 90 min. After that, the resultant mixture was transferred into the pressing and defibering machine for pressing and defibering, and the obtained corn stalks were transferred into the reaction kettle again and processed in the same way. Upon completion of the processing, the mixture was added into a disk mill for grinding, the corn stalk and wheat stalk fiber obtained by filtration were repeatedly washed with water, and the washed corn stalk and wheat stalk fiber were dispersed in water at a concentration of 10% to form a suspension. 100 g of the above corn stalk and wheat stalk fiber suspension were heated to 95°C under stirring, and 2.5 g of agar, 30 g of sodium sulfate decahydrate, 20 g of disodium hydrogen phosphate dodecahydrate, 6 g of sodium chloride, 6 g of potassium chloride and 10 g of sodium silicate decahydrate were added into the suspension until fully dissolved. Subsequently, the temperature of the mixed liquid was lowered to 80 °C, 0.4 g of polyacrylamide and 0.8 g of hydroxymethyl cellulose were added under stirring. After they were fully dissolved, the mixed liquid was slowly cooled down to room temperature to obtain the solid-state temperature control material. The material prepared in this example had a phase change temperature of 32.5 °C and an enthalpy of phase change of 238.6 J/g. There is no significant degradation in performance after 500 phase change cycles. Comparative Example 1 A solid-state phase change temperature control material without agricultural and forestry waste fiber was prepared. This example was compared with Example 1 in order to know the influence of the physical fiber network on the performance of the phase change temperature control material. 90 g of water was heated to 90°C, and 1.5 g of agar, 40 g of sodium sulfate decahydrate, 5 g of ammonium chloride and 5 g of borax were added into the water until fully dissolved. Subsequently, the temperature of the mixed liquid was lowered to 70°C, 1 g of polyacrylamide was added under stirring. After the polyacrylamide was fully dissolved, the mixed liquid was slowly cooled down to room temperature to obtain the solid-state temperature control material. The comparative sample had a phase change temperature of 31.4 °C and an enthalpy of phase change of 225.0 J/g. During 300 phase change cycles, the comparative sample had a gradual decrease in heat storage performance and a final enthalpy of phase change changed to 172.6 J/g, and agglomeration occurred. As can be seen from Comparative Example 1, the agricultural and forestry waste fiber could effectively improve the use performance of the material. This was due to the fact that, the agricultural and forestry waste fiber can construct a hybrid fiber network through hydrogen bonds and macromolecules or polymers, which could effectively prevent the sedimentation, aggregation and growth of undissolved particles after the inorganic hydrous salt was molten, inhibit the occurrence of phase separation, and further improve the recycling performance of the phase change material. In addition, compared with inorganic fibers such as asbestos, the agricultural and forestry waste fiber is more environment-friendly, and can further promote waste recycling as well. Comparative Example 2 A solid-state phase change temperature control material with no agar added was prepared. This example was compared with Example 1 in order to know the influence of the agar on the phase change temperature control material. The preparation process was the same as in Example 1, except that "1.5 g of agar" was removed. This comparative sample could not maintain a solid state at room temperature, and was tested to have a phase change temperature of 31.2°C and an enthalpy of phase change of 225.2 J/g. The comparative sample had a slight decrease in performance and an enthalpy of phase change changed to 213.8 J/g after 500 phase change cycles, indicating that the agar also had a certain effect of maintaining the recycling performance of the inorganic hydrous salt phase change material. As can be seen from Comparative Example 2, the agar could help to prepare a solid-state phase change temperature control material, and also to enhance the recycling performance of the material to a certain extent. Based upon a characteristic that gel was formed subsequent to cooling of an agar solution, the inorganic hydrous salt could still maintain a solid state within a certain temperature range after heat-absorbing phase change. Moreover, the gel had good strength and toughness, facilitating practical processing and application of the phase change material. In addition, the agar gel was formed owing mainly to the action of agarose. The agarose was a linear polymer that could also form a certain fiber network to prevent phase separation and improve the recycling performance of the phase change material. Comparative Example 3 A solid-state phase change temperature control material without a filler was prepared. This example was compared with Example 1 in order to know the influence of the filler on the phase change temperature control material. The preparation process was the same as in Example 1, except that the step that "Subsequently, the temperature of the mixed liquid was lowered to 70 °C, 1 g of polyacrylamide was added under stirring. After the polyacrylamide was fully dissolved, the mixed liquid was slowly cooled down to room temperature" was replaced with "Subsequently, the mixed liquid was slowly cooled down to room temperature". The comparative sample had a phase change temperature of 31.4 °C and an enthalpy of phase change of 224.6 J/g. The comparative sample had a slight decrease in performance and an enthalpy of phase change changed to 206.5 J/g after 500 phase change cycles. As can be seen from Comparative Example 2, the filler used in the invention is a macromolecule or polymer, and mainly functions to assist the agricultural and forestry waste fiber in forming a hybrid fiber network, in order to further improve the recycling performance of the material. A certain decrease in the performance of the material could occur if no filler is added. Crystalline hydrous salt is a phase change agent that, when being heated, absorbs heat and melts into water and salt to store the heat, and when being cooled, releases the heat so that the water and the salt restore to the hydrous salt. It is a main component that plays a temperature control role in the phase change material in the invention. The crystalline hydrous salt having a low phase change temperature is seen as a medium- and low-temperature material among phase change heat storage materials, and has the advantages of high thermal conductivity, large heat of solution, low price and easy availability. The phase change temperature regulator can control the phase change temperature of the crystalline hydrous salt between 25°C and 35°C, which better conforms to actual temperature control requirements. The main mechanism of the phase change temperature regulator is that the phase change agent and the phase change temperature regulator in appropriate proportions can form a eutectic salt, thereby changing the phase change temperature. In the material, the nucleating agent serves mainly to avoid the occurrence of supercooling that still had no solidification when the temperature of the phase change agent was lowered to the solidifying point. If serious supercooling is seen, the substances, within a certain temperance range, can not undergo liquid-solid phase change and thus release no stored heat, indicating that the phase-change temperature control effect cannot be achieved. The nucleating agent added into the temperature control material acts as a heterogeneous crystal nucleus. When the phase change agent is crystallized on the surface of the nucleating agent, the energy required is low, and it is easier to achieve crystallization and heat release, avoiding the occurrence of supercooling. According to the invention, a gradient fiber network is constructed using agricultural and forestry wastes and the filler, thereby effectively improving the problems of crystalline hydrous salt phase change materials such as supercooling and phase separation. Meanwhile, solid-state materials that can be processed into any shape and have a small phase change volume change is prepared on the basis of a characteristic that agar solutions can solidify at normal temperature. The solid-state temperature control material of the invention has simple production process and good temperature control effect, can be widely used in the fields of building energy conservation, solar thermal storage or the like, and has great economic values and social benefits. Finally, it is to be noted that the foregoing examples are merely for explaining the technical solutions of the invention, rather than limitations thereto. Although the invention has been detailed with reference to the foregoing examples, a person of ordinal skill in the art shall understand that modifications can still be made to the technical solutions documented in the foregoing examples, or equivalent substitutions can be made to some of the technical features therein. Such modifications or substitutions do not deviate the nature of the corresponding technical solutions from the spirit and scope of the technical solutions embodied in the examples of the invention.

Claims

1. A solid-state temperature control material, characterized in that the material comprises the following raw materials by mass percentage: 4% to 10% of agricultural and forestry waste fiber, 50% to 60% of water, 0.5 to 1.8% of agar, % to 40% of a crystalline hydrous salt, 2% to 8% of a phase change temperature regulator, 3%

to 6% of a nucleating agent, and 0.2% to 1% of a filler.

2. The solid-state temperature control material according to claim 1, characterized in that a raw material for the agricultural and forestry waste fiber is one or more of bagasse, wheat straw, and corn stalk.

3. The solid-state temperature control material according to claim 1, characterized in that the crystalline hydrous salt is one or more of sodium sulfate decahydrate, calcium chloride hexahydrate, sodium carbonate decahydrate, disodium hydrogen phosphate dodecahydrate, and zinc nitrate hexahydrate.

4. The solid-state temperature control material according to claim 1, characterized in that the phase change temperature regulator is one or more of sodium chloride, potassium chloride, ammonium chloride, ammonium sulfate, and magnesium chloride hexahydrate.

5. The solid-state temperature control material according to claim 1, characterized in that the nucleating agent is one or more of borax, urea, sodium silicate decahydrate, barium hydroxide octahydrate and nano-silica.

6. The solid-state temperature control material according to claim 1, characterized in that the filler is one or more of hydroxymethyl cellulose, bacterial cellulose, polyacrylamide, and polyacrylamide-sodium polyacrylate copolymer.

7. A preparation method of the solid-state temperature control material according to claim 1, characterized in that the method comprises the following steps of: (1) adding washed agricultural and forestry waste into hot water at 90-95°C for soaking for 30-60 min, and then transferring the agricultural and forestry waste into a pressing and defibering machine and processing the same until a defibered state is achieved; (2) transferring the defibered agricultural and forestry waste into a reaction kettle, adding an aqueous solution containing sodium hydroxide, hydrogen peroxide and a chelating agent, and heating to 75-80°C under stirring for purposes of reacting for 60-120 min; then, transferring the resultant mixture into the pressing and defibering machine for pressing and defibering, transferring the obtained agricultural and forestry waste into the reaction kettle again and processing the obtained agricultural and forestry waste in the same way;

(3) adding the processed mixture into a disk mill for grinding, filtering off the mixture by suction, repeatedly washing the agricultural and forestry waste fiber obtained by suction filtration to remove excess reaction reagents, and finally dispersing the agricultural and forestry waste fiber in water to obtain a fiber suspension; (4) heating the fiber suspension to 90-95°C, and adding the agar, the crystalline hydrous salt, the phase change temperature regulator and the nucleating agent into the fiber suspension until fully dissolved; and (5) lowering the temperature of the mixed liquid to 70-75°C, adding the filler into the mixed liquid, and finally, slowly cooling the mixed liquid to room temperature under stirring, to obtain the solid-state temperature control material jointly stabilized by the agricultural and forestry waste fiber and the agar.

8. The preparation method according to claim 7, characterized in that in the aqueous solution of the step (2), the sodium hydroxide has a mass concentration of 3% to 6%, the hydrogen peroxide has a mass concentration of 4% to 7%, and the chelating agent has a mass concentration of 0.2%

to 0.5%.

9. The preparation method according to claim 7, characterized in that the chelating agent in the step (2) is one or more of ethylenediaminetetraacetic acid, diethylenetriaminepentaacetic acid, and diethylenetriamine pentamethylene phosphonic acid.

10. The preparation method according to claim 7, characterized in that the fiber suspension in the step (3) has a mass percentage concentration of 8% to 15%.