储热材料的研究_范文大全网

范文一：储热材料的研究

High latent heat storage and high thermal conductive phase change materials using exfoliated graphite nanoplatelets

Sumin Kim a, ?, Lawrence T. Drzal b

a Department of Architecture, College of Engineering, Soongsil University, Seoul 156-743, Republic of Korea

Composite Materials and Structures Center, College of Engineering, Michigan State University, East Lansing, MI 48824-1226, USA

a r t i c l e i n f o

Article history:

Received 24April 2008Received in revised form 8September 2008

Accepted 16September 2008

Available online 1November 2008Keywords:

Exfoliated graphite nanoplatelets (xGnP)Phase change material (PCM)Paraf?n wax

Latent heat storage Thermal conductivity

a b s t r a c t

Using exfoliated graphite nanoplatelets (xGnP),paraf?n/xGnPcomposite phase change materials (PCMs)were prepared by the stirring of xGnP in liquid paraf?n for high electric conductivity, thermal conductivity and latent heat storage. xGnP of 1, 2, 3, 5and 7wt%was added to pure paraf?n at 751C. Scanning electron microscopy (SEM)morphology showed uniform dispersion of xGnP in the paraf?n wax. Good dispersion of xGnP in paraf?n/xGnPcomposite PCMs led to high electric conductivity. The percolation threshold of paraf?n/xGnPcomposite PCMs was between 1and 2wt%in resistivity measurement. The thermal conductivity of paraf?n/xGnPcomposite PCMs was increased as xGnP loading contents. Also, reproducibility of paraf?n/xGnPcomposite PCMs as continuous PCMs was manifested in results of electric and thermal conductivity. Paraf?n/xGnPcomposite PCMs showed two peaks in the heating curve by differential scanning calorimeter (DSC)measurement. The ?rst phase change peak at around 351C is lower and corresponds to the solid–solid phase transition of the paraf?n, and the second peak is high at around 551C, corresponding to the solid–liquid phase change. The latent heat of paraf?n/xGnPcomposite PCMs did not decrease as loading xGnP contents to paraf?n. xGnP can be considered as an effective heat-diffusion promoter to improve thermal conductivity of PCMs without reducing its latent heat storage capacity in paraf?n wax.

1. Introduction

Solid–liquid phase change materials (PCMs)are often used for heat-storage applications. Examples include water, salt hydrates, paraf?ns, certain hydrocarbons and metal alloys. Salt hydrate PCMs used for thermal storage in space heating and cooling applications have low material costs, but high packaging costs. A more economic installed storage may be possible with medium priced, high latent heat organic materials suitable for low-cost packaging, i.e. those that are insoluble in water and un-reactive with air and some of the common packaging ?lms [1,2].

In recent times, several candidate inorganic and organic PCMs and their mixtures have been studied as PCMs for latent heat thermal energy storage (LHTES)applications [3–6]. PCMs that are used as storage media in latent thermal energy storage can be classi?ed into two major categories:inorganic and organic compounds. Inorganic PCMs include salt hydrates, salts, metals and alloys, whereas organic PCMs are comprised of paraf?n, fatty acids/estersand polyalcohols. Paraf?n is taken as the most

promising PCM because it has a large latent heat and low cost, and is stable, non-toxic and not corrosive [7,8]. Among the investigated PCMs, paraf?ns have been widely used for LHTES applications due to their large latent heat and proper thermal characteristics such as little or no super cooling, varied phase change temperature, low vapor pressure in the melt, good thermal and chemical stability, and self-nucleating behavior [5,9–11]. Portable electronic devices such as notebook computers and wearable electronic devices possess unique characteristics that nearly eliminate the use of traditional methods of thermal management [12]. Cooling by heat transfer to PCMs is one of the promising directions. This cooling technology has been widely regarded overseas in recent years, and it also has had certain applications in some high-tech systems, such as aviation, micro-electronics and military electronic systems [13]. Therefore, electric conductivity of PCMs is one of the important factors for electric device application.

In spite of these desirable properties of paraf?ns, the low thermal conductivity (0.21–0.24W/mK) is its major drawback decreasing the rates of heat stored and released during melting and crystallization processes which in turn limits their utility areas [14]. These drawbacks reduce the rate of heat storage and extraction during the melting and solidi?cation cycles and restrict their wide applications, respectively

Contents lists available at ScienceDirect

journal homepage:www.elsevier.com/locate/solmat

Solar Energy Materials &Solar Cells

?Corresponding author. Tel.:+8228200665; fax:+8228163354.

E-mail address:skim@ssu.ac.kr(S.Kim).

Solar Energy Materials &Solar Cells 93(2009)136–142

[8]. To overcome the low thermal conductivity problem of paraf?n as PCMs, studies have been carried out with the purpose of developing LHTES systems with un?nned and ?nned con?gurations, dispersing high conductivity particles and inserting a metal matrix into paraf?n wax [14–16].

Expanded graphite (EG)is generally produced by using H 2SO 4–graphite intercalation compounds (GICs).H 2SO 4–GICs are widely used for the exfoliation process, because they can give a high expansion volume during the thermal treatment. The electrochemical intercalation of H 2SO 4as well as the chemical one were described in the Tryba’s works [17,18]. The EG maintains the layered structures similar to natural graphite ?ake but produces tremendously different sizes of pores and nanosheets with very high aspect ratio [19,20].

Research in the Drzal group has shown that exfoliated graphite nanoplatelets (xGnPTM ), which combine the layered structure and low price of nanoclays with the superior mechanical, electrical and thermal properties of carbon nanotubes, are very cost effective and can simultaneously provide a multitude of physical and chemical property enhancements [21–23]. Nanocomposites prepared with xGnP in thermosetting and thermoplastic polymer systems showed excellent mechanical properties and electrical conductivity [24–26].

To increase thermal conductivity, EG has been used to insert into the paraf?n wax [3,8,12,27,28]. However, Zhang and Fang studied the effect of the EG addition on the thermal properties of the paraf?n (m.p.:48–501C)/EGcomposite prepared as form-stable PCM, and they reported that the latent heat capacity of the PCM decreased with increase of the mass fraction of the graphite [8,14]. This study aimed to prepare the composites of paraf?n (n -docosane, m.p.:42–441C)/xGnPTM with low mass fraction of xGnP to obtain a form-stable composite PCM and to investigate the effect of xGnP addition on thermal conductivity and melting time, melting temperature, and latent heat capacity of the paraf?n, especially to keep latent heat of the paraf?n as xGnP contents is the main purpose of this research.

2. Experimental

2.1. Materials

xGnP TM are prepared from sulfuric acid-intercalated expand-able graphite (3772)obtained from Asbury Graphite Mills, Inc., NJ, USA by applying a cost-and time-effective exfoliation process initially proposed by Drzal’s group [21,26]. The sulfuric acid-based GIC was fabricated from natural graphite through chemical oxidation in the presence of concentrated sulfuric acid. It is composed of layered, but compactly fastened nanoplates of graphite shown in Fig. 1a. Fig. 1b shows a worm-or accordion-like expanded structure of GIC which was exfoliated up to 300–500times in their initial volume by rapid heating in a microwave environment. Multi-pores structure is observed from high magni?cation (?350) of EG shown in Fig. 1c. Pulverization using an ultrasonic processor is employed to break down the worm-like structure and to reduce its size, resulting in individual graphite nanoplatelets that are o 10nm thick and have an average diameter of 15m m as shown in Fig. 1d. This xGnP is denoted as

Fig. 1. Scanning electron microphotographs of (a)acid-intercalated graphite, (b)expended graphite by microwave EG (?50), (c)expended graphite by microwave EG (?350), and (d)exfoliated graphite; xGnP15.

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xGnP-15and was used in this study as a reinforcement in paraf?n matrix for PCM. The Brunauer–Emmet–Teller (BET)surface area of the xGnP was measured using an auto N 2absorption instrument (ASAP2010, Micrometrics, USA). The measurement results showed that the BET surface area of the EG was around 30m 2/g.Paraf?n wax (n -docosane) with melting temperature of 53–571C was purchased from Sigma-Aldrich company. 2.2. Preparation of paraf?n/xGnPcomposite PCM

To establish the relationship between thermal conductivity of the composite PCM and the mass fraction of xGnP and determine the minimum mass fraction of xGnP that is adequate to obtain paraf?n/xGnPcomposite as form-stable PCM, the composite PCMs were prepared by stirring of xGnP in liquid paraf?n with mass fraction of 1%,2%,3%,5%and 7%.The paraf?n was melted by heating it at 751C, and then, the xGnP was mixed into the liquid paraf?n. After being ?ltered and dried, the paraf?n/EGcomposite PCM was obtained. To check the availability of PCMs as continuous PCMs, the samples were remelted for measuring electrical and thermal conductivity. 2.3. Characterization techniques

2.3.1. Scanning electron microscopy (SEM)

The morphology of intercalated graphite, exfoliated graphite, xGnP and paraf?n/xGnPcomposite PCMs were observed by SEM at room temperature. A JEOL (modelJSM-6400) SEM with accelerating voltage of 12kV and working distance of 15mm was used to collect SEM images. To compare images by gold coating, non-coating and gold coating samples were prepared. A gold coating of a few nanometers in thickness was coated on samples.

2.3.2. Electrical property measurement

The resistivity of paraf?n/xGnPcomposite PCMs was mea-sured, with a Gamry instrument under FAS2TM Femtostat plug system and potentiostatic mode, along the ?ow direction, in case of the injection-molded samples, using impedance spectroscopy by applying the two-probe method at room temperature. Samples with dimensions of 5?3?12mm 3were cut from the middle portion of ?exural bars, and the resistivity was measured along the thickness direction (5mm). The two surfaces that were connected to the electrodes were ?rst treated with O 2plasma (14min, 550W) in order to remove the top surface layers which are rich in polymer, to ensure good contact of the sample surface with the electrodes. The resistance of sample was measured in the

frequency range of 0.1–1,00,000Hz and converted to conductivity by taking into account the sample dimensions. Eq. (1)can be used to calculate the resistivity of the sample. R ?I ?S =T

(1)

where I is the impedance value at 1Hz, R is the resistivity, S is the intercept surface area, T is the thickness of the sample.

2.3.3. Thermal conductivity measurement

The thermal conductivity of paraf?n wax and paraf?n/xGnPcomposite PCMs were measured using a UNITHERM TM machine-UNITHERM TM Model 2022(AnterCorporation, Pittsburgh, PA). The tests were performed according to ASTM E1530(Standardtest method for evaluating the resistance to thermal transmission of materials by the guarded heat ?ow meter method technique). Specimens of 1in diameter were prepared with stainless mold as shown in Fig. 2. Hot liquid sample was put into the mold and cooled down by liquid nitrogen. In order to ensure that the sample thickness was within the recommended range, 3–5discs were stacked-up for the composites with higher xGnP loading. The samples were tested at 201C under an applied load of 30psi. Reported results represent the average of three measurements for each xGnP loading.

2.3.4. Differential scanning calorimeter (DSC)

The melting and heat storage behaviors of the paraf?n/xGnPcomposite PCMs were studied using a TA Instruments 2920DSC equipped with a cooling attachment, under a nitrogen atmo-sphere. The data were collected with a scan rate of 101C min –1over a temperature range of à50–1101C. The measurement was made using a 5–10mg sample on a DSC sample cell after the sample was quickly cooled to à501C from the melt of the ?rst scan.

2.3.5. Thermogravimetric analysis (TGA)

TGA was conducted with a TA Instruments TGA 2950that was ?tted to a nitrogen purge gas from 30to 6001C. This unit has the ability to decrease the ramp rate when an increased weight loss is detected in order to obtain better temperature resolution of a decomposition event. The general ramp rate was 41C/minwith a weight loss detection sensitivity set to 4.0in the furnace control software. Approximately 5–15mg of cut samples was used to determine the decomposition temperatures.

Fig. 2. Mold for paraf?n/xGnPcomposite PCM and thermal conductivity test sample:(a)mold for paraf?n/xGNPcomposite PCM and (b)mold and sample for thermal conductivity.

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3. Results and discussion

3.1. Morphology of paraf?n/xGnPcomposite PCMs

The cryogenically fractured surface of the paraf?n/xGnP composite PCMs was studied by SEM. Fig. 3shows the SEM photographs of the paraf?n/xGnPcomposite PCMs of 2%and 5%xGnP loading contents with gold coating and non-coating (magni?cation of ?2000). It is observed from Fig. 3a and c that the dispersions of the xGnP in the paraf?n wax are uniform. xGnP was well-dispersed in paraf?n. Actually, it looks like paraf?n covered slightly on the xGnP surface. It is a different morphology compared to the Author’s result in which xGnP is dispersed in LLDPE polymer matrix [26]. We can easily recognize the existence of xGnP by its uniform shape. As shown in Fig. 3b and d the dispersion of xGnP in paraf?n is indicated by the clear white plate phase even it was not coated by gold. From this morphology of non-coating samples, it can be expected that these 2%and 5%xGnP-loaded PCM composites are electrically conductive because SEM can detect only electrically conductive materials by the electric beam. The dispersion of xGnPs covered by paraf?n is more signi?cantly indicated in high magni?cation (?5000) as shown in Fig. 7. Furthermore, the uniform xGnP particle size is indicated with these ?gures. Although xGnP loading contents were low, 2and 5wt%,xGnP are well embedded and dispersed enough to show their existence. 3.2. Resistivity of paraf?n/xGnPcomposite PCMs

This high electrical conductivity was detected by the resistivity of paraf?n/xGnPcomposite PCMs as xGnP loading content as shown in Fig. 5. The incorporation of xGnP can greatly decrease the resistivity of composites with a sharp transition from an electrical insulator to an electrical conductor. For the purpose of ?nding a percolation threshold for the resistivity 1, 2, 3, 5and 7wt%xGnP loaded samples were measured. The percolation threshold of xGnP-LLDPE nanocomposite by solution mixing and injection molded was between 1and 2wt%. This percolation threshold is extremely low compared to the Author’s result [26]of xGnP dispersed into low linear density poly propylene. It was between 12and 15wt%.Resistivity of 1wt%of xGnP loaded was high around 109O cm, even second melted sample. However, resistivity was down to 104O cm from 2wt%of xGnP. The percolation threshold for the resistivity depends very much on the geometry of the conducting ?llers. Fillers with elongated geometry such as sheets can be used to achieve very low percolation threshold value, due to the fact that sheets with higher aspect ratios have great advantage over spherical or elliptical ?llers in forming conducting networks in polymer matrix. As we check the morphology in Figs. 3and 4, xGnP were connected to each other to make electric conductivity. As continuous PCMs, resistivity of second melted sample was measured. Resistivity of second melted samples showed similar behavior with ?rst melted samples. Reproducibility of paraf?n/xGnPcomposite PCMs as continuous PCMs was manifested (Fig. 5).

Fig. 3. Scanning electron microphotographs of 5and 2wt%of paraf?n/xGnPcomposite PCMs by coating condition for SEM (lowmagni?cation, ?2000):(a)paraf?n/xGNP 5%— gold coating, (b)paraf?n/xGNP5%— non-coating, (c)paraf?n/xGNP2%— gold coating, and (d)paraf?n/xGNP2%— non-coating.

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3.3. Thermal conductivity of paraf?n/xGnPcomposite PCMs The thermal conductivity results of pure paraf?n and the paraf?n/xGnPcomposite PCMs are shown in Fig. 6. It can be found that the thermal conductivities of the composite PCMs improve evidently compared to that of pure paraf?n. When the thermal conductivity of pure paraf?n is 0.26W/mK,the thermal con-

ductivity of the composite PCM including mass 7wt%xGnP is found to be 0.8W/mK.These results are comparable to Zhang’s report [29]. Ten percent of graphite mass fraction in the shape-stabilized PCM showed 0.229W/mK,while 20%of cases were 0.482W/mKin this report. Theoretically, the thermal conductiv-ities will increase continually with increasing additive quantity of exfoliate graphite. The thermal conductivity of paraf?n/xGnPcomposite PCMs was increased as xGnP loading contents. Reproducibility of paraf?n/xGnPcomposite PCMs as continuous PCMs for thermal conductivity also reappeared like electric conductivity, even second samples were little higher than the ?rst samples.

3.4. Thermal storage performance and thermal stability of paraf?n/xGnP composite PCMs

The heating and freezing curves by DSC measurements of the paraf?n and the paraf?n/xGnPcomposite PCMs are presented in Fig. 7. It can be seen from the heating curve in Fig. 7(a)that the paraf?n has two phase change peaks. The ?rst phase change peak at about 35.31C is lower and corresponds to the solid–solid phase transition of the paraf?n, and the second peak is very high at around 55.21C, corresponding to the solid–liquid phase change. These peaks are matched with pure paraf?n peaks in the previous report [30]. The DSC curve of the paraf?n/xGnPcomposite PCMs is shown in Fig. 7b–d. There are also two peaks around 35and 551C in the DSC curve of the paraf?n/xGnPcomposite PCMs, and the thermal characteristics of the composite PCM are very close to

Fig. 4. Scanning electron microphotographs of 5and 2wt%of paraf?n/xGnPcomposite PCMs by coating condition for SEM (highmagni?cation, ?5000):(a)paraf?n/xGNP5%— gold coating, (b)paraf?n/xGNP5%— non-coating, (c)paraf?n/xGNP2%— gold coating, and (d)paraf?n/xGNP2%— non-coating.

101

102103104105106107108

10910

R e s i s t i v i t y (o h m ? c m )

xGnP loading content (wt%)

34567

Fig. 5. Resistivity of paraf?n/xGnPcomposite PCMs by melting times.

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those of the pure paraf?n. This is because there is no chemical reaction between the paraf?n and the EG in the preparation of the composite PCM [8]. The latent heat of the paraf?n is obtained as the total area under the peaks of the solid–solid and solid–liquid transitions of the paraf?n in the composite by numerical integration. From the Fig. 8, it can be seen that the latent heat of the paraf?n/xGnPcomposite PCMs approach those of the pure paraf?n. The latent heat of paraf?n/xGnPcomposite PCMs did not decrease as loading xGnP contents to paraf?n. There is no signi?cant difference of the latent heat between paraf?n and paraf?n/xGnPcomposite PCMs. Previous results showed a decrease of latent heat as graphite loading contents increased [8,30]. In these results, due to graphite and EG, although thermal conductivity of paraf?n/graphitecomposite PCM was increased, the latent heat of PCM was decreased as graphite loading contents. They explained the reason that the three-dimensional net structure con?nes the molecular heat movement of the paraf?n in the PCM composites. However, in the case of xGnP, there was no problem because of good dispersion of xGnP in paraf?n with high surface area.

0.2

0.30.40.50.60.70.80.9T h e r m a l C o n d u c t i v i t y (W /m K )

xGnP loading content (wt%)

Fig. 6. Thermal conductivity of paraf?n/xGnPcomposite PCMs by melting times.

H e a t f l o w (w /g )

Temperature (°C)

paraffin only

51.1°C

55.2°C

32.9°C

35.3°C

26.2J/g

128.8J/g

29.2J/g

128.1J/g

405060

H e a t f l o w (w /g )

Temperature (°C)

paraffin/xGnP1%

51.0°C

55.1°C °C

35.1°C

26.8J/g

134.0J/g

27.8J/g

131.0J/g

4050

H e a t f l o w (w /g )

paraffin/xGnP3%

51.4°C

55.1°C

33.0°C

34.9°C

27.8J/g

132.4J/g

28.1J/g

127.6J/g

H e a t f l o w (w /g )

paraffin/xGnP5%

50.8°C

54.9°C

°C

35.4°C

27.4J/g

131.5J/g

27.4J/g

130.0J/g

4050

Fig. 7. The heating and freezing curves by DSC of paraf?n/xGnPcomposite PCMs.

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The mass loss of the paraf?n/xGnPcomposite PCMs is shown in Fig. 9. As the xGnP loading increased, the thermal stability of the composites did not signi?cantly differ until 3wt%,while at 5wt%it slightly increased. After thermal decomposition, after 3001C, we can check the xGnP contents of each composite with weight percent of remaining materials. xGnP of 1, 3and 5wt%was exactly loaded in the paraf?n/xGnPcomposite PCMs. 4. Conclusion

Paraf?n/xGnPcomposite PCMs were prepared for high electric conductivity, thermal conductivity and latent heat storage. The paraf?n/xGnPcomposite PCMs can be easily prepared by stirring of xGnP in liquid paraf?n. From the cryogenically fractured

surface of the paraf?n/xGnPcomposite PCMs, xGnP was well-dispersed into paraf?n wax, and it led to high electric conductivity and thermal conductivity. As increasing xGnP loading contents, electric conductivity and thermal conductivity were increased. The results clearly indicated an almost linear relationship between thermal conductivity and mass fraction of xGnP in composite PCM. The percolation threshold of Paraf?n/xGnPcomposite PCMs on resistivity was between 1and 2wt%.This low percolation threshold was caused by well dispersion of high aspect ratio of xGnP. On the other hand, latent heat was not decreased as xGnP loading contents. xGnP of uniform high surface area showed improved thermal storage performance. As a result, xGnP can be considered as an effective heat diffusion promoter to improve thermal conductivity of PCMs without reducing its latent heat storage capacity.

Acknowledgement

This work (SuminKim) was supported by the Soongsil University Research Fund. References

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3040110120130140L a t e n t h e a t c a p a c i t y (J /g )

xGnP loading content (wt%)

12345

Fig. 8. Latent heat storage performance of paraf?n/xGnPcomposite PCMs at phase transition and phase change by DSC.

020406080100W e i g h t c h a n g e (%)

Temperature (°C)

paraffin only

paraffin/1% xGnP PCMsparaffin/3% xGnP PCMsparaffin/5% xGnP PCMs

100

200300400500600

Fig. 9. Thermal decomposition behavior of paraf?n/xGnPcomposite PCMs by TGA.

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范文二：相变储热材料的有关学习

无机非金属材料工程专业学生论文 2014年6月相变储热材料的有关学习

李珍勤

（无机非金属材料工程1302班)

摘要: 介绍了相变储热材料的概念与特点；以及相变材料的分类和各类相变材料的性能，储能机理及其优缺点；

介绍了相变材料在太阳能利用、建筑节能等领域的应用;展望了未来相变材料的发展方向和应用前景。

关键词:储热材料；相变储热；制备；

1 相变材料的概念和机理，以及特点

相变材料（PCM）是一类在其本身发生相变的过程中，可以吸收环境的热（冷）能，并在需要时向环境发出热（冷）能，从而达到控制周边环境温度的目的的材料。其相变机理是：相变材料从液态向固态转变时，要经历物理状态的变化。在这两种相变过程中，材料要从环境中吸热，反之，向环境放热。在物理状态发生变化时可储存或释放的能量称为相变热，发生相变的温度范围很窄。物理状态发生变化时，材料自身的温度在相变完成前几乎维持不变。大量相变热转移到环境中时，产生了一个宽的温度平台。该温度平台的出现，体现了恒温时间的延长，并可与显热和绝缘材料区分开绝缘材料只提供热温度变化梯度。

相变材料在热循环时，储存或释放显热。相变材料在熔化或凝固过程中虽然温度不变但吸收或释放的潜热却相当大。以冰——水相变的过程为例。对相变材料在相变时所吸收的潜热以及普通加热条件下所吸收的热量作一比较：当冰熔解时，吸收 3 3 5 J ／ g的潜热，当水进一步加热，温度每升高 1 ℃它只吸收大约4坛的能量。冈此，由冰到水的相变过程中所吸收的潜热几乎比相变温度范围外加热过程的热吸收高 8 0多倍。除冰水之外，已知的天然和合成的相变材料超过 5 0 0种，且这些材料的相变温度和储热能力各不相同。把相变材料与普通建筑材料相结合，还可以形成一种新型的复合储能建筑材料这种建材兼备普通建材和相变材料两者的优点。

然而绝大多数无机物相变材料具有腐蚀性，相变过程中存在过冷和相分离的缺点。为防止无机物相变材料的腐蚀性。储热系统必须采用不锈钢等特殊材料制造，从而增加了制造成本：为抑制无机物相变材料在相变过程中的过冷和相分离，需通过大量试验研究，寻求好的成核剂和稳定剂。而有机物相变材料则热导率较低。相变过程中的传热性能差，在实际应用中通常采用添加高热导率材料如：铜粉，铝粉或石墨等作为填充物以提高热导率。或采用翅片管换热器，依靠换热面积的增加来提高传热性能，但这些强化传热的方法均未能解决有机相变材料热导率低的本质问题。

相变过程一般是一等温或近似等温过程 ,相变过程中伴有能量的吸收或释放 ,这部分能量称为相变潜热 ,利用相变过程的这一特点开发了许多相变储能材料。与显热储能材料相比 ,潜热储能材料不仅能量密度较高 ,而且所用装置简单、体积小、设计灵活、使用方便且易于管理。另外 ,它还有一个很大的优点 ,即这类材料在相变储能过程中 ,材料近似恒温 ,可以以此来控制体系的温度。同时，相变材料还具有以下几个特点：凝固熔化温度窄，相变潜热高，导热率高，比热大，凝固时无过冷或过冷度极小，化学性能稳定，室温下蒸汽压低。此外，相变材料还需与建筑材料相容，可被吸收。

利用储能材料储能是提高能源利用效率和保护环境的重要手段之一 ,可用于解决热能供给与需求失配的矛盾 ,在能源、航天、军事、农业、建筑、化工、冶金等领域展示出十分广泛和重要的应用前景 ,储热材料研究目前已成为世界范围内的研究热点。

2 相变储热材料的种类及其优缺点

相变储能材料的相变形式一般可分为四类:固 ——固相变、固——液相变、液 ——气相变和固——气相变。相变储能材料按相变温度的范围分为高温(大于 250℃)，中温(100～250℃)和低温(小于100℃)储能材料;按材料的组成成分又可分为无机类、有机类(包括高分子类)及无机——有机复合相变储能材料。相变材料是由多组份构成的 ,包括主储热剂、相变点调整剂、防过冷剂、防相分离剂、相变促进剂等组份。

2.1 无机类

无机类固——液相变材料有结晶水合盐类、熔融盐类、金属 (包括合金) 和其它无机类相变材料 (如水) 。其中最典型的是结晶水合盐类 , 结晶水合盐提供了熔点从几摄氏度到一百多摄氏度的可供选择的相变材料。它们有比较大的熔解热和固定的熔点(实际是脱出结晶水的温度 ,脱出的结晶水使盐溶解而吸热 ,降温时其发生逆过程 ,吸收结晶水而放热)。相变机理如下:

AB· mH2O加热( T > Tm)冷却( T

其中 , m 和p为结晶水的个数, Tm为熔点 ,Q为溶解热。

结晶水合盐通常是中、低温相变储能材料中的重要一类 ,有如下的优点:使用范围广，价格较便宜、导热系数大(与有机类相变材料相比)、熔解热较大，密度大、一般呈中性。但是这类材料通常存在着两个问题 ,一是过冷现象 ,解决的方法有: a1 加成核剂 ,如加入微粒结构与盐类结晶物相类似的物质; b1 冷指法 ,保持一部分冷区 ,使未熔化的一部分晶体作为成核剂。另一个问题是相分离 ,解决的办法有: a1 加增稠剂;b1 加晶体结构改变剂;c1 盛装相变材料的容器采用薄层结构; d1 摇晃或搅动。徐伟亮研究了水合乙酸钠相变储热的性能 ,发现硼砂对过冷现象

有明显的抑制 ,羟甲基纤维素、聚丙烯酰胺等对相分离现象有明显的抑制。

固——固相变储热材料的无机盐类主要是利用固体状态下不同种晶型的变化而进行吸热和放热的 ,主要有层状钙铁矿、 Li2SO4、 KHF2 等代表性物质。通常它们的相变温度较高 ,适合于高温范围内的储能和控温之用 ,而中、低温的材料较少 ,因此不能完全满足人们的需要、目前在实际中应用也不是很多。

2.2 有机类

典型的有机类相变材料有:石蜡、酯酸类、高分子化合物等。有机类相变材料具有固体成型好、不易发生相分离及过冷现象、腐蚀性较小、性能稳定等特征，但与无机类相比，其导热性较差，熔点较低，溶解热较小且易挥发、易燃烧。

石蜡主要由直链烷烃混合而成 ,可用通式 CnH2n + 2表示。短链烷烃熔点较低 ,链增长时 ,熔点开始增长较快 ,而后逐渐减慢。一般说来 ,同系物的相变温度和相变焓会随碳链的增长而增大 ,这样可以得到具有一系列相变温度的储能材料 ,但随着碳链的增长 ,相变温度的增加值会逐渐减小 ,其熔点最终将趋于一定值。石蜡是混合物 ,因此不象低分子量的物质有一个熔融尖峰。石蜡作为相变储能材料的优点是:无过冷及析出现象 ,性能稳定 ,无毒 ,无腐蚀 ,价格便宜。缺点是:导热系数小 ,密度小 ,单位体积储热能力差。

高分子化合物类的相变材料 ,由于是具有一定分子量分布的混合物 ,并且分子链较长 ,结晶并不完全 ,因此它的相变过程也有一个熔融温度范围 ,没有熔融尖峰。酯酸类也是一种有机储热相变材料 ,其分子通式为 CnH2nO2 ,其性能特点与石蜡相似。为了得到相变温度适当、性能优越的相变材料 ,常常需要将几种有机相变材料复合以形成二元或多元相变材料,以弥补二者的不足 ,得到性能更好的相变材料 ,以使之得到更好的应用。

3 应用

3.1 建筑方面

由于相变物质在其物相变化过程(熔化或凝固)中，可以从环境吸收或放出大量热量，同时保持温度不变，可以多次重复使用等优点，将其应用于建筑节能领域不但可以提高墙体的保温能力，节省采暖能耗，而且可以减小墙体自重，使墙体变薄，增加房屋的有效使用面积，因而具有广阔的应用前景。

相变储热材料具有调节室内温度，降低混凝土水化反应温度的作用：（1）复合PCEM具有普通建材无法比拟的热容，对于房间内气温的稳定及空调系统工况的平稳是非常有利的。当室外温度有较大波动(波峰与波谷的距离较大)时，墙体温度波动不大，这样室内温度波动也不大，同时，相变房间的热流密度也明显比普通房间低，因此相变储能材料起到了调节室内温度的效果。（2）由于混凝土水化反

应时释放出大量的反应热，导致混凝土内温度升高，使混凝土开裂、强度降低，尤其是在大体积混凝土更为明显，甚至可能造成结构破坏等严重的工程事故。而加入适当的相变材料，可以吸收水化反应释放的热量，发生相变，使混凝土内部温度稳定在某一范围内，在反应结束时热量才逐渐传递出来，不会造成混凝土内部温度过高，达到降低混凝土水化反应温度的目的。

3.2 其他应用

相变储能材料的应用涉及面很广。选用MgNO3· 6H2O作为主储热材料 ,MgCl·6H2O作为添加剂调节相变温度 ,可以用于处理发热发电系统产生的城市废热(温度在 60～100℃)。在冷藏系统中 ,用主要为 Na2SO4·10H2O、NH4CL和 KCl 的混合物作为相变储能材料代替传统的换热体系,能够提高冷藏系统的性能 ,有利于缓解高峰制冷负荷、克服开门期间的能量损失和满足较长停电期间的制冷需要。另外相变储能材料在纺织服装、温室种植等领域都有应用。

4 展望前景

今后相变储热材料主要应用于几个方面：

(a ) 进一步筛选符合环保的低价的有机相变储能材料，如可再生的脂肪酸及其衍生物。对这类相变材料的深入研究，可以进一步提升相变储能建筑材料的生态意义： (b )开发复合相变储热材料是克服单一无机或有机相变材料不足，提高其应用性能的有效途径； (c ) 针对相变材料的应用场合，开发出多种复合手段和复合技术，研制出多品种的系列复合相变材料是复合相变材料的发展方向之一： (d ) 开发多元相变组合材料。在同一蓄热系统中采用相变温度不同的相变材料合理组合，可以显著提高系统效率，并能维持相变过程中相变速率的均匀性。这对于蓄热和放热有严格要求的蓄能系统具有重要意义：(e ) 进一步关注高温储热和空调储冷。美国NA S A L e w i s 研究中心利用高温相变材料成功的实现了世界上第一套空间太阳能热动力发电系统 2 k W电力输出，标志这一重要的空间电力技术进入了新的阶段。太阳能热动力发电技术是一项新技术，是最有前途的能源解决方案之一，必将极大地推动高温相变储热技术的发展。另外，低温储热技术是当前空调行业研究开发的热点，并将成为重要的节能手段；(f )纳米复合材料领域的不断发展，为制备高性能复合相变储热材料提供了很好的机遇。利用纳米材料的特点制备新型高性能纳米复合相变储热材料是制备高性能复合相变材料的新途径。

范文三：相变储热材料的制备与应用

相变储热材料的制备与应用

摘要：热能储存可以通过蓄热材料的冷却、加热、熔化、凝固。气化、化学反应等方式实现。它是一种平衡热能供需和使用的手段。热能储存按储热方式可分为三类，即显热储能、潜热储能和化学反应储热。

关键词：相变；储热；复合材料

一、相变材料在国内外的发展状况国外对相变储能材料的研究工作始于20世纪60年代。最早是以节能为目的，从太阳能和风能的利用及废热回收，经过不断的发展，逐渐扩展到化工、航天、电子等领域。近年来最主要的研究和应用集中在建筑物的集中空调、采暖及被动式太阳房等领域。国外研究机构和科研人员对蓄热材料的理论研究工作，尤其是对蓄热材料的组成、蓄热容量随热循环变化情况、相变寿命、储存设备等进行了详细的研究，在实际应用上也取得了很大进展。相对于已经进入实用阶段的发达国家，我国在20世纪70年代末80年代初才开始对蓄热材料进行研究，所以国内相变储能材料的理论和应用研究还比较薄弱。上世纪90年代中期以来，国内研究重点开始转向有机相变材料和复合定形相变材料的研究开发。

二、相变储热材料的分类

（1）从材料的化学组成来看，主要分为无机类相变材料和有机类相变材料，而在课堂上我们主要讲解的是有机类相变材料。无机相变材料包括结晶水合盐、熔融盐和金属合金等无机物。与无机类相变储能材料相比，有机类相变储能材料具有无过冷及析出，性能稳定，无毒，腐蚀等优点。其中石蜡类相变潜热量大、相变温度范围广、价格低，所以在相变储能材料的研究使用中受到广泛的重视。但石蜡类相变储能材料热导率较低，也限制了其应用范围。为有效克服石蜡类有机化合物相变储能材料的缺点，同时改善相变材料的应用效果及拓展其应用范围，复合相变储能材料应运而生。复合相变材料由较稳定的有机化合物和具有较高导热系数的无机物颗粒制备而得，因而复合相变材料具有稳定的化学性质，无毒无腐蚀性或毒性和腐蚀性小。同时它的导热能力较有机物有较大的改善。

(2)根据使用的温度不同又可以分为高、中、低温相变储热材料。一般使用温度高于100℃的相变储热材料称为高温相变储热材料。以熔融盐、氧化物和金属及其合金为主。使用温度低于100℃为中、低温相变储热材料，这类相变材料以水合盐、石蜡类、脂酸类为主，在低温类中也有利用液-气相变型的，如液氮、氦。

（3）从蓄热过程中材料相态的变化方式来看，可分为固液、固气、液气、固固四种相变。由于固气和液气两种方式相变是有大量气体产生，使材料的体积变的很大，所以实际中很少采用这两种方式。

三、相变材料的分类选择因素

（1）合适相变温度；

（2）较大的相变潜热；

（3）合适的导热性能；

（4）性能稳定，可反复使用而不发生熔析和副反应；

（5）相变的可逆性，过冷度要尽量小；

（6）符合绿色化学要求:无毒、无腐蚀、无污染；

（7）使用安全、不易燃。易爆或氧化；

（8）蒸汽压要低使之不易挥发损失；

（9）材料密度较大，从而确保单位体积储热密度较大；

（10）体积膨胀较小；

（11）成本低廉，原料易得。

实用型的相变储热材料需要满足以上各项基本原则，但选用时也可以结合实际的应用情况，在满足主要条件之下，采用一定的技术和手段来克服其缺点和不足。

四、相变材料的应用领域

（1）在太阳能方面的应用

太阳能清洁、无污染，而且取用方便。利用太阳能是解决能源危机的重要途径之一。但是到达地球表面的太阳辐射能量密度偏低，且受到地理、季节、昼夜及天气变化等因素的制约，表现出稀薄性、间断性和不稳定性等特点。为了保证供热或供电装置的稳定不问断的运行，需要利用相变储能装置，在能量富裕时储能，在能量不足时释能。

（2）在工业余热方面的应用

在冶金、玻璃、水泥、陶瓷等部门都有大量的各式高温窑炉，它们的能耗非常之大，但热效率通常低于30％，节能的重点是回收烟气余热。传统的做法是利用耐火材料的热熔变化来储热，这种储热设备的体积大、储热效果不明显。如果改用相变储热系统，则储热设备体积可减小30％～50％，同时可节能15％～45％，还可以起到稳定运行的作用。

（3）在建筑节能方面的应用

有关资料显示，社会一次能源总消耗量的1／3用于建筑领域。提高建筑领域能源使用效率，降低建筑能耗，对于整个社会节约能源和保护环境都具有显著的经济效益和社会影响。利用相变储能建筑材料可有效利用太阳能来蓄热或电力负荷低谷时期的电力来蓄热或蓄冷，使建筑物室内和室外之间的热流波动幅度减弱、作用时间被延迟，从而降低室内的温度波动，提高舒适度，以及节约能耗

（4）在其他方面的应用

相变储热材料有着优异的储能性能，如果这种性能能利用到电池当中去，就会引起电池的一场变革。现在用的电池不管是一次性电池还是可充式电池，都是利用化学反应来实行放电和充电的。化学电池中含有大量的重金属，电池废弃后如处理不当，会对环境人的身体健康造成很大的危害。相变材料大多为无机非金属材料制成，这类材料无害无毒非常适合用来制造绿色电池。

五、相变材料使用目前存在的问题

（1）耐久性

相变材料在循环过程中热物理性质的退化问题；相变材料易从基体的泄漏问题；相变材料对基体材料的作用问题。

（2）经济性

如果要最大化解决上述问题，将导致单位热能储存费用的上升，必将失去与其他储热法或普通建材竞争的优势。相变储能建筑材料经过20多年的发展，其智能化功能性的特点勿容置疑。随着人们对建筑节能的日益重视，环境保护意识的逐步增强，相变储能建筑材料必将在今后的建材领域大有用武之地，也会逐渐被人们所认知，具有非常广阔的应用前景。

六、结论

相变储热材料有着优异的储热性能，这一性能在能源的利用上有着广阔的前景。相变储热材料大多数为无机非金属材料，原料易得，易于制备，无污染，是一种良好的绿色资源。随着能源的短缺，各国都在努力提高资源利用率和寻找可再生的绿色能源。相变材料开始受到人们的重视，相变材料在太阳能，工业余热利用，绿色建筑，航天航空领域有着广泛的应用。随着对相变材料研究的深入，相变材料会应用在更多的领域。

范文四：相变潜热式储热材料的研发

2011-08-09 05:54 来源：钢联资讯作者：一员试用手机平台

随着太阳能热发电技术的研究和应用在世界范围内迅速发展，储热材料的研究日益显示出重要的现实意义。太阳能热发电中所需要的储热材料必须具有高的储热密度，且能构成紧凑的储热装置结构。然而，相变储能技术之所以难于实现长期和稳定的工程应用，最重要的原因是采用的相变材料性能不高。

潜热储热材料主要有无机盐和金属及合金。目前应用于太阳能热发电的中高温储热材料主要是熔融盐，如NaNO3等。美国的太阳2号塔式电站就是采用硝酸盐作为储热材料。但熔融盐类储热材料存在腐蚀性、毒性和不稳定性等问题，容易造成热交换管道及附属设施的腐蚀。

据研究，富含Al、Si元素的二元或多元合金具有较高的储热密度，相变潜热在400kJ/kg左右，同时还具有较高的导热系数且价格适中，是目前作为研发方向的比较理想的合金相变储热材料。与其它相变储能材料相比，硅含量为7~24%的铝硅合金作为相变储能材料具有以下优点：（1）相变温度和相变潜热较高。如含硅12.5%的铝硅共晶合金的相变温度为580°C，相变潜热为490~510kJ/kg.。（2）成分和结构的变化对其相变温度和相变潜热影响很小，一般相变温度的变化在12℃之内。（3）在反复熔化-凝固的热循环过程中，氧化的影响很小。（4）在反复熔化-凝固热循环后，相变潜热的降幅较小，而相变温度和过冷度基本保持不变。如A1-13%Si合金在经过反复720次熔融-凝固热循环后，其相变潜热降幅只有10.5%，而相变温度则基本保持稳定。（5）铝硅原料资源丰富，性价比较高。因此，铝硅合金是很有希望的相变储能材料。

相变储能材料的性能指标主要反映在其单位质量的储热量上。据测试，Al-7%Si合金单位质量的储热量可达1066kJ/kg，相当可观；其相变温度为570~619℃。从实用角度看，单位体积的储热量更为重要。在这方面，含Cu、Zn较多的系列合金储热材料优于其它成分的铝合金储热材料。虽然Cu、Zn合金元素的添加实际上使储热材料单位质量的相变潜热下降，但由于提高了储热材料的密度，所以单位体积相变潜热可以达到较高水平。据测试，Al-24.5%Cu-12%Mg-18%Zn合金单位体积的储热量可达3610J/cm3。另外，当合金中产生熔点较高的中间相时，相变温度范围会扩大。Mg、Zn的添加可以有效调节相变温度，如Al-24.5%Cu-12%Mg-18%Zn合金的相变温度范围为460~624℃，这是合金中生成MgZn2的结果。

铝基合金储热在高温时的储热性能优于无机盐，且储能容量大，热导率和稳定性良好，不足的是合金液态的化学活性较强，易与储热容器材料反应。但总的来说，铝基高温相变储热材料在相变温度、储热密度、使用寿命等方面均适合于大规模太阳能热发电储热系统要求，并具有较好的综合储热性能，在高温储热方面具有广泛的应用前景。(一员)

范文五：无机相变储热材料

无机相变储热材料的探究

赵程程

武汉大学化学与分子科学学院 2010级化类一班

摘要：介绍Na2SO4·10H20用作相变材料的储能特性，综述了针对Na2SO4·10H20过冷和

相分离现象的解决方法以及Na2S04·lOH20某些共晶盐的研究。

关键词：相变材料、十水硫酸钠、共晶盐、过冷相分离

引言：Na2S04·10H20是一种典型的无机水合盐相变储能材料。它属于低温储热材料，有

较高的潜热(254kJ／kg) 和良好的导热性能、化学稳定性好、无毒、价格低廉，是许多化工产品的副产品，来源广，因合适的相变温度，能用于贮存太阳能、各种工业和生活废热，与其它无机盐(如NaCI) 形成的低共熔盐的相变温度可控制在20～30"C 范围内。因此Na2S04·10H20以其优越的性能，成为很具吸引力的储热材料。

实验原理：

1.Na2S04·10H20的相变储热循环过程为：Na2S04·10H20(S)+饱和溶液=Na2SO4·10H2O （l ）

2. 过冷：即液相的水溶液温度降低到其凝固点以下仍不发生凝固。这样就使释热温度发生变

动。在其储热后由结晶态变为液态时，因过冷不结晶就不能释放出所储存的潜热，而且由于过冷，液体随温度降低粘度不断增加，阻碍了分子进行定向排列运动，从而使其在过冷程度很大时形成非晶态物质，相应减小相变潜热。

3. 相分离：即指结晶水合盐在使用过程中的析出现象。当(AB·mH20) 型无机盐水合物受热时，

通常会转变成含有较少摩尔水的另一类型AB ·pH20的无机水合盐，而AB ·pH20

会部分或全部溶解于剩余的水中。加热过程中，一些盐水混合物逐渐地变成无水盐，并可全部或部分溶解于水(结晶水) 。若盐的溶解度很高，则可以全部溶解，但如果盐的溶解度不高，即使加热到熔点以上，有些盐仍处在非溶解状态，此时残留的固态盐因密度大沉到容器底部而出现固液相分离。

实验过程：

1. 解决Na2SO4·10H20过冷现象：添加成核剂法和冷指法。

●成核剂可作为结晶生成中心的微粒，使在凝固点时顺利结晶，减少或避免过冷的发生。可作Na2S04·10H20成核剂的物质有硼砂等。

●冷指法即相变过程中保留部分固态Na2S04·10H20，以这部分未融化的Na2s04·10H20作

为成核剂。

为了防止在熔化时固液相的分层需要加入一定量的增稠剂或悬浮剂。

→增稠剂的作用：提高溶液的粘度从而阻止水合盐聚集，但并不妨碍相变过程；常用的增稠剂是活性白土、PCA(聚羧酸) 、YDS 一1、cMc(羧甲基纤维素) 等。

→悬浮剂的作用：将析出的无水Na2SO4和成核剂均匀地分散在体系中，使它们与溶液充分接触。常用的悬浮剂有木屑和白碳黑等。

2.Na2S04·10H20的一些低共熔混合物的性能：

低共熔混合物即共晶盐相变材料，是指2种或2种以上物质组成的具有最低熔点的混合物。低共熔混合物具有与纯净物一样的明显的熔点，在可逆的固一液相变中始终保持相同的组分。是相变储能材料中比较理想的材料。

●在Na2S04·10HzO 中加入能与其形成共熔混合物杂质NaCI ，制备了一系列NaCI 含量不同的储热材料样品。

→随NaCI 质量百分比的不断增大，材料的相转变点不断降低，储热能力也相应降低，但是在NaCI 含量为13％左右时，出现例外，其储热量骤然增大。

→在Na2S04·10H20、NaCI 、硼砂、CMC 、木屑、HMP 盐、水等，形成低共熔混合体系，最佳组成为4％硼砂+7％木屑+2％CMC+0．2％HMP 以及一定量的NaCl 。

结论：

(1)Na2SO。·10H20无机共晶盐的研究主要有Na2S04·10H20-Na ：HP04·12H20、

Na2S04·10H20-NFLCl 、Na2S04· 10H20-NaCl以及Na2SO4·10H20—NaN03等体系，取得了较好的研究效果。主要表现在过冷现象基本控制，熔化潜热较大。

(2)整体研究水平大都还停留在试验阶段，商业化应用不多。

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