# Preparation and Characterization of CPCM for Thermal Energy Storage Unit
## 2 CPCM Preparation and Characterizing Instruments
### Characterization Instruments  
In order to select a suitable CPCM for the TESU used in the ASHP heating system, the following tests are carried out to assess the phase transformation temperature, latent heat of phase transformation, thermal conductivity, thermal stability, chemical groups, microstructure, and density of CPCMs.  
SEM analysis: A Mira4 scanning electron microscope with an accuracy of 5-10 nm is first used. The LA/EG composite phase change material is ground into powder, a small amount is placed on the conductive glue made by surface gold spraying treatment, and the surface topography is observed with the scanning electron microscope. The topography and microstructure of EGs with different meshes and CPCM3 are observed with the transmitted scanning electron microscope with a magnification of 6000 times.  
Hot disk thermal conductivity test: The transient plane heat source method and a 3500S type thermal conductivity tester (Hot Disk Ltd., Uppsala, Sweden) with an accuracy of 3% is used in this paper whose probe, consisting of a temperature sensor and a heating source, is a continuous double-helix structure sheet formed by the conductive metal nickel after an etching treatment. Conductivities of 10 solid CPCMs and LA are obtained in 30 \({}^{\circ}\)C conditions, and those of 11 liquid samples are obtained in 50 \({}^{\circ}\)C conditions.  
DSC test: A TA DSC25 differential scanning calorimeter (TA Inc., New Castle, DE, USA) with an E type thermocouple of 0.1% accuracy is used. Each sample in an aluminum crucible is 5 mg. In turn, 11 samples are heated from 30 \({}^{\circ}\)C to 85 \({}^{\circ}\)C at a rate of 5 \({}^{\circ}\)C/min with nitrogen protection and then cooled from 85 \({}^{\circ}\)C to 30 \({}^{\circ}\)C for a cycle. Based on the test results, the phase transformation temperature and latent heat of 10 CPCMs and LA are obtained.  
TG thermogravimetric analysis: A TA Q600 type thermogravimetric analyzer (TA Corporation, New Castle, DE, USA) with an accuracy of 2% is used to indicate the temperature-mass change relationship of the CPCMs in this paper. Based on the tests and analysis above, LA, CPCM3, and CPCM9 are selected for this test. The sample powder is placed in the furnace cavity first, and then heated from 20 \({}^{\circ}\)C to 500 \({}^{\circ}\)C at a rate of 10 \({}^{\circ}\)C/min with nitrogen protection.  
FT-IR test: A THERMO 50 Fourier transform infrared spectrometer (Thermo ScientificTM, Waltham, MA, USA) with an accuracy of 0.005 cm\({}^{-1}\) is used. Based on the tests and analysis above, LA, CPCM3, and CPCM9 are selected for this test. A total of 1 mg powder of each sample is added into a potassium bromide diluent and ground evenly, and then, the spectrum is collected by the Fourier infrared spectrometer, in which an infrared light source is used, whose beam splitter is XT-KBr, wave number is 500-4000 cm\({}^{-1}\), and resolution is 2 cm\({}^{-1}\).  
Density test: Solid density is determined by measuring its mass and volume at a room temperature of 20 \({}^{\circ}\)C. Based on Archimedes' law and the equilibrium state of floating objects on a liquid surface at a constant temperature of 60 \({}^{\circ}\)C in a water bath, the liquid density of the CPCMs is obtained by using a hydrometer of ET-120D (Beijing ETuno Electronic Technology Co., Ltd., Beijing, China).