# Preparation and Properties of Epoxy Composites with Multi-Scale BN Sheets
## 2 Materials and Methods
### Performance Characterization  
1. Thermal conductivity test  
The samples adopt the NETSCHL FA467 laser flash diffusion thermal analyzer to test the diffusion coefficient (\(\alpha\)). The diameter of sample is 1 cm, and the thickness of them is about 2 \(\pm\) 0.1 mm. The sides of samples spraying a thin layer of graphite powder. Then calculate the thermal conductivity according to Formula (1) [27; 28], where \(c\) is the heat capacity, \(\rho\) is the density, and T is the temperature. This experiment uses a fixed temperature of 30 \({}^{\circ}\)C. The average value of the three sample was made.  
\[\lambda(\text{T})=\alpha(\text{T})^{*}\rho(\text{T})^{*}c(\text{T}) \tag{1}\]  
2. Breakdown performance test  
Place a disc with a thickness of 1 \(\pm\) 0.1 mm and a diameter of 3 cm on a breakdown tester (the test instrument is Huayang Instrument and Equipment Factory, Jilin Province, with a boosted rate of 1200 V/s).  
The dielectric breakdown strength of the EP nanocomposites is analyzed within the framework of Weibull statistics. Weibull statistical distribution is the most important method for processing data breakdown strength, which reflects the probability of the material at certain field strength (\(E\)) or the probability of failure or breakdown at a certain time (\(t\)). Weibull cumulative distribution breakdown field strength can be described as follows.  
\[p=1-\exp[-(\frac{E}{\alpha})^{\beta}] \tag{2}\]  
where \(p\) is the cumulative probability of the dielectric breakdown \(E\) is the breakdown strength for testing; \(\beta\) is the shape parameter that reflects the scatter of the breakdown strength \(E\); and \(\alpha\) is the characteristic breakdown strength during the cumulative probability of the dielectric breakdown is 63.2%. After twice taking the logarithm, Equation (2) can be deformed as follows.  
\[\log[-\ln(1-p)]=\beta\log(E)-\beta\log(\alpha) \tag{3}\]This point log [\(-\)ln (1 \(-\)\(p\))] and log (\(E\)) form a linear relationship in the Cartesian coordinate system. According to IEEE Standard 930-2004, when the number of samples is less than 25, \(p\) should be computed by the formula as follows.  
\[p_{i}=\frac{i-0.44}{n+0.25}\times 100\% \tag{4}\]  
where \(n\) is the breakdown times or voltages in order from smallest to largest and assign them a rank from \(i\) = 1 to \(i\) = \(n\), and \(n\) is 12.  
SEM characterization  
The scanning electron microscope (S-4700) of Hitachi Company was used to study the microscopic morphology of the samples. Firstly, the samples were sprayed with gold, and then the surface of the particles and the fracture morphology of the samples were observed. In the preparation of the material cross-section, the sample needs to be cooled in liquid nitrogen for 2 h, and then the brittle fracture is made using a clamp.  
FT-IR test  
The structures of modified BN sheets were analyzed by the Fourier transform infrared spectral (FT-IR, Nexus 670).  
Mechanical properties  
Static uniaxial in-plane tensile tests were conducted through a dynamic mechanical analyzer (DMA Q800, TA Instruments). The maximum tensile force is 18 N, which is sufficient to do the in-plane tensile tests of samples with rectangle stripe shape (length: 20 mm, width: 10 mm, thickness: 2 mm). The tensile strength was extracted from the curve before failure.  
DC conductivity measurements were conducted in the room temperature (25 \({}^{\circ}\)C). DC conduction path testing system including the high voltage DC power with continuously adjustable output voltage within 0-10 kV, pA ammeter (measuring range of 10\({}^{-2}\)-10\({}^{-14}\) A), oven temperature (shielding box, the maximum operating temperature, 25 \({}^{\circ}\)C) and multi-branching switch was used to investigate electrical properties of the insulating materials. The samples with equal thickness each time were put into the oven. The output voltage was increased progressively. When the current is not change for 20 min, the data was read every 30 s. The mean data of 40 point were seen as the experiment data.