# Terahertz Time Domain Spectroscopy of Transformer Insulation Paper after Thermal Aging Intervals

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## Abstract

**:**

## 1. Introduction

## 2. Materials and Methods

#### 2.1. Sample Preparation

#### 2.2. THz spectral Acquisition

#### 2.3. THz Data Processing

## 3. Analysis and Discussion

#### 3.1. THz Time-Domain Waveforms and Frequency-Domain Spectra

#### 3.2. Analysis of Absorption and Refraction Characteristics

^{2}= 0.976) was used to analyze the DP vs. refractive index, which was shown in Figure 4b. It found that there is a good linear relationship between the DP and refractive index of paper.

#### 3.3. Molecular Geometric Configuration

_{12}H

_{22}O

_{11}) repeating unit. It is well known that DFT has great advantages in geometric optimization of molecular structure. Therefore, to analyze the paper spectrum, a quantum chemical calculation of cellobiose was performed via Gaussian software (Gaussian Inc., Wallingford, CT, USA). The geometry structure of the isolated molecule of cellobiose was optimized using the hybrid functional model of Becke, three-parameter, Lee-Yang-Parr (B3LYP) with 6-311G+ (d, p) basis set in Gaussian software. The optimized molecular structure was drawn in GaussView (Gaussian Inc., Wallingford, CT, USA). And Figure 5 shows the calculated molecular structure in atomic coordinates. There was no imaginary frequency in the calculation, which means that the obtained molecular conformation was stable. And the atomic coordinate can be used as input for calculating vibration modes.

#### 3.4. Comparison of Experimental and Theoretical Spectra

#### 3.5. Assignment of Absorption Peaks

## 4. Conclusions

## Author Contributions

## Funding

## Conflicts of Interest

## References

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**Figure 2.**The THz spectra of reference and the unaged insulation paper. (

**a**) The time-domain waveform; (

**b**) The frequency-domain spectra.

**Figure 3.**Optical parameters of transformer insulation paper. (

**a**) Absorption coefficients of unaged paper; (

**b**) Refractive indices of unaged paper and paper aged for 3, 20, and 40 days at 403 K.

**Figure 4.**(

**a**) Curves of refractive index and degree of polymerization (DP) of insulation paper; (

**b**) DP vs. refractive index of paper.

**Figure 6.**Comparison of theoretical and experimental spectra of the paper (the experimental spectra vertically displaced by 10 a.u.).

**Figure 7.**The assignment of simulated absorption peaks of insulation paper. (

**a**) peak at 0.18 THz; (

**b**) peak at 0.82 THz; (

**c**) peak at 1.47 THz; (

**d**) peak at 1.53 THz.

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**MDPI and ACS Style**

Wang, L.; Tang, C.; Zhu, S.; Zhou, S.
Terahertz Time Domain Spectroscopy of Transformer Insulation Paper after Thermal Aging Intervals. *Materials* **2018**, *11*, 2124.
https://doi.org/10.3390/ma11112124

**AMA Style**

Wang L, Tang C, Zhu S, Zhou S.
Terahertz Time Domain Spectroscopy of Transformer Insulation Paper after Thermal Aging Intervals. *Materials*. 2018; 11(11):2124.
https://doi.org/10.3390/ma11112124

**Chicago/Turabian Style**

Wang, Liang, Chao Tang, Shiping Zhu, and Shengling Zhou.
2018. "Terahertz Time Domain Spectroscopy of Transformer Insulation Paper after Thermal Aging Intervals" *Materials* 11, no. 11: 2124.
https://doi.org/10.3390/ma11112124