Inspection of Microfibril Angle of Sugi Wood by THz-TDSH.Wang, S.Tsuchikawa, and T.Inagaki | Graduate School of Bioagricultural Sciences, Nagoya University

wanghan.bio@gmail.com

ACKNOWLEDGEMENT: The authors would like to acknowledge the financial support from JSPS (KAKENHI, No.16H02559).

INTRODUCTION

MATERIALS & EXPERIMENT

● Two sugi wood(Cryptomeria japonica) blocks were cut out along theradial direction from same log. One of them was provided to measureproperties by SilviScan analyze system as reference (X-ray system:FPInnovations, Canada), another was cut in 117 pieces of 32 mm (L:longitudinal) ×29mm (T: tangential) ×3 mm (R: radial) as samples anddivided into 5-1 (57 pieces) and 5-2 (60 pieces) two groups by theposition of pith.

● Group 5-1 were air-dried and group 5-2 were dried at 70℃ then takeTHz measurement through the RT-plane, after measurement, all of thesamples were oven-dried then measured by THz again (samplesrotated for 0-90° by 5° ).

● For predicting of MFA, group 5-1 were air-dried and take THzmeasurement (samples rotated for 0-180° by 5°).

Fig. 1. Workflow of the experiment. L (longitudinal), T (tangential), R (radial) directions showed in rectangular coordinates.

● THz is very well suited to investigate the wood materials because woodis high transparent and exhibits strong birefringence combined withdiattenuation at THz region.

● The previous measurement methods of MFA are time-consuming andlaborious, THz radiation is expected to be able to estimate MFA andother properties non-destructively, safely and quickly.

● Wood as a biological material is wildly used inindustry, but wood has large differences inindividual properties. Investigation of theproperties quickly and non-destructively is veryimportant in production. Particularly, microfibrilangle (MFA) is a structure present in the S2layer of cell wall providing 85% or morethickness of the cell wall, which is largelydetermined the stiffens of wood.

θ: MFA

S2 layer

S1 layer

S1 layer

Primarywall

Sugi wood log cut along the dot lines

Group 5-1 (57 pieces 3mm) Air-dry

Group 5-2 (60 pieces 3mm) 70℃

SilviScanR

LT

THzOven-dry

θ

θ1: 0-90°θ2: 0-180°Rotate by 5°

EXPERIMENTIAL DEVICE

RESULT & DISCUSSION

1. Calculation of refractive index and absorption coefficientRefractive index and theabsorption coefficient of woodsamples were calculated fromthe time difference and theintensity ratio between referencespectrum and sample. Based onthe measured peak information,the refractive index and theabsorption coefficient werecalculated using Equation 1.

Fig. 2. Relationship between measured time of THz wave and electric field

𝒏𝒉 =𝒄 𝒕𝑺 − 𝒕𝒓𝒆𝒇

𝑳+ 𝟏

𝒂𝒉 =−𝟐 𝒍𝒐𝒈 Τ𝝂𝒔 𝒗𝒓𝒆𝒇

𝑳

Equation 1. nh: Refractive index ah: Absorption coefficient (m-1) c: Speed of light (m/s) L: Sample thickness (m) t: Detection time (s) ν: Wavelength intensity

2. Prediction by multiple regression analysis

Ⅰ. Prediction of density and moisture content

that were got form the SilviScan.Multiple linear regression (MLR)was performed with the real andimaginary parts of the complexrefraction index as explanatoryvariables, density and moisturecontent (MC) as objectivevariables, showed in Fig.4.

Ⅱ. Inspection of MFA by THz-TDSTara M. Todoruk (2005)concluded that birefringencehave two different sources:intrinsic birefringence (seen incrystalline materials) and formbirefringence (seen in orderedparticles arrangement). The twosources contribute about half ofbirefringence. In wood materials,MFA decreases with growth asshown in Fig.5 that got form theSilviScan. However, in Fig.5 MFAstarted to increase form 100mmthat can be considered as theFig. 6. Positional deviation of the refractive index

minimum

theoretically be correlated to the MFA. However in this experiment, resultis not ideal as shown in Fig.7(A). Further, multiple regression analysis wasperformed with the refractive index and absorption coefficient asexplanatory variables, MFA as objective variables. There is a correlationbetween the predicted and the measured MFA showed in Fig.7(B). Howeverfor more accurate prediction, further experiment and analysis are needed.

THz emitter

Sample holder

Time delay module

Femtosecond laser

THz detector

Tera Prospector-Kit model, Nippo Precision Co.Frequency Range: 0.1-4.0. THz Resolution: ≤ 6GHz. S/N: ≥2500. Dynamic range: ≥5 digits

Fig. 3. Density increases with the growth.

The density increases with the growth oftrees, density of latewood larger thanearlywood in per growth ring, and thesample density of the bark side is higherthan that of the pith side as shown in Fig.3

influence of reaction wood.When the microfiber isparallel to THz radiation(that is, MFA = 0°, formbirefringence is minimum),the refractive index is theminimum, and thepositional deviation of therefractive index minimum(showed in Fig.6) should

.

Fig. 7. (A) Correlation between positional deviation and MFA. (B) The best prediction of MFA in this experiment.

Fig. 4. Prediction of density and moisture content

Fig. 5. MFA decreases with the growth.

𝒅𝒆𝒏𝒔𝒊𝒕𝒚 = 𝟏. 𝟎𝟎𝟑𝒏𝒉 + 𝟑. 𝟏𝟖𝟓 × 𝟏𝟎−𝟔𝒂𝒉 − 𝟎. 𝟖𝟕𝟕 𝑴𝑪 = 𝟑𝟏. 𝟏𝟐𝟐𝒏𝒉 + 𝟎. 𝟎𝟎𝟔𝒂𝒉 − 𝟑𝟓. 𝟒𝟒𝟐

Equation 2. MLR equation