Abstract—Frequency dependence of the complex dielectric constant at 2 % and 4 % moisture content for 4 wood species in longitudinal and transverse fiber directions were measured using a free space method. Anisotropic properties of wood, notably manifested at 0.1 – 0.5 THz are marked.

I. INTRODUCTION

HE spectra of permittivity of wet wood allow to obtain new knowledge about the electromagnetic characteristics of biological molecules, as well as bound and free water.

Information about these spectra is required for more accurate interpretation of the results of remote sensing of forest cover and non-destructive evaluation in the production of quality wood products1, 2.

In this work we have investigated the interaction of electromagnetic radiation at 0.1 – 0.5 THz with four wood species, such as birch (Betula pendula Roth), spruce (Picea obovata), cedar (Pinus sibirica) and pine (Pinus silvestris L.). The longitudinal and transverse fiber directions of the wood were considered. The moisture content was 2 % and 4 %.

II. RESULTS

The complex permittivity was measured by quasioptical Mach-Zahnder Interferometer with backward-wave oscillator as a radiation source3.

Samples used for measuring are smoothly polished wood plates with different thicknesses ranging from 3 mm to 8 mm. Samples were dried in an oven at a temperature of 85 ° C to constant weight. Moisture of wood was fixed after measuring. The measurements were performed at room temperature, which did not leave the interval 21 ÷ 23° C. Wood samples were prepared in longitudinal and transverse directions relative to the fibers of wood.

The spectra of permittivity for longitudinal slice have not noticeable features. We can see weak decrease of permittivity and rise of imaginary component with frequency increasing. Area of anomalous dispersion is observed with a maximum of the dielectric loss at 250 GHz for a sample with transverse fiber directions of the wood. Comparison of the results shown in Figures 1 suggests that the selective absorption of electromagnetic energy in a cross section appears due to a resonance effect, which can be explain by the equality of the wavelength of electromagnetic radiation and the size of basic wood cells – tracheids – in this direction, since the transverse dimension of such cells is from 0.01 to 0.1 mm. In the longitudinal direction the length of ones is much greater than the wavelength – 0.5 ÷ 10 mm.

The comparative analysis shows that the values of the relative permittivity for longitudinal sections of higher than the transverse by 10 ÷ 15 %. As for dielectric loss assessment is difficult because of its small values.

Fig. 1. Frequency dependence of relative permittivity, ε′, and dielectric loss, ε″ of pine. Label (P) and label (T) means polarization direction parallel and perpendicular to fiber direction of wood, respectively. Moisture content is 2.4 % for (P) and 2.1 % for (T).

The spectra of permittivity and dielectric loss for cross and

longitudinal slices of spruce samples have similar character as in the previous case, what is shown in Figures 2.

Fig. 2. Frequency dependence of relative permittivity, ε′, and dielectric loss, ε″ of spruce. Label (P) and label (T) means polarization direction parallel and perpendicular to fiber direction of wood, respectively. Moisture content is 1.9 % for (P) and 2.0 % for (T).

The difference is that the maximum of the imaginary

component of permittivity for cross section in this case is placed at 170 GHz. Shift of the anomalous dispersion can be caused by the fact that the transverse dimensions of tracheids in the sample spruce about 1.5 times greater than that of the investigated sample of pine, which to some extent corresponds to the difference in the densities of pine ρ = 0.52 g/cm3, spruce ρ = 0.42 g/cm3. Slight change in the frequency dependence of the relative permittivity under the additional humidification

Valentin I. Suslyaev, Tatiana D. Kochetkova, Grigorii E. Dunaevskii, and Kirill V. Dorozhkin National Research Tomsk State University, Tomsk, 634050 Russia

Research of Dielectric Properties of Wood at Frequencies 0.1 ÷ 0.5 THz

T

(up 4.1%) as is shown in Figure 3 indicates that the main role plays cell shape anisotropy in the observed selective absorption, but not the presence of water.

Fig. 3. Frequency dependence of relative permittivity, ε′, and dielectric loss, ε″ of spruce. Moisture content is 2.0 % (solid symbols) and 4.1% (crossed symbols).

Table 1 shows average values of ε´ and ε″ at 100 GHz. Here

the symbol “║” represents polarization direction parallel to fiber direction of wood in transverse sample, and the symbol “┴” – perpendicular in a longitudinal sample.

Table 1. Relative permittivity, ε′, and dielectric loss, ε″, at 100 GHz.

Figure 4 shows the spectra of relative permittivity of all

four wood species. In this case samples were freshly cut from a living tree wood with thicknesses ranging from 3 mm to 4 mm. Sample surface were not good polished due to big moisture content. Plates were air-dried.

It can be seen that the species differ from each other according to their density: the birch – 0.65 g/cm3, cedar – 0.62 g/cm3, pine – 0.52 g/cm3, spruce – 0.45 g/cm3. This result is consistent with the data obtained by Tanaka et all2.

III. SUMMARY

Thus, the main role in the observed selective absorption plays the presence of water and cell shape anisotropy. These measurement results can be used for the non-destructive evaluation of wood and the modeling of permittivity for the remote sensing of the Earth.

This work was made within the framework of the Federal

program of the Russian Federation for the priority direction «Environmental management».

(a)

(b)

Fig. 4. Frequency dependence of relative permittivity, ε′ (a), and dielectric loss, ε″ (b) for different species of wood. Polarization direction was parallel to fiber direction of wood. Moisture content is 7.8 ÷ 7.9 %.

REFERENCES

[1]. R Magagi, M. Bernier, and Chhun-H. Ung “Analysis of RADARSAT SAR Data Over a Sparse Forest Canopy,” IEEE Transactions on Geoscience and Remote Sensing, vol. 40, pp. 1301–1313. [2]. S. Tanaka, Y. Fujii, K. Shiraga, and Y. Ogawa, “Relation between anisotropic relative permittivity and density of wood evaluated using THz time domain transmission spectroscopy,” 38th IRMMW-THz International Conference, TU14-3, Mainz on the Rhine, Germany, Sept. 1-6, 2013. [3]. E.V. Emelyanov, V.I. Suslyaev, G.E. Dunaevskii, V.A. Zhuravlev, V.L. Kuznetsov, S.I. Moseenkov, and I.N. Mazov “Terahertz transmission spectra of composite materials based on MWNT with different time of ultrasonic processing,” 37th IRMMW-THz International Conference, 6380147, Wollongong, Australia, Sept. 23-28, 2012.

Sample ε´ (moisture, %) ε″ (moisture, %) Birch ┴ 1.96 (1.8 %) 2.05 (3.8 %) 0.06 (1.8 %) 0.09 (3.8 %) Birch ║ 1.79 (2.2 %) 1.85 (3.6 %) 0.06 (2.2 %) 0.06 (3.6 %) Spruce ┴ 1.51 (1.9 %) 1.90 (4.2 %) 0.04 (1.9 %) 0.07 (4.2 %) Spruce ║ 1.81 (2.0 %) 1.55 (4.1 %) 0.03 (2.0 %) 0.04 (4.1 %) Cedar ┴ 1.79 (1.5 %) 1.88 (4.1 %) 0.07 (1.5 %) 0.12 (4.1 %) Cedar ║ 1.55 (2.3 %) 1.53 (3.9 %) 0.02 (2.3 %) 0.02 (3.9 %) Pine ┴ 1.69 (2.1 %) 1.76 (4.2 %) 0.07 (2.1 %) 0.09 (4.2 %) Pine ║ 1.49 (2.4 %) 1.53 (4.1 %) 0.06 (2.4 %) 0.11 (4.1 %)