Wood As An Orthotropic Dielectric Material


  • Richard T. Lin


Tsuga heterophylla, grain angle, moisture content, AC resistivity, moisture meter


When wood is treated as an orthotropic dielectric material, the relation between electrical displacement vector {D} and electrical field intensity vector {E} can be expressed as {D} = ϵ0 [k] {E}, where [k] is diclectric constant matrix. The transformation of the permittivity matrix then is [k] = [A] [k] [A]', where [A] and [A]' are the rotational matrix and its transpose.

The validity of the transformation equation was tested on western hemlock (Tsuga heterophylla [Raf.] Sarg.) specimens of various grain angle in the longitudinal-radial (LR), longitudinal-tangential (LT), and radial-tangential (RT) planes at 1 kHz and room temperature from green to oven-dry. The transformation equation applied to wood below 15% moisture content with a negligible error. The maximum dielectric constant of wood appears at the grain angle of 30 degrees in the LR plane and 15 degrees in the LT plane.

Discontinuity in the plot of the logarithm of dielectric properties versus moisture content was observed at 6 to 10% and 30 to 40% moisture content. Density of wood has little effect on dielectric properties of wood.


Brown, J. H., R. W. Davidson, and C. Skaar. 1963. Mechanism of electrical conduction in wood. For. Prod. J. 13(10):455-459.nHojendahl, K. 1946. Measurements of dielectric constant and dielectric loss of different wood species. K. Vet. Hojsk. Aarsskr. (Roy. Vet. Agr. Coll. Yearbook), Copenhagen.nJames, W. L., and D. W. Hamill. 1965. Dielectric properties of Douglas-fir measured at microwave frequencies. For. Prod. J. 15(2): 51-56.nKollmann, F. F. P., and W. A. Côté, Jr. 1968. Principles of wood science and technology. Springer-Verlag., New York.nKröner, K., and L. Jungs. 1953. Über das Ver-halten des dielektrischen Verlustfaktors von Naturholz im grossen Frequenzhereich. Holzforschung 7(1): 12-18.nMcLauchlan, T. A., J. A. Norton, and D. J. Kusec. 1973. Slope-of-grain indicator. For. Prod. J. 23(5):50-55.nNye, J. F. 1957. Physical properties of crystals, pp. 68-81. Oxford Press.nRafalski, J. 1966. [On the dielectric properties of differently compressed solid red beech wood.] Holztechnologie 7(2): 118-122.nSchniewind, A. P., and J. D. Barrett. 1972. Wood as a linear orthotropic viscoelastic material. Wood Sci. Technol. 6:43-57.nSkaar, C. 1948. The dielectric properties of wood at several radio frequencies. New York State College of For. Tech. Publ. No. 69.nSkaar, C. 1972. Moisture in wood. Syracuse University Press, Syracuse, New York.nSpalt, H. A. 1958. Fundamentals of water vapor sorption by wood. For. Prod. J. 8(10):288-295.nStamm, A. J. 1964. Wood and cellulose science. The Ronald Press Co., New York.nStone, J. E., and A. M. Scallan. 1967. The effect of component removal upon the porous structure of the cell wall of wood. II. Swelling in water and the fiber saturation point. Tappi 50(10):496-501.nTang, R. C. 1973. The microfibrillar orientation in cell-wall layers of Virginia pine tracheids. Wood Sci. 5(3): 181-186.nTinga, W. R. 1969. Multiphase dielectric theory—applied to cellulose mixtures. Ph.D. Thesis, University of Alberta, Canada.nUyemura, T. 1960. Dielectric properties of wood as the indicator of the moisture. Bull. Gov. For. Exp. Sta. No. 119, 95-172, Tokyo, Japan.nVan Beek, L. K. H. 1967. Dielectric behavior of heterogeneous systems. Prog. Dielectric 7: 69-114. Heywood and Co., London.n






Research Contributions