Dielectric Properties of Typical Australian Wood-Based Biomass

Jul 27, 2010 - Dielectric Properties of Typical Australian Wood-Based Biomass ... Received May 19, 2010. Revised Manuscript Received July 7, 2010...
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Energy Fuels 2010, 24, 4534–4548 Published on Web 07/27/2010

: DOI:10.1021/ef100623e

Dielectric Properties of Typical Australian Wood-Based Biomass Materials at Microwave Frequency Shanmuganathan Ramasamy and Behdad Moghtaderi* Priority Research Centre for Energy, Chemical Engineering, The University of Newcastle, Callaghan, NSW 2308, Australia Received May 19, 2010. Revised Manuscript Received July 7, 2010

Recent progress in microwave characterization of wood based materials has created a need for better understanding of dielectric properties of these materials, which influence the absorption of electromagnetic energy. In the present study the dielectric measurements were carried out for typical Australian wood species such as Slash pine (Pinus elliottii, a softwood species), and Spotted gum (Eucalyptus maculata, a hardwood species), based on Von Hippel’s transmission line method. The influence of extractive removal was also studied and compared with the virgin wood samples. Measurements were performed at 9.47 GHz for both virgin and extractive-free wood samples. Experiments were carried out at atmospheric pressure under a range of ambient temperatures between 20 and 25 °C. The dielectric properties of wood species were determined as a function of controlling factors such as density, moisture content, and fiber directions. Moisture content varied from 0% to 13% for virgin wood samples at atmospheric equilibrium conditions. Results indicated that the dielectric properties of both wood species were affected by density, moisture content, and fiber directions. In general, for virgin wood samples, the dielectric property was found to increase with density and moisture content. The values of the dielectric property in parallel direction to the stem were generally higher than those in the perpendicular direction for virgin sample of both wood species. However, such a trend was not observed for extractive-free wood samples.

of manufacturing because they are extensively used as raw materials in the manufacturing of various products particularly furniture.10 Many of the processes associated with the above application areas (i.e., power generation, fire safety, and manufacturing) are underpinned by physical3 (e.g., density, moisture content, permeability) and structural properties11,12 (e.g., fiber and cellular structures, grain direction, porosity, evolution of porous structure during reactions) of wood. Determination of these properties is paramount in developing energy efficient and safer technologies for utilization of wood-based materials. In general, porous materials such as woody biomass do not lend themselves to visual data acquisition methods and as such invasive methods, such as thermocouple probing and gas sampling, are often the norm. Among available characterization techniques, methods based on nondestructive evaluation of woody biomass13 are very attractive as they enable the determination of material properties at nearly any point, line, surface or volume element of interest and at nearly any stage during the processing of the material. The use of microwaves for nondestructive testing (NDT) of wood materials has received an ever increasing interest in recent years due to the expanding materials technology and continuous introduction of microwave active and passive components with lower sizes, lower power requirements, and lower costs. The microwave based techniques,

Introduction This study is concerned with dielectric properties of woodbased biomass materials (or woody biomass), with the aim of facilitating a more widespread deployment of microwave based techniques for characterization of these materials. Woody biomass is of significant importance in a number of application areas including (i) power generation, (ii) fire safety, and (iii) manufacturing. Within the context of power generation, woody biomass fuels are of particular interest1-5 not only because biomass currently forms the world’s forth largest primary energy resource after coal, gas and oil but also due to the fact that biomass is considered to be CO2 neutral6,7 (i.e., does not contribute to global warming). Woody biomass is also important in the context of fire safety8 since wood-based materials are widely used as building materials and hence constitute the bulk of fuel load in building fires.9 Wood-based materials are also important in the context *To whom correspondence should be addressed. Telephone: þ61 2 4985 4411. Fax: þ61 2 4921 6893. E-mail: [email protected]. (1) Wu, H.; Fu, Q.; Giles, R.; Bartle, J. Energy Fuels 2008, 22, 190– 198. (2) Yu, Y.; Bartle, J.; Li, C.-Z.; Wu, H. Energy Fuels 2009, 23, 3290– 3299. (3) Abdullah, H.; Wu, H. Energy Fuels 2009, 23, 4174–4181. (4) Bridgwater, A. V. Fuel 1995, 74, 634. (5) Zhang, Y.; Mckechnie, J.; Cormier, D.; Lyng, R.; Mabee, W.; Ogino, A.; Maclean, H. L. Environ. Sci. Technol. 2010, 44, 538–544. (6) Evengelos, C. P.; Costas, P. P. Energy Fuels 2009, 23, 1055–1066. (7) Heath, L. S.; Maltby, V.; Miner, R.; Skog, K. E.; Smith, J. E.; Unwin, J.; Upton, B. Environ. Sci. Technol. 2010, 44, 3999–4005. (8) Dai, J.; Yang, L.; Zhou, X.; Wang, Y.; Zhou, Y.; Deng, Z. Energy Fuels 2010, 24, 609–615. (9) Bernd, S. R. T. Appl. Geochem. 2002, 17, 129–162. r 2010 American Chemical Society

(10) Pakarinen, T. Forest Prod. J. 1999, 49 (9), 79–85. (11) Shen, D. K; Gu, S; Luo, K. H; Bridgwater, A. V. Energy Fuels 2009, 23, 1081–1088. (12) Asadullah, M.; Zhang, S.; Min, Z.; Yimsiri, P.; Li, C.-Z. Ind. Eng. Chem. Res. 2009, 48, 9858–9863. (13) Bucur, V. Non-Destructive Characterisation and Imaging of Wood; Springer: New York, 2003.

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of energy that can be stored in the material is related to its dielectric constant ε0 . The greater the polarization of the material, the greater will be the dielectric constant.26 The mean dipole moment of a material is expressed through the sum of different types of polarization. The loss factor ε00 is a measure of the energy that can be dissipated in the form of heat (sometimes called the “dielectric absorption”27). These two values are related to each other as follows:

which until recently were referred to as emerging techniques, are now of current use in many wood industries for treatment of wood14-19 and diagnostic purposes.20 In all these applications, however, a fundamental knowledge of dielectric properties is required so that the interaction of the complex microwave field with wood is better understood. The dielectric properties of wood depend markedly on the chemical and fine structures as well as on the macroscopic structure of wood. The investigation on dielectric properties of wood, therefore, provides an important insight to the correlation between the properties and the structure of wood. The dielectric properties of wood have both theoretical and practical significance. The theoretical significance arises from a better understanding of the molecular structure of wood and of wood-water interactions that may result from the knowledge of dielectric behavior. The behavior of water with the constituents of wood such as cellulose and lignin can be understood more clearly by studying dielectric properties.21 The dielectric properties are also of practical importance because (1) their relationship to the density and moisture content of wood offers a potential method for determining moisture content and density by a nondestructive electrical measurement and (2) the dielectric properties of wood may be an important design factor where wood is employed in a structure subjected to electromagnetic fields.22,23 The mechanism of microwave on wood involves the transport of the electrical charges by the ions present in the wood cell wall and in cellulose.24 The randomly oriented dipoles in a dielectric material align themselves in a direction opposite to that of the applied external electric field. In this configuration, the energy supplied by the field is stored in the molecules as potential energy. The rise in vibrational motions of molecules in a microwave field is determined by the mechanism of ionic conduction and by dipole rotation, that is, a rotation of polar molecules under the influence of the external electric field. The electric field applied to wood at microwave frequencies induces a dissociation of molecules and a migration of ions, by delivering ordered kinetic energy. Dipole rotation is dependent on temperature and frequency.25 It also depends on several wood physical parameters, such as species type, density, moisture content, temperature, and structural orientation versus the anisotropic axes. When microwaves are directed toward a material, part of the energy is reflected, part is transmitted through the surface, of which a smaller part is absorbed by the material. The proportions of energy, which fall into these three categories, have been defined in terms of the dielectric properties. The amount

ε ¼ ε0 - jε00

ð1Þ

where ε* is the complex dielectric constant, while ε0 and ε00 , respectively, form the real and imaginary part of ε* (Note j is the numerical symbol representing the imaginary part of a complex number). The ratio ε00 /ε0 is known as the loss tangent tan δ.28,29 The values of ε0 and tan δ plays an important role in all the calculations associated with the interactions between a microwave and wood and depend on the tree species, wood density, moisture content, and temperature as well as on the operating frequency and its orientation in relation to the direction of the grain.30 The part of the power that is dissipated in the wood in the form of heat, and the absorbed power is given by28,31 the following formula: P ¼ 2πfE 2 ε0 tan δ

ð2Þ

where P is the absorbed power, f is the frequency, δ is the loss factor, and E is the electrical field strength. Despite the recent progress in the field, the interplay between dielectric properties of wood-based materials and controlling parameters such as moisture content, structural properties, and density are not well understood, particularly for typical Australian wood species. The study presented in this paper is an attempt to address this shortcoming. The significance of the present study is that it provides a fundamental insight into the dielectric characteristics of wood species. Such fundamental knowledge is essential for the development of microwave base diagnostic tool (e.g., moisture meter, density meter, porous meter, etc.). This is of great importance given that microwave diagnostic tools have unique advantages over other nondestructive tools due to their efficient coupling through air, no physical contact to the test material, deep penetration into nonmetals, and low power requirements. The paper summarizes the results of dielectric measurements of extractive-free and virgin wood samples of a hardwood (Spotted gum or “Eucalyptus maculate”) and a softwood species (Slash pine or “Pinus elliottii”), which are widely used for industrial applications in Australia. The dielectric measurements were conducted at 9.47 GHz for virgin and extractivefree samples. The effect of moisture content (with the exception of extractive-free samples) and grain direction were studied for all samples.32

(14) Wang, X.; Chen, H.; Luo, K.; Shao, J.; Yang, H. Energy Fuels 2008, 22, 67–74. (15) Robinson, J. P.; Kingman, S. W.; Barranco, R.; Snape, C. E.; Al-Sayegh, H. Ind. Eng. Chem. Res. 2010, 49, 459–463. (16) Boldor, D.; Kanitkar, A.; Terigar, B. G. ; Leonardi, C.; Lima, M.; Breitenbeck, G. A. Environ. Sci. Technol. 2010, 44, 4019–4025. (17) Lei, H.; Ren, S.; Julson, J. Energy Fuels 2009, 23, 3254–3261. (18) Banowetz, G. M.; Griffith, S. M.; Steiner, J. J.; El-Nashaar, H. M. Energy Fuels 2009, 23, 984–988. (19) Liu, C.-Z.; Cheng, X.-Y. Energy Fuels 2009, 23, 6152–6155. (20) Olmi, R.; Bini, M.; Ignesti, A.; Riminesi, C. J. Microwave Power Electromagn. Energy 2000, 35, 135–143. (21) Norimoto, M.; Yamada, T. Wood Research 1972, 52, 30–43. (22) James, W. L.; Hamill, D. W. Forest Prod. J. 1965, 15, 51–56. (23) Kabir, M. F.; Daud, W. M.; Khalid, K. B.; Sidek, A. H. A. Wood Fiber Sci. 2000, 32, 450–457. (24) Skaar, C. Wood in Water Relations; Springer: New York, 1988, p 283. (25) Brown, J. H.; Skaar, C. Dielectric Properties of Wood and Wood Adhesives. FAO, Fifth Conference on Wood Tech, U.S. Forest Products Laboratory, Madison, WI, 1963; Vol. 63, p 8.

(26) Netushil, A. V.; Zhukhovitskiy, B. J.; Kudin, V. N.; Parini, E. P. Gosenergoizdat 1959, 38–154. (27) Lin, R. T. Forest Prod. J. 1967, 17, 61–66. (28) Peyskens, E.; Pourcq, M.; Stevens, M.; Schalck, J. Wood Sci. Technol. 1984, 18, 267–280. (29) Kabir, M. F.; Khalid, K. B.; Daud, W. M.; Aziz, S. H. A. Wood Fiber Sci. 1997, 29, 319–324. (30) Torgovnikov, G. I. Dielectric Properties of Wood and Wood Based Material; Springer: Berlin, Heidelberg, Germany, 1993. (31) Brelid, P. L.; Simonson, R.; Risman, P. O. Holz Roh-Werkst. 1999, 57, 259–263. (32) Ramasamy S. Microwave Characterisation of Typical Australian Wood-based Biomass Materials. Ph.D. Thesis, The University of Newcastle, Australia, 2010.

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Figure 1. Equipment used to perform the dielectric measurement.

Apparatus, Methods, and Materials

of 9.47 GHz. It was important to make sure the sample inserted into the waveguide was closely adjoining the short circuit plate as the wave reflected from the short circuit plate and the incident waveform forms a standing wave. The slotted line with a crystal detector (which is connected to NI data acquisition) was moved until the node of the standing wave nearest to the waveguide was found. The position of the node was estimated with an error not exceeding 0.05 mm. At the same time, the wavelength was checked by measuring the distance between the nodes. This made it possible to find the position of the minimum reading with high accuracy. The dielectric loss factor was measured from the node width with and without the sample. It was ensured that the distance of the node width did not exceed 0.002-0.005 of the guided wavelength value. The loss factor in the wood specimen was found to be comparatively higher than the waveguide loss. The node width of the empty waveguide was taken into account for the correct interpretation of the loss tangent (tan δ) measurement. The selected wood species for dielectric measurements were Australian Slash pine (Pinus elliottii) and Spotted gum (Eucalyptus maculata). The main reasons for these selections were their commercial importance for the Australian market and industry and factors related to the wood itself, such as density and anatomical features. Selection of samples for dielectric measurement is carried out in two steps. Initially, several slabs were cut from large blocks of wood sawed out of the sap region. The required test samples were then cut from relevant slabs. Samples were cut along two different fiber directions, namely, parallel and perpendicular direction to stem (see Figure 2). In the case of parallel direction, the samples were cut along radial (LR) and tangential (LT) plains so that the influence of the molecular structure of the cell wall can be investigated. The samples had a cross section of approximately

The dielectric properties of the test materials were determined using a slotted waveguide and a data acquisition system (NI USB9233). The method was developed for the measurement of solid dielectrics in a shorted transmission line (ASTM D2520).33 Frequency used in this study was 9.47 GHz, and it was chosen because of its importance in potential diagnostic applications. Figure 1 shows the experimental setup of the shorted transmission line. The equipment consists of a gunn oscillator (microwave source, Terabeam-HXI, model HGV9.5221990I) capable of operating at X-band frequency (9-10 GHz) and delivering 19 dBm of power. The source is connected to (i) a unidirectional isolator device which protects the microwave source from the back reflected power, (ii) an attenuator which controls the power level in the transmission line, (iii) a square wave modulator to modulate at 1 kHz frequency for data acquisition, (iv) a frequency meter which detects the frequency in the transmission line, (v) a slide screw tuner for obtaining a uniform standing wave, (vi) a slotted line with traveling crystal detector to measure the standing wave, and (vii) a short circuit plate where the sample is placed for measurement. The output of the traveling detector was connected to a data acquisition card (NI USB-9233), and the output voltage is measured using a lab view program. The experimental procedure involved a number of steps. Before starting the measurements, the gunn source was tuned to a frequency (33) ASTM D 2520-01. Standard Test Methods for Complex Permittivity (Dielectric Constant) of Solid Electrical Insulating Materials at Microwave Frequencies and Temperatures to 1650°C; American Society for Testing and Materials: Conshohocken, PA, 1986.

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the moisture content during the test. A series of calculations were carried out for each test sample using the measured data (e.g., sample dimensions, the test frequency wavelength, etc) to determine its dielectric property.

Results and Discussion Dielectric Properties of Oven-Dried Wood-Based Materials. Samples used in this part of study were oven-dried to a constant mass in air at a temperature equal to 103 ( 2 °C. Research has shown that the dielectric parameters of oven-dried wood are considerably influenced by the wood density, temperature, the external field frequency, and orientation of vector E relative to the direction of the fibers. In addition, wood from different tree species possess different dielectric properties. An increase in the field strength affects the dielectric constant ε0 and loss tangent tg δ values of oven-dried wood inconspicuously.35 This section provides an overview of the dielectric properties of oven-dried samples of typical Australian wood species. Experiments were performed at oven-dried conditions for virgin wood samples at a frequency of 9.47 GHz for various density and directions by maintaining the temperature constant. For better understanding, the effect of fiber direction was studied in parallel and perpendicular directions to the stem. Previous research has found that the dielectric parameter of oven-dried wood is highly influenced by (volume of cell wall substances) density and air in the vector E direction.36-38 Figure 3a shows the mean dielectric constant of oven-dried Slash pine (coniferous) and Spotted gum (angiosperm) virgin wood samples in parallel and perpendicular directions to the stem at a frequency of 9.47 GHz. The most significant feature is that the dielectric constant is very high in parallel direction to the stem for both wood species. It has been found experimentally that the dielectric anisotropy is caused by both molecular structures of wood substances as well as the macroscopic structures of the wood cell, where the magnitude of the dielectric constant is very high in the parallel direction to the stem than in the perpendicular.21 It was also found that there is a 50% rise in the dielectric constant of Spotted gum in both directions compared to Slash pine wood species. The difference in the dielectric behavior among the wood species is due to the intrinsic characteristics of each species such as density and permeability. Figure 3b shows the dielectric constant of three different sets of samples of Slash pine and Spotted gum at various density and fiber directions. The sets designated by “1” and “2” represent Slash pine samples obtained from two different climatic conditions (i.e., different growth conditions) while designation 3 represents Spotted gum samples. Sets 1 and 3 were analyzed for both fiber directions whereas set 2 was only analyzed for the parallel direction. The gross values of the densities for the tree sets of sample used in this part of the study ranges between 592 and 658 kg/m3 for set 1, 617658 kg/m3 for set 2, and 880-932 kg/m3 for set 3. The density of Slash pine for both sets was within 20% of the full range, and the density of Spotted gum was around 5055% of the range. As can be seen from Figure 3b compared with Slash pine samples, Spotted gum samples exhibit a sharper rise in the dielectric constant when the density is increased. This trend, however, is not clearly observed within Slash pine

Figure 2. Specimen configuration (shown within waveguide). Table 1. Moisture Content of Slash pine and Spotted Gum Wood Species at Different Humidity Levels moisture content % at 25 °C 40% humidity. (virgin)

80% humidity. (virgin)

wood species

parallel to stem

perpendicular to stem

parallel to stem

perpendicular to stem

Slash pine Spotted gum

5.7 5.4

5.9-6.4 3.8-5.4

13 12.9

12.7-13.1 11.8-12.1

2.28 cm 1.02 cm with the third dimension varied between 2 and 3 cm for the test. The configuration of the test specimen is given in Figure 2. One group of virgin specimens was dried at 103 ( 2 °C for 24 h, and other two groups were conditioned using an environmental chamber (Steridium, Australia) at two different humidity levels of 40% and 80% at 25 °C until they reached their respective equilibrium moisture content (Table 1). As the wood samples swell during the process of humidification from the oven-dried state to the fiber saturation point, it was necessary to prepare separate test samples for each moisture content level. Extractive-free samples were prepared according to the ASTM standard D1105-9634 test method for preparation of extractivefree wood. However, a slightly different procedure was employed because of the large physical dimensions of the test samples in this study. This, in particular, required adjustments in the extraction time to values 3 times of those recommended by the ASTM standard. Three different extraction mediums (ethanol, toluene, and hot water) were employed so that the effect of organic and inorganic compounds of wood can be better studied. The samples were oven-dried before and after each extraction process for dielectric measurement, and they were repeated for different directions. Every sample was weighed prior to each test and then oven-dried and reweighed at the conclusion of each experiment to determine

(35) (36) (37) (38)

(34) ASTM D 1105-96. Standard Test Methods for Preparation of Extractive-Free Wood; American Society for Testing and Materials: Conshohocken, PA, 1996.

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Nanassy, A. J. J. Wood Sci. Technol. 1979, 6, 67–77. Lichtenecker, K. Phys. Z. 1926, 27, 115. Buchner, A. Wiss. Veroff Siemens-Werken 1939, 18, 84–96. Trapp, W.; Pungs, L. Holzforschung 1956, 103, 65–68.

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Figure 3. Mean dielectric constant of oven-dried Slash pine and Spotted gum virgin wood samples as a function of (a) fiber direction and (b) density for both fiber directions and different tree sample sets.

sets of samples where the dielectric constant of the second set in the parallel direction is relatively low at higher densities. It is obvious that the macroscopic and microscopic structures of wood differ within the same wood species based on the growth place. In addition, there is a difference in the electric field incident between the samples in the parallel direction. This effect was not that significant within the same sample sets for the studied range under oven-dried conditions. Higher differences can only be observed if greater density variations exist for a given wood species.

Figure 4a,b shows the relationships between the dielectric constant and dielectric loss factor for oven-dried samples of Slash pine and Spotted gum wood species at different fiber directions. Figure 4a shows that regardless of the fiber direction, the dielectric constant of Spotted gum is higher than Slash pine. Given that the density of Spotted gum is higher than that of the Slash pine, the above observation implies that the dielectric constant increases with the rise in density. The above observation also confirms that the dielectric constant of Spotted gum is around 1.3 times higher (for both 4538

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Figure 4. (a) Dielectric constant and (b) dielectric loss factor of oven-dried Slash pine and Spotted gum virgin wood samples as a function of density for both fiber directions at 9.47 GHz. Table 2. Chemical Composition of Slash pine and Spotted gum Wood Species39 Slash pine (%)

Spotted gum (%)

portion of the wood

cellulose

hemi cellulose

lignin

cellulose

hemi cellulose

lignin

cross-section heartwood sapwood

52 40.6 52.3

18.5 14.7 17.9

25.4 27.2 26.3

58.5 57 63.7

11.1 9.9 11.9

17.2 17.9 16.5

of hemicellulose and lignin are higher in Slash pine. However, the influence of dielectric polarization is very high with cellulose compared to hemicellulose and lignin.40 The sample with higher polar groups accompanying the dielectric polarization will have a higher dielectric constant, and it is clear that the low-density sample will have few polar groups. Note

fiber directions) than that of the Slash pine, which is influenced by the intrinsic characteristic and chemical composition of the wood species (Table 2).39 Table 239 shows that the percentage of cellulose is high in Spotted gum compared to Slash pine, whereas the percentages (39) Ximenes, F. A.; Gardner, W. D.; Kathuria, A. Forest Ecol. Manage. 2008, 256, 335–346.

(40) Norimoto, M. Wood Res. 1976, 59/60, 106–152.

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Figure 5. (a) Dielectric constant and (b) dielectric loss factor of oven-dried Slash pine virgin wood samples as a function of density for different sample sets in the parallel direction to the stem at 9.47 GHz.

that the average density used in this part of study for Slash pine and Spotted gum samples in the parallel direction were 613 and 914 kg/m3 and in the perpendicular direction were 643 and 890 kg/m3, respectively. Figure 4b clearly shows that the dielectric loss factor of Spotted gum was higher for both fiber directions than in Slash pine. It also shows a distinct difference in the dielectric loss factor value in Slash pine wood species compared to Spotted gum for both fiber directions. However, the dielectric loss factor is highly scattered in the case of Spotted gum for both fiber directions. This behavior is due to the intrinsic characteristics of the wood species where the cell structure is more uniform in Slash pine and is much more complex in Spotted gum.

Figure 5a,b shows the dielectric constant and dielectric loss factor in the parallel direction to the stem for two different sets of oven-dried Slash pine samples. The two sets clearly exhibit different values of the dielectric constant and dielectric loss factors, which confirm that for the same wood species there is a significant variation of properties from one tree to another due to climatic and growth conditions (e.g., sample sets 1 and 2 in this study). Figure 5a shows a stable value of the dielectric constant in the studied range of density for both sets of samples. Torgovnikov30 reported that the dielectric constant is highly influenced by the wood density. While this statement can be true when one compares two different wood species (see Figure 3), 4540

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Figure 6. Dielectric constant of oven-dried Slash pine virgin wood samples as a function of density for different fiber directions and tree sample sets at 9.47 GHz.

Figure 7. Dielectric constant of oven-dried Spotted gum virgin wood samples as a function of density for different fiber directions at 9.47 GHz.

the variation in density for a given wood species does not necessarily leads to changes in the dielectric constant (see Figure 5a). The above observations indicate that for a given wood species, the dielectric constant in the parallel direction to the stem is more influenced by the climatic conditions (i.e., the original tree and its growth conditions) and, as such, it may vary from tree to tree. Figure 5b shows a similar trend and behavior as in Figure 5a. The difference in dielectric loss factor is roughly 2 times higher in set 1 compared to set 2. A comparison between parts a and b of Figure 5 reveals that unlike the dielectric constant, the dielectric loss factor exhibits some scattering. The level of this scatter for set 1 is by and large higher than that of set 2. This might be partly assigned to the variation in the anisotropy of cell wall substances and the percentage of chemical constituents which differ from set 1 to set 2. Figure 6 shows the dielectric constant of oven-dried Slash pine wood samples as a function of density at three different fiber directions (i.e., LR, LT, and RT planes, see Figure 2). While there is a significant difference between the dielectric constants of different fiber directions, for each orientation the dielectric constant appears to be a week function of the density (i.e., does not vary significantly). The difference in the dielectric constant is roughly 1.3 times higher in the LT plane compared to the RT plane for set 1 samples in spite of their relatively low densities. It also shows that for set 2, the dielectric constant is low for the denser LR plane compared to the set 1 lighter LT plane. This behavior and trend is identical to that shown earlier in Figure 5a. The difference in the dielectric property for the fiber direction is due to the specific molecular structure of the cell wall.41 In addition, the hydroxyl groups of cellulose have more freedom of rotation in the parallel direction (i.e., LR and LT planes) to the stem than in the perpendicular one (i.e., RT plane27). Figure 7 shows the dielectric constant of oven-dried Spotted gum wood samples as a function of density for various

fiber directions. Evidently, Spotted gum shows a similar trend to that observed earlier for Slash pine (see Figure 6). In the case of Spotted gum, as Figure 7 indicates the dielectric constant in the LT plane is roughly 1.05 times higher than that of the LR plane for a small rise in density. However, it is 1.2 times higher in the LT plane compared with the RT plane under the same set of conditions. This observation confirms that the difference in the dielectric constant between the LR and LT planes for Spotted gum is less than Slash pine. Overall, it appears that the difference in the dielectric constant is highly significant between the LT, RT and LR, RT planes and it is comparatively less significant between the LT and LR planes. Dielectric Properties of Moist Wood. Dielectric measurement of moist wood is highly complicated compared to other measurements in wood. The absolute value of the moisture content in wood can reach up to 50-60% and even higher. Dielectric property of free moisture (i.e., capillary water) is very high compared to the wood itself. Free moisture can create a significant change in the interaction between wood and electromagnetic fields. These result in changes of the dielectric properties of the wood and the frequencies at which water possess anomalously high dielectric parameters. Water is an ingredient that changes the dielectric properties of wood which is observed in many studies. However, the theoretical studies were limited in determining the physical factors that influence the moisture content on the dielectric property of wood. The present study only gives a qualitative estimation of dielectric data obtained for different moisture contents through experimental research. Experiments were performed in a constant temperature environment on virgin wood samples at a frequency of 9.47 GHz for different wood species, moisture contents, densities, and fiber directions. The effect of fiber direction was only studied for the LT plane in the case of the parallel direction to the stem and the RT plane in the case of the perpendicular direction. Figure 8a shows a bar chart for the mean dielectric constant in moist wood as a function of fiber direction and wood

(41) Norimoto, M.; Yamada, T. Wood Res. 1971, 51, 12–32.

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Figure 9. Dielectric constant of moist Slash pine and Spotted gum virgin wood samples as a function of moisture content for both fiber directions at 9.47 GHz.

Figure 8 shows a sharp rise in the dielectric constant for the studied range of moisture content in both wood species. The influence of moisture content for both fiber directions is more pronounced in Slash pine which shows a 20% rise in the dielectric constant over the studied range of moisture content whereas it is only 10% in Spotted gum. The difference in dielectric behavior between the wood species is due to the response rate of moisture content in each species. Figure 9 shows the variation of the dielectric constant of Slash pine and Spotted gum virgin wood samples at different moisture contents and fiber directions. As explained earlier for moist wood, the effect of fiber direction was only studied for the LT plane in the case of the parallel direction to the stem and the RT plane in the case of the perpendicular direction. As noted earlier, the average moisture content used in this study ranges from 0 to 13% (Table 1). The figure shows a sharp rise in the dielectric constant with an increase in the moisture content for both wood species. The impact of the moisture content on the dielectric behavior is more pronounced in Slash pine compare to Spotted gum species. The trends in Figure 9 show that there is a direct linear relationship between the dielectric constant and moisture content. The higher the moisture content ,the greater the dielectric constant. However, the slopes of the linear plots for Slash pine samples appear to be steeper than their Spotted gum counterparts for both fiber directions. This observation confirms that the dielectric constant is highly influenced by the moisture content in Slash pine and by chemical constituents for Spotted gum (Table 2). The response rate to moisture content is more rapid in Slash pine than in denser wood like Spotted gum. This can be partly assigned to the fact that Slash pine is highly affected by the atmospheric changes because of the nature of its porous structure. The dielectric behavior in moist wood is also influenced by the amount of water within the wood matrix, where the dielectric values of water is very high compared to other components in the wood species. In addition, the polar groups in the cell wall and cellulose have more freedom of rotation at

Figure 8. Mean dielectric constant of moist Slash pine and Spotted gum virgin wood samples as a function of (a) fiber direction and (b) moisture content.

species. The most significant feature in this figure is that for both wood species, the dielectric constant attains very high values in the parallel direction to the stem compared to the perpendicular direction. The graph shows a 20% rise in the dielectric constant for the Spotted gum in both fiber directions compared to Slash pine. Also for each species, a 20% uniform rise in the dielectric constant is observed between the parallel and perpendicular directions. Figure 8b shows the mean dielectric constant at three different moisture levels for the two wood species under investigation as a function of fiber direction. The average moisture content used in this study ranges from 0 to 13% for both wood species. The trend shown in Figure 8b in terms of fiber direction and wood species is almost identical to that of Figure 8a. 4542

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22,27

higher moisture contents. As observed earlier, the value of the dielectric constant was comparatively high in the parallel direction than in the perpendicular direction for both wood species. This is due to the arrangement of cell wall substances21 in which the orientation of the cellulose is approximately the same as the fiber axis and where the hydroxyl group’s possess more freedom in the parallel direction.27 The trend lines from Figure 9 show that the slope of the plot of the dielectric constant in the perpendicular direction is steeper than the parallel direction for both wood species. Given that the electric conductivity of wood-based materials depends on the concentration of ions (e.g., hydroxyl groups) and on their mobility,30 the above observation suggests that as the moisture content is increased, the mobility of ions in the parallel direction is greatly reduced. The reduction in ion mobility in the parallel direction manifests itself in a much more gradual rise in the dielectric constant compared with that of the perpendicular direction (see Figure 9). Therefore, it can be concluded that the moisture content has a more substantial impact (i.e., retardation effect) on the dielectric constant in the parallel direction.42 As noted earlier, the graph shows a 10% rise in the dielectric constant for Slash pine in both fiber directions compare to Spotted gum. Also for each species, a 20% uniform rise in the dielectric constant is observed between the parallel and perpendicular directions at lower moisture contents, whereas it is only 10% at higher moisture contents. The equations below show the correlations between the dielectric constant and moisture content for both fiber directions and wood species. Slash pine ε0 parallel ¼ 0:0393M:C: ð%Þ þ 1:4901

Figure 10. Dielectric constant of moist Slash pine and Spotted gum virgin wood samples as a function of density for both fiber directions at 9.47 GHz.

ðR2 ¼ 0:981Þ ð3Þ

ε0 perpendicular ¼ 0:053M:C: ð%Þ þ 1:132

ðR2 ¼ 0:999Þ

ð4Þ Spotted gum ε0 parallel ¼ 0:0168M:C: ð%Þ þ 1:9514

ðR2 ¼ 0:988Þ ð5Þ

ε0 perpendicular ¼ 0:03M:C: ð%Þ þ 1:6364 ðR2 ¼ 0:994Þ ð6Þ Figure 10 shows the dielectric constant of moist wood as a function of density for both fiber directions and wood species. The behavior and trend over the dielectric constant for the studied range of density in Figure 10 shows that the impact of density on the dielectric constant is similar to that of moisture content shown earlier in Figure 9. Note that the density used in this part of study for both fiber directions in Slash pine and Spotted gum ranges from 592 to 806 kg/m3 and 871 to 1071 kg/m3, respectively. The variation in density for each species is therefore around 200 kg/m3. Over this range, the impact of density is clearly more pronounced for the Slash pine than Spotted gum despite the fact that the density variation is identical in both cases. Yet again, such a difference in dielectric behavior can be assigned to the prominent role of the moisture content. Figure 11 shows the difference in dielectric loss factor of Slash pine and Spotted gum virgin wood samples as a function of moisture content for the parallel and perpendicular directions.

Figure 11. Dielectric loss factor of moist Slash pine and Spotted gum virgin wood samples as a function of moisture content for both fiber directions at 9.47 GHz.

The dielectric loss factor behavior and trend (see Figure 11) is almost identical to that shown earlier in Figure 9 for the dielectric constant. The graph shows around 15-20% uniform rise in the dielectric loss factor for Slash pine over the range of moisture contents studied here in both fiber directions, whereas the rise is 20% in the parallel direction and 5-10% in the perpendicular direction for Spotted gum. As observed, the difference in dielectric loss factor is more distinct between the parallel and perpendicular directions in the Spotted gum wood species. Also the data is highly scattered in the case of Spotted gum for both fiber directions as also observed earlier in Figure 4b for the oven-dried samples. These observations indicate that the dissipation factor toward

(42) Sahin, H.; Nurgul, A. Y. J. Wood Sci. 2004, 50, 375–380.

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Figure 12. Dielectric loss factor of moist Slash pine and Spotted gum virgin wood samples as a function of density for both fiber directions at 9.47 GHz.

electromagnetic energy is very high for Spotted gum samples (note that the dielectric loss factor is highly influenced by the chemical constituents and the intrinsic characteristics for Spotted gum wood species). In addition, the volume of the samples (length) and moisture contents are not identical for both fiber directions and wood species. The eqs 7-10 represent the correlations between the dielectric loss factor and moisture content for both fiber directions and wood species. Slash pine ε00 parallel ¼ 0:0076M:C: ð%Þ þ 0:0449

ðR2 ¼ 0:9375Þ ð7Þ

ε00 perpendicular ¼ 0:0089M:C: ð%Þ þ 0:0035 ðR2 ¼ 0:9845Þ

ð8Þ

Figure 13. Loss tangent of moist Slash pine and Spotted gum virgin wood samples for both fiber directions as a function of (a) moisture content and (b) density at 9.47 GHz.

Spotted gum ε00 parallel ¼ 0:011M:C: ð%Þ þ 0:0568

ðR2 ¼ 0:957Þ

ε00 perpendicular ¼ 0:0059M:C: ð%Þ þ 0:0401

ð9Þ

and density for the parallel and perpendicular directions. Loss tangent is numerically equal to the ratio between the active current and the reactive current in the material or to the real and respective powers ratio. The trends observed in Figure 13a,b are almost identical to those shown earlier in Figures 11 and 12, implying the impact of density and moisture content on the dielectric loss factor and the dielectric loss tangent are very similar for the wood species studied here. The dielectric loss tangent measurement is more complicated than other dielectric measurements where the magnitude of error is always high. These are highly influenced by the moisture content, chemical constituents, and the intrinsic characteristics of the wood species. This varies from sample to sample for different wood species, especially in Spotted gum. Previous research has shown that the data cannot be reproduced for the same species. Note that the average loss tangent observed in this study ranges from 0.0014 to 0.10 for both wood species at different fiber directions and moisture contents. This observation confirms that the selected wood species in this study is a low-loss material.

ðR2 ¼ 0:802Þ

ð10Þ Figure 12 illustrates the dielectric loss factor of moist wood as a function of density for both fiber directions and wood species. The dielectric loss factor is more widely spread in the parallel and perpendicular directions for Slash pine, whereas this feature is only observed in the parallel direction for Spotted gum. Regardless of the value of the density, Spotted gum samples show very similar dielectric loss factors in the perpendicular direction. This is due to the variation of the moisture content, which is not that distinct as in Slash pine, and the chemical constituents of the wood species itself. The above observations indicate that the dielectric loss factor at high frequencies is more influenced by the density and the fiber directions of the wood species, which are in turn influenced by the moisture content. Figure 13a,b shows the loss tangent of Slash pine and Spotted gum virgin wood samples as a function of moisture content 4544

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Figure 14. Dielectric constant of moist Slash pine and Spotted gum virgin wood samples as a function of moisture content for different fiber directions at 9.47 GHz.

The correlations presented in eqs 11-14 show the relationship between the dielectric loss tangent and moisture content for both fiber directions and wood species. Slash pine tan δparallel ¼ 0:0033M:C: ð%Þ þ 0:0304

ðR2 ¼ 0:897Þ

ð11Þ tan δperpendicular ¼ 0:0048M:C: ð%Þ þ 0:0049

ðR2 ¼ 0:982Þ

ð12Þ Spotted gum tan δparallel ¼ 0:0046M:C: ð%Þ þ 0:0311

ðR2 ¼ 0:943Þ

Figure 15. (a) Dielectric constant and (b) dielectric loss factor of oven-dried Slash pine and Spotted gum wood species as a function of density for the different extraction process and fiber directions at 9.47 GHz.

ð13Þ tan δperpendicular ¼ 0:0026M:C: ð%Þ þ 0:0251

ðR2 ¼ 0:705Þ

wall and cellulose have more freedom of rotation in the LR plane at higher moisture contents for both wood species. The difference in the dielectric behavior between the LR and LT planes is more pronounced in Slash pine and is less significant for Spotted gum. The above observations indicate that the dielectric constant increases with a rise in moisture content, and it is more predominant in the LR and LT planes than in the RT plane. Also, the influence of moisture is highly significant in the LR plane for both wood species. Dielectric Properties of Extractive-Free Wood. In addition to cellulose, hemicellulose, and lignin, wood also contains organic matters which are commonly referred to as extractives (e.g., polyphenols, oils, fats, gums, resins, waxes, or starch) and inorganic matters (minerals, etc). These components normally contribute only a small amount of mass in wood but they contribute to color, taste, odor, density, durability, flammability, and hygroscopicity. In the present study, ethanol and toluene were used as organic and inorganic solvents to remove the extractives and minerals from the wood samples. In addition, hot water was used to extract tannins, gums, sugars, starches, and coloring

ð14Þ Figure 14 shows the dielectric constant of Slash pine and Spotted gum virgin wood samples as a function of moisture content at three different fiber directions (i.e., LR, LT, and RT planes, see Figure 2). Specifically, the effect of fiber direction was studied for the LR and LT planes in the case of the parallel direction to the stem and the RT plane in the case of the perpendicular direction. There is a significant difference between the dielectric constants at different fiber directions. The data set shows a sharp rise in the dielectric constant with an increase of moisture content for each orientation. The most significant feature is that the value of the dielectric constant in the RT plane is distinctively different from those of the LR and LT planes. This behavior is due to the arrangement of cell wall substances, which is completely different in the RT plane than in the LR and LT planes.41 In addition, the trends observed in Figure 14 show that the value of the dielectric constant is relatively high at the LT plane for lower moisture contents, while for higher moisture contents, the dielectric constant appears to be dominant in the LR plane. This observation confirms that the polar groups in the cell 4545

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Figure 16. (a) Dielectric constant, (b) dielectric loss factor, (c) loss tangent of Slash pine and Spotted gum virgin wood samples as a function of the moisture content between the fiber directions at 9.47 GHz.

matters. Experiments were performed in a constant temperature environment on virgin wood samples to check the permittivity of the wood species with respect to the removal of extractives in each species at both parallel and perpendicular fiber directions. Dielectric measurements were performed on oven-dried extraction free samples. Extractions were performed according to the ASTM standard (ASTM D1105-96).34 As noted earlier, the effect of fiber direction was only studied for the LT plane in the case of the parallel direction to the stem and the RT plane in the case of the perpendicular direction. Figure 15a,b shows the dielectric constant and dielectric loss factor of oven-dried Slash pine and Spotted gum extractive-free and virgin wood samples as a function of density at the parallel and perpendicular directions. There is a significant

difference in the dielectric constant between the parallel and perpendicular directions for both wood species (see Figure 15a). Also, at both fiber directions there is a little difference among the dielectric constants of extractive-free Slash pine samples obtained from different extraction processes. To the contrary, the dielectric constant of Spotted gum samples show some sensitivity to the extraction method used. The difference in the dielectric constant for Spotted gum is particularly high for hot water and toluene extraction processes in the perpendicular direction. Unlike the dielectric constant, the loss factor in the extractionfree samples appears to be very sensitive to the extraction method employed. Figure 15b, for example, shows a significant difference in the dielectric loss factor of Spotted gum 4546

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wood species for each extraction process in the parallel and perpendicular directions. This difference is more distinct in the parallel direction to the stem than in the perpendicular direction. Also, the dielectric loss factor is very high for extracted samples (i.e., ethanol extraction, hot water, and toluene extraction) compared to their counterparts in the parallel direction, whereas this effect was not clearly observed in the perpendicular direction. This behavior is due to the arrangement of the cell wall substances, where the dielectric properties differentiate themselves clearly in the parallel direction to the stem than in the perpendicular direction. In addition, during the extraction process of the Spotted gum wood species, the extractives might have relocated to critical areas blocking various cell structures and thereby reducing the permeability of the wood species.43 However, the above observations were not clearly observed for the Slash pine wood species. It is assumed that the percentage of extractives and minerals in Slash pine is much less than Spotted gum wood species. Evidently, the dielectric loss factor of Slash pine is inconsistent due to the variation in the composition of extractives and minerals for each sample (Figure 15b). The above observations imply that the impact of the extraction process is insignificant on the dielectric properties of the Slash pine wood species. However, the density has a greater impact than any of the other factors in the wood species.44 From the present study, it can be concluded that the effect of extractives and minerals in the wood species is trivial at higher frequencies. Regression Analysis. A regression analysis was done for the measured dielectric properties of oven-dried and moist Slash pine and Spotted gum virgin wood samples as a function of moisture content in the parallel (i.e., LR plane) and perpendicular (i.e., RT plane) directions. As noted earlier, the average moisture content used in this study ranges from 0 to 13% (see Table 1). A linear dielectric (i.e., dielectric constant (ε0 ), dielectric loss factor (ε00 ), and loss tangent (tan δ)) relationship between the fiber directions was observed for the Slash pine and Spotted gum wood species. The slopes of the trend lines in Figure 16 indicate that the dielectric properties (i.e., dielectric constant, dielectric loss factor, and loss tangent) of Slash pine are more influenced by the fiber direction than in Spotted gum Moreover, the properties of the Slash pine are highly influenced by the moisture content and the intrinsic characteristic of the wood itself. However, in terms of the absolute values, the dielectric properties of Spotted gum are generally much higher than those of the Slash pine because the impact of the density is more substantial than any other factors. For example, as Figure 16a shows, the dielectric constant for Spotted gum is 1.32 times higher than that of the Slash pine in the parallel fiber direction and 1.42 times higher in the case of the perpendicular direction. Also, the dielectric constant in the parallel direction is 1.27 times higher than the perpendicular direction in the case of Slash pine and 1.20 times higher in the case of Spotted gum. However, for both wood species, the ratio of dielectric constants corresponding to the two fiber directions decreases with the increase in moisture content.

Figure 17. Dielectric constant of Spotted gum virgin wood samples as a function of moisture content between the LR and LT planes at 9.47 GHz.

Similar trends are also observed for the dielectric loss factor and loss tangent. For example, Figure 16b shows that the dielectric loss factor in the parallel direction is 10 times higher than the perpendicular direction in the case of Slash pine and 1.72 times higher in the case of Spotted gum wood species. In addition, the dielectric loss factor in Spotted gum wood species is 1.38 higher than Slash pine in the case of the parallel direction and 6.74 times higher in the case of the perpendicular direction. The ratio of dielectric loss factors corresponding to the two fiber directions sharply decreases with the rise in moisture content for Slash pine while exhibits a gradual decrease in the case of Spotted gum. Figure 16c is almost identical to that shown in Figure 16b, implying that the impact of fiber direction, density, and moisture content on the dielectric loss factor and loss tangent for both wood species are very similar. Equations 15-20 express the relationships among dielectric properties and the fiber directions for the Slash pine and Spotted gum wood species. Slash pine ε0 ðparallelÞ ¼ 0:7753ε0 ðperpendicularÞ þ 0:5955

ð15Þ

ε00 ðparallelÞ ¼ 0:9179ε00 ðperpendicularÞ þ 0:0365

ð16Þ

tan δðparallelÞ ¼ 0:7292 tan δðperpendicularÞ þ 0:023

ð17Þ

Spotted gum ε0 ðparallelÞ ¼ 0:5355ε0 ðperpendicularÞ þ 1:0858

ð18Þ

ε00 ðparallelÞ ¼ 1:396ε00 ðperpendicularÞ þ 0:0102

ð19Þ

tan δðparallelÞ ¼ 1:2576 tan δðperpendicularÞ þ 0:0044

ð20Þ

The dielectric behavior for the anisotropy of the given wood species was also studied for various parameters. In particular, the magnitude of the dielectric constant at parallel direction for the LR and LT planes in the Spotted gum wood species was examined through a regression analysis. For example, Figure 17 shows the relationship between the dielectric constants at the LR and LT planes at three different moisture contents for Spotted gum virgin wood samples. As noted earlier, the average moisture content used in this

(43) Ellwood, E. L.; Ecklund, B. A. The Role of Extractives in Wood Permeability; Report Wood Technology, Forest Products Laboratory: Richmond, CA, 1961. (44) Sehlstedt-Persson, M. The Effect of Extractive Content on Moisture Diffusion Properties for Scots Pine and Norway Spruce. European Cost E15 Workshop on Wood Drying, Helsinki, Finland, 2001.

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study ranges from 0 to 13%. Figure 17 shows the existence of a linear relationship between the dielectric constant in the LR and LT planes. The graph shows that the dielectric constant in the LT plane is 1.05 times higher than the LR plane in the case of moisture level 1. However the ratio of dielectric constants between the LR and LT planes is negligible at higher moisture contents (i.e., moisture level 2 and moisture level 3). The following equation shows the dielectric relationship between the LR and LT planes for the Spotted gum wood species. ð21Þ ε0 ðLTplaneÞ ¼ 0:6095ε0 ðLRplaneÞ þ 0:081

Conclusions

substantial effect on the dielectric constant. The values of dielectric constant in the parallel direction to stem were generally higher than those in the perpendicular direction. The dielectric properties of Spotted gum, which is a hardwood species, were found to be generally higher than those of the Slash pine (a softwood species). From a microwave diagnostics point of view, the samples of softwood and hardwood species studied in the present investigation were found to be of low loss material. The effect of the extraction process was found to be insignificant for Slash pine whereas some changes were observed for Spotted gum. However, the examination of extractive-free samples revealed that by and large the effect of extractives and mineral content were not significant at frequencies of interest in microwave diagnostics. As such, microwave based diagnostic techniques can be successfully applied for determination of structural properties of wood species without significant interference by extractives.

In this paper, the dielectric properties of virgin, conditioned (i.e., moist), and extractive-free samples of Slash pine (Pinus elliottii) and Spotted gum (Eucalyptus maculata) were studied using Von Hippel’s transmission line method at a microwave frequency of 9.47 GHz. The current results suggest that the dielectric constant of virgin wood samples increase with moisture content and density. The structural directions also have a

Acknowledgment. We thank for the financial support that was received for the project from the Priority Research Centre for Energy, The University of Newcastle, Australia. We also like to thank Prof. Krishnan Balasubramaniam of the Indian Institute of Technology Madras, Chennai, India, for providing the experimental facility and finally Dr. Venkatesh and Mr. Chris Stevanov for their valuable comments.

The above correlation indicates that the difference in dielectric constant is almost insignificant at high moisture contents whereas a small difference is observed at oven dry conditions.

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