| 概要 |
木材細胞壁中の微細空げきは，水分や薬液の細胞壁内通導の通路であり，さらに，水分吸着によって引き起こされる膨潤・乾縮とも重要な関係を持っている．したがって，木材の性質・挙動と細胞壁の構造との有機的な関係づけが極めて大切な研究課題である．ところで，電気伝導の主要な通路は細胞壁中に形成される水分の吸着層であり，しかも含水率が電気伝導率に与える影響は極めて大きいことがよく知られている．そこで，この研究では...，木材の吸・脱湿に伴う含水率および細胞壁構造の変化を考え併せながら，異なった電場条件下に置かれた木材の電気的刺激に対する応答性，すなわち①電気伝導の周波数依存性，②電気伝導に与える印加電圧の影響，③電気伝導に与える電場方向の影響を検討し，ひいては電気伝導のメカニズムと細胞壁中での水分の存在状態および細胞壁構造とのかかわりを考察した．高周波（1MHz）から極めて低い周波数（0．0001Hz）にわたる範囲で，含水率と温度のそれぞれが低いほど，周波数の低下に伴う電気伝導率の減少がより低い周波数に至るまで見られた．常温下でかつ約5％以上の含水率範囲では，低い周波数の側での電気伝導率の周波数依存性は印加電圧によって影響を受け，イオン伝導に基づく非可逆的な分極によるものと推定した．この電気伝導率および電気伝導の挙動に与える電圧の影響は，含水率が増加するほど大きくなり，吸着イオンが吸着点から脱着される量と脱着後の移動速さが電圧によって異なることに基づくと考えた．他方，電気伝導率の周波数依存性に関係する分極現象は含水率5％以下で可逆的であり，しかも電圧による影響を認めなかった．したがって，水分が単分子層で吸着されている低含水率の範囲では，吸着イオンの脱着および解離とその移動が，電気伝導にかかわりを持たないことを推定した．電気伝導の異方性は，電圧および周波数によって影響を受け，印加電圧が大きくかつ周波数が低いほど，軸方向と横方向との間の電気伝導の異方度が大きくなる傾向を認めた．これは，細胞壁中での電気伝導の異方性が電荷担体である伝導イオンの移動距離と関係し，細胞壁内の空げき構造がイオンの移動度に与える影響とも関係することを示唆している．ちなみに，含水率が電気伝導率およびその異方度に与える影響は，含水率約8％を境にしてその上下の範囲で異なった挙動が得られ，とくに水分吸着に伴う細胞壁中の構造変化と密接なかかわりを持つことを推定できた．低周波から底流領域で電場条件（周波数，印加電圧，電場の方向）を変えたとき，電気的挙動とその機構は含水率の範囲によって異なっている．これは水分子と木材実質との間の吸着力がイオンの解離度および移動度に与える影響，そして細胞壁構造がイオンの移動度に与える支配的な影響に起因すると推定した。このように，含水率の変化に伴って生じる細胞壁構造と水分との相互的な作用が電気伝導のメカニズムと密接に関係するために，木材の電気伝導を論じるには水分の量だけでなく，細胞壁中の空げき構造を含めた水分の存在状態の影響を十分に考慮しなければならない．
The transient micro-capillaries, as formed in the wood cell wall with moisture uptake by wood, make the conducting path of various liquids in the wood cell walls and are also associated with a mechanism of swelling/shrinkage of wood. Therefore an importance of studying the relationship of physical properties and behaviours with the cell-wall structure should be pointed out. The mechanism of electrical conduction in wood, of which dominant current path is formed by the water-molecule layers in the cell walls as revealed by a remarkable effect of moisture content on the electric conductivity, must be dependent on the cell wall structure as well as moisture content. The purpose of the paper is to study the mechanism of response under the influence of electric stimulus through the dependence of conduction on frequency, applied voltage, and field direction, with relation to moisture content by using sugi (Clyptomeria japonica D. Don), and to discuss the dependence of the mechanism of electrical conduction on the mechanism of moisture adsorption includes the structural change of the cell walls with moisture content. The decrease of conductivity with decreasing frequency was extended to lower frequency range as moisture content and temperature were decreased, in the range from high (1 MHz) down to very low frequency (0.0001 Hz). The frequency dependence of conductivity in lower frequency range was affected by the applied voltage in the moisture・content range above 5% at 20℃, and it was therefore thought to be due to the irreversible polarization owing to the ionic conduction. The voltage effect on conductivity and conductive behaviour were developed with moisture content increased over 5%, and that were assumed to be due to the difference in quantity and mobility of ions forced to break from the bonding sites with applied voltage. On the other hand the behaviour of polarization worked for the decreasing conductivity with frequency below the moisture content of 5% was reversible, and it was not affected by the applied voltage. Thus the electrical conduction in the wood cell walls where the bound water molecules make just the mono-layer of water molecule does not accompany the ionic breakage from the bonding sites following the dissociation and migration of ions. The electrical anisotropy expressed as the ratio of longitudinal to transverse conductivity tended to increase with decreasing frequency and increasing voltage at air-dried condition which were supposed to affect the ionic mobility and the number of conducting ions. respectively. The anisotropy was also affected by the moisture content; it was increased with moisture content above 8% and was decreased below 8%. These anisotropic behaviours suggested to relate to the effect of intra-cell wall structure on mobility of conducting ions, or to the structural change of cell-wall structure with moisture content. especially of the inter-connecting structure of transient intra-cell wall capillaries. The electrical behaviours and those mechanism resulting from the conditional change of the field with frequency, intensity, and direction were different among moisture-content ranges, and it was therefore confimed that not only the quantity but the nature of bound water surrounded by the cell walls should be considered as far as the electrical conduction in wood is discussed. Thus the mechanism of electrical conduction in wood should be closely related to the interdependence of moisture and cell wall structure with the change of moisture content, through the effect of the attractive force induced among water molecules or between the water molecules and the wood substance on the degree of dissociation or the mobility of adsorbed ions, and through the dominant effect of intra-cell wall structure on the ionic mobility.続きを見る
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