Electrical properties of biomorphic SiC ceramics and SiC/Si composites fabricated from medium density fiberboard

Elec ical p ope ies o biomo phic SiC ce amics and SiC/Si composi es ab ica ed om medium densi y ibe boa d T.S. O lo a (a), V.V. Popo (a), J. Quispe Cancapa (b), D. He nández Maldonado (b), E. En ique Maga ino (b), F.M. Va ela Fe ia (b), A. Ramí ez de A ellano (b), J. Ma ínezFe nández (b) (a)Io ePhysico echnical Ins i u e, Russian Academy o Sciences, Poli ekhnicheskayaul. 26, S . Pe e sbu g, 194021 Russia (b) Dp o. Física de la Ma e ia Condensada-ICMSE, Uni e sidad de Se illa-CSIC, Se illa 41080 Spain Abs ac A s udy has been made o he dependences o he elec ical esis i i y and he Hall coe icien on he empe a u e in he ange 1.8–1300 K and on magne ic ields o up o 28 kOe o he biomo phic SiC/Si (MDF-SiC/Si) composi e and biomo phic po ous SiC (MDF-SiC) based upon a i icial cellulosic p ecu so (MDF – medium densi y ibe boa ds). I has been shown ha elec ic anspo in MDF-SiC is e ec ed by ca ie s o n- ype wi h a high concen a ion o ∼1020 cm−3 and a low mobili y o ∼0.4 cm2 V−1 s−1. The speci ic ea u es in he conduc i i y o MDF-SiC a e explained by quan um e ec s a ising in diso de ed sys ems and equi ing quan um co ec ions o conduc i i y. The TEM s udies con i med he p esence o diso de ing s uc u al ea u es (nanoc ys alline egions) in MDF-SiC. The conduc i i y o MDF-SiC/Si composi e o igina es p ima ily om Si componen in he empe a u e ange 1.8–500 K and since ∼500 o 600 K he con ibu ion o MDF-SiC ma ix becomes dominan . Keywo ds C. Elec ical p ope ies; Biomo phicSiC;SiC/Si composi e; Elec on mic oscopy 1. In oduc ion The s udies o elec ical p ope ies o silicon ca bide (SiC) ce amics and SiC-based composi es ha e emendous scien i ic signi icance and p ac ical in e es as well. Due o hei high pe o mance cha ac e is ics a high empe a u es and good esis ance o co osi e en i onmen s he SiC ma e ials a e p omising o be used o hea e elemen s, esis ance he mome e s and he moelec ic powe gene a o s in space, au omo i e and ene gy ans o ma ion indus ies.1 The main ab ica ion p ocesses o SiC ce amics a e ca bo he mic educ ion o SiO2,2 eac i e compac ion,3 and 4 and ho sin e ing.5 and 6 Recen ly, na u al wood-de i ed biomo phic SiC (bio-SiC) ce amics ha e been a ma e o in e es .7, 8, 9, 10, 11, 12, 13 and 14 The p ocessing echnique o po ous SiC ce amics om wood in ol es he py olysis o na u al wood p ecu so s, 1 ollowed by he in il a ion o mol en silicon o o m silicon ca bide (SiC), e aining he ini ial wood po ous s uc u e.9, 10, 11, 12, 13 and 14 Some esidual Si emains in cellula po es o ming he eby SiC/Si composi e. The pu e po ous ma e ial, bio-SiC, is p oduced om he SiC/Si composi e by e ching silicon ou o he channel po es.15 Bo h biomo phic SiC/Si composi e and po ous SiC ce amic ( he composi e ma ix) ha e highly aniso opic s uc u e, eplica ing he cellula s uc u e o na u al wood (cellula channel po es elonga ed along he ee g ow h di ec ion). Some amoun o un eac ed ca bon is also p esen in biomo phic SiC/Si composi e as well as in po ous bio-SiC.13 The bio-SiC has se e al ad an ages o e con en ional SiC. I p esen s ela i ely ligh ma e ial wi h open po ous s uc u e, which possesses excellen mechanical p ope ies in comp ession and lexu e.16 The echnology o i s ab ica ion allows o ela i ely easy p oduc ion o complex shapes by using p elimina y machining o biomo phic ca bon p ecu so .12 The p ocessing o bio-SiC is much cheape because i occu s a empe a u es ha a e much lowe han hose equi ed o SiC sin e ing o ho -p essing echniques. The la e equi es empe a u es exceeding 2000 °C.4 and 5 The s uc u e and esul ing p ope ies, mechanical ones a leas , o bio-SiC and bio-SiC/Si s ongly depend on he o iginal wood p ecu so used.17 In es iga ion o he elec ical anspo p ope ies o his new class o ma e ials – biomo phic na u al wood-de i ed SiC/Si composi es and po ous bio-SiC has been ecen ly a ac ing conside able a en ion. Fo example, he elec ic esis i i y ρ o biomo phic SiC/Si composi es de i ed om sapele,18 whi e eucalyp us 19, 20 and 21 and beech22 was measu ed in he empe a u e ange 5–300 K. In Re .23, he empe a u e ange o measu emen o he dependence ρ(T) was ex ended o 950 K o he SiC/Si composi es ab ica ed om whi e eucalyp us. These s udies e ealed ha he elec ical esis i i y o biomo phic SiC/Si is aniso opic, i.e. he e is a di e ence be ween he esis i i y ρII measu ed along he axial di ec ion (pa allel o he ee g ow h di ec ion) and he esis i i y ρ⊥ measu ed along he ans e se di ec ion. In he empe a u e ange 4.2–300 K, bo h empe a u e dependences ρII(T) and ρ⊥(T) ollow semime allic pa e n: he elec ical esis i i y a ies insigni ican ly a low empe a u es and inc eases s a ing om ∼100 K. ρII o he biomo phic SiC/Si composi es a oom empe a u e a ies in he ange 10−3 o 10−2 Ω cm. The conduc i i y o biomo phic SiC/Si composi es (de i ed om eucalyp us21 and beech22) was ound o o igina e p ima ily om Si componen and o be p o ided by ca ies o p- ype wi h concen a ion o he o de o 1019 cm−3 and high mobili y (∼103 cm2 V−1 s−1 a 4.2 K).22 All kinds o wood aken as ini ial p ecu so s in he abo e conside ed wo ks belong o he wood wi h open po osi y.24 Howe e , ρ(T) dependences measu ed o bamboo wood-de i ed SiC/Si composi e exhibi ed a semiconduc ing pa e n o e a wide empe a u e ange (25–1000 K)25 ha is con a y o me allic ρ(T) beha iou o biomo phic SiC/Si om sapele,18 eucalyp us 19, 20 and 21 o beech.22 This di e ence seems o be explained by he speci ici y o bamboo s uc u e. As i was no ed in Re .24 ca bon p e o ms om bamboo con ain a combina ion o channel po es and closed cells ha impede he comple e in il a ion o Si. As a esul in e connec ing Si- illed channels ne wo k p obably is no o med. This can also lead o a la ge amoun o un eac ed ca bon which can o m own conduc ing pe cola ion pa hs. 2 The esis i i y o po ous bio-SiC ( om eucalyp us21 and beech22) a low empe a u es (4.2– 300 K) is se e al o de s o magni ude highe han he esis i i y o he espec i e SiC/Si composi es. The ρ(T) dependences o he bio-SiC demons a e semiconduc ing beha iou in he ange 4.2–300 K. The elec ical anspo in beech-de i ed bio-SiC is e ec ed by n- ype ca ie s wi h a high concen a ion o ∼1019 cm−3 and a low mobili y o ∼1 cm2 V−1 s−1.22 I has been concluded22 ha he elec ical anspo in he na u al beech-de i ed SiC occu s simila ly o he anspo in s ongly diso de ed semiconduc o s and could be desc ibed wi h heo y o quan um co ec ions o he conduc i i y.26 Nowadays p ecu so s which could be al e na i e o he na u al wood a e conside ed o be p omising o p ocessing bio-SiC and bio-SiC/Si ma e ials. These a e a i icial compac ed ibe boa ds o , mo e speci ically, medium densi y ibe boa ds (MDF). P ocessing echnology o MDF-based biomo phic SiC (MDF-SiC) and espec i e SiC/Si (MDF-SiC/Si) composi e is simila o ha used o na u al wood p ecu so s.27 and 28 The MDF-SiC and MDF-SiC/Si ha e some ad an ages compa ed wi h simila ma e ials p ocessed om na u al wood p ecu so . Upon ca boniza ion, he homogenei y and consis ency o ibe -boa ds esul in he o ma ion o a consis en , low-coas ha d ca bon.29 Because o be e homogenei y ca bonized ibe boa ds can be mo e easily machined in o complica ed shapes compa ed wi h ca bonized woods.29 Since MDF boa ds a e o med unde con olled condi ions, MDF-SiC can possess ep oducible homogenous s uc u e unlike na u al wood-de i ed SiC ma e ials he s uc u e o which depends on yea ly ings, clima e ac o s, e c. P ocessing o MDF by p essing allows a choice o densi y and po osi y o he p ecu so ha can modi y unc ional cha ac e is ics o inal MDF- SiC. The e a e only a ew pape s de o ed o mechanical p ope ies o MDF-SiC and MDF-SiC/Si composi es.27 and 28 Physical p ope ies, speci ically elec ical ones, o MDF-SiC ha e no been s udied, a all. The aim o he p esen wo k is o in es iga e elec ical p ope ies o po ous bio-SiC ce amic and bio-SiC/Si composi es bo h ob ained om MDF p ecu so s. Measu emen s ha e been made o empe a u e dependences o he elec ical esis i i y, as well as he Hall coe icien in a empe a u e ange o 1.8–300 K and magne ic ields o up o 28 kOe. The ype and concen a ion o ca ie s in MDF-SiC we e de e mined om measu emen s o he Hall coe icien . Elec ical p ope ies o MDF-SiC/Si and MDF-SiC a e compa ed wi h hose o simila biomo phic ma e ials ab ica ed om na u al wood (eucalyp us, beech, bamboo). In addi ion, we also p esen some o ou p elimina y da a on esis i i y- empe a u e beha iou o MDF-SiC and MDF-SiC/Si ma e ials a ele a ed empe a u es ( om 300 o 1300 K). 2. Expe imen al p ocedu e Po ous biomo phic SiC ce amic and SiC/Si composi e, bo h de i ed om MDF boa ds, ha e been p epa ed. A comme cial medium densi y (0.6–0.8 g/cm3) MDF boa ds we e chosen as p ecu so . The p ocessing echnique o MDF-based SiC (he eina e MDF-SiC) and SiC/Si (MDF- SiC/Si) composi es included se e al s ages. The p ocessing echnology is p esen ed in de ails in Re s.27 and 28. B ie ly, i s a MDF piece o 50 mm × 30 mm × 30 mm was py olized a 1050 °C in lowing a gon. As a esul , he bioca bon p e o m (MDF-C) was ob ained. 3 Unlike he bioca bon ob ained by py olysis o he na u al wood (eucalyp us, beech, sapele, pine), which has open channel po es along he ee g ow h di ec ion, he MDF-C has mo e homogeneous s uc u e.28 An example o he mic os uc u e o he MDF-C is shown in Fig. 1. Samples o MDF-C we e cu wi h dimensions o 3 mm × 3 mm × 20 mm, he long dimension being pe pendicula o he di ec ion o he load applica ion in MDF p essing. Each sample o MDF-C pe o m was in il a ed wi h an excess o Si o he s oichiome ic amoun needed o all he C amoun in he pe o m. The inal SiC/Si composi es we e o med. Pu e biomo phic MDF-SiC samples de i ed om MDF boa ds we e ob ained by e ching silicon away wi h a mix u e o hyd o luo ic and ni ic acids.15 The dependences o elec ical esis i i y and o he Hall cons an R on empe a u e and magne ic ield H we e measu ed by he s anda d ou -p obe echnique. In hese measu emen s he elec ic cu en was passed along he long side o he sample, i.e., pe pendicula o he p essing di ec ion in he pa en MDF boa d p ocessing. The concen a ion and mobili y o cha ge ca ie s we e de e mined by Hall me hod. The mic os uc u e o bio SiC (MDF) sample was cha ac e ized by scanning and ansmission elec on mic oscopy (SEM and TEM) using a Philips XL30 scanning elec on mic oscope and a Philips CM-200 ansmission elec on mic oscope espec i ely (Elec on Mic oscopy Se ice, CITIUS, Uni e si y o Se ille, Spain). Samples o SEM and TEM obse a ions we e ab ica ed ollowing con en ional echniques. 3. Resul s and discussion 3.1. Low- empe a u e s udies Low- empe a u e (4.2–300 K) s udies we e made on he samples ob ained on he base o he ini ial MDF boa d wi h densi y 0.8 g/cm3. The excess o Si aken o he il a ion was de e mined by a io PSi = 3.5 Pc whe e PSi and Pc a e he weigh ac ions o Si and MDF-C. Such a io p o ides app oxima ely 30 ol.% o emaining Si in he esul ing SiC/Si composi es a e he in il a ion. 3.1.1. MDF-de i ed ca bon p ecu so Since all biomo phic SiC/Si composi es as well as pu e po ous biomo phic SiC ma e ials de i ed om na u al wood o om MDF con ain some amoun o esidual ca bon,11 i s we s udied esis i i y- empe a u e dependences ρ(T) o MDF-C p e o ms which we e used o p ocessing MDF-SiC/Si composi es and po ous MDF-SiC ce amics. In Fig. 2 he esis i i y- empe a u e ρ(T) dependences o MDF-C p e o ms a e compa ed wi h hose o bioca bon p e o ms py olized om na u al wood (eucalyp us30 and pine31) a he same empe a u e o ca boniza ion ∼1000 °C. Fig. 2a co esponds o he po ous biomo phic ca bon samples, whe eas in Fig. 2b he ρ(T) dependences a e ecalcula ed wi h co ec ion o he po osi y. To make co ec ion o he po osi y i is necessa y o e e he esis ance o he wo king olume (c oss-sec ion) pa icipa ing in cha ge ans e . In he case o he wood-de i ed bioca bon samples (eucalyp us30 and pine31) wi h channel po osi y, he esis i i y along he ee g ow h di ec ion wi h co ec ion o he po osi y was ound acco ding o he exp ession.23 4 ρ1=ρ(1−P). He e, ρ and ρ1 a e he esis i i ies wi hou and wi h he co ec ion o he po osi y o he sample, espec i ely, and P is he po osi y. Fo MDF-C samples, in a ough app oxima ion, we could assume ha in he c oss-sec ion o he sample also only an a ea S1 = (1 − P)S ans e s he cu en . He e S and S1 a e he c oss-sec ions o he sample wi hou and wi h co ec ion o he po osi y, espec i ely. The ob ained ρ1(T) dependence o MDF-C wi h allowance o po osi y is shown in Fig. 2b (cu e 1). All he ρ(T) dependences demons a e simila cha ac e o semiconduc ing pa e n. The alues o ρ1 in he empe a u e ange 5–300 K o MDF-C a e e y close o hose o he pine-de i ed bioca bon. I is known ha comme cial MDF boa ds a e mainly p oduced om pine ibe p ecu so s. 32 and 33 3.1.2. MDF-de i ed SiC/Si composi e and po ous MDF-SiC Fig. 3a displays ypical empe a u e dependence o elec ical esis i i y ρ o MDF-SiC/Si composi e. The alue o ρ g ows wi h inc easing empe a u e om abou 50 o 300 K. Such beha iou is a signa u e o me allic conduc ion. Howe e o lowe empe a u es MDF-SiC/Si composi e shows weak semiconduc ing beha iou . Simila beha iou was also obse ed o biomo phic SiC/Si composi e p ocessed on he base o na u al wood o di e en ype. Fo compa ison, ρ(T) dependences o biomo phic SiC/Si composi es de i ed om na u al wood (eucalyp us21 and beech22) a e p esen ed in Fig. 3b. Fo SiC/Si composi es de i ed om he same ype o wood, he alue o esis i i y a oom empe a u e, ρ300, a ies om 0.002 o 0.02 Ω cm. 18, 19, 20 and 21 I is dependen on he olume ac ion o Si and esidual po osi y in he composi e.21 As a whole, he ρ(T) dependences and he esis i i y a oom empe a u e o MDF-SiC/Si composi es a e e y simila o hose o biomo phic SiC/Si de i ed om he wood wi h open po osi y, like eucalyp us,21 beech,22 sapele,18 bu e y di e en om hose o biomo phic SiC/Si de i ed om Indian bamboo.25 The ρ(T) dependence o bamboo-de i ed SiC/Si composi es was o a semiconduc ing ype in he wide empe a u e ange 25–1073 K.25 As i was no ed in he In oduc ion, bamboo-wood is cha ac e ized by he p esence o closed cells ha impede he comple e in il a ion o Si and mos p obably p e en he o ma ion o in e connec ing Si- illed channel ne wo k. Thus, he ρ(T) dependence o MDF-SiC/Si is simila o ha o he SiC/Si composi es de i ed om he wood wi h open po osi y. As demons a ed below, he elec ical esis i i y o he MDF-SiC/Si composi e is mo e han 2 o de s o magni ude lowe han ha o he MDF-SiC ob ained a e ex ac ion o esidual silicon. Hence simila o he case o biomo phic na u al wood-de i ed-SiC/Si composi es, 21 and 22 he esis i i y o MDF-SiC/Si composi e is de e mined mainly by silicon. Typical dependences o ρ(T) o MDF-SiC a e shown in Fig. 4a. To compa e ρ(T) dependences o MDF-SiC and o bio-SiC de i ed om na u al wood, i is necessa y o co ec o hei po osi y. We assume ha simila o he case o na u al wood p ecu so s, 21 and 22 he po osi y o he inal MDF-SiC is abou he same as ha o he MDF-C p ecu so . We es ima ed ρ1(T) dependence o MDF-SiC in he same way as i was done o he MDF-C samples. The ob ained ρ1(T) dependence o he MDF-SiC is shown in Fig. 4b (cu e 3). The ρ1(T) dependence o MDF- SiC is simila o ha o he wood-de i ed bio-SiC (eucalyp us21 and beech22) (Fig. 4b). A oom empe a u e, he elec ical esis i i y ρ300 o di e en MDF-SiC samples ab ica ed om 5 MDF (0.8 g/cm3 densi y) was ound o be in he ange 0.12–0.2 Ω cm ha is consis en wi h he ρ300 alues o he wood-de i ed bio-SiC (Table 1). The ca ie concen a ion N can be de i ed om Hall coe icien (R) measu emen s in a magne ic ield. Fig. 5 plo s he dependence o he Hall ol age UR on magne ic ield H measu ed a 77 K o MDF-SiC. Gene ally he alue o he Hall coe icien R is de i ed om he low- ield pa s whe e R ∼ UR/H ≈ cons .22 Then he concen a ion o ca ie s can be ound as N = 1/eR (e is he elec on cha ge) and hei mobili y as μ = R/ρ. The es ima ion o ca ie concen a ion om he linea pa o he UR(H) dependence yields N ≈ 1.6 1020 cm−3 ha is app oaching o ‘me allic’ concen a ion. The ob ained N- alue in MDF-SiC is highe by abou one o de o magni ude han he ca ie concen a ion in he beech wood-de i ed SiC (Table 1). The he mopowe measu emen s showed elec on conduc ion (n- ype) in MDF-SiC. Es ima ion o in eg a ed mobili y μ = R/ρ a 4.2 K yields e y low alue μ ≈ 0.4 cm2 V−1 s−1 which is compa able wi h he mobili y in beech wood-de i ed SiC (Table 1). High ca ie concen a ion (app oaching me allic concen a ion) in combina ion wi h elec ical esis i i y g ow h wi h dec easing empe a u e canno be explained by classical sca e ing mechanism. Simila elec ical anspo cha ac e is ics we e ound also o he pine wood- de i ed bioca bon31 and he beech wood-de i ed bio-SiC22 and we e explained by quan um e ec s which a ise when cha ge anspo occu s in diso de ed sys ems. The s uc u al inhomogenei ies can es ic d ama ically he ee pa h o elec ons leading he eby o he necessi y o ake in o accoun he quan um co ec ions o he conduc i i y, such as in e e ence o elec on wi h i sel ( he co ec ion o weak localiza ion) o mu ual elec on in e e ence ( he co ec ion o elec on-elec on in e ac ion).26 S uc u al inhomogenei ies we e iden i ied as nano-c ys alli es in nano-amo phous bioca bon30 and 31 and as ‘colonies’ o nano-sized SiC g ains in he beech wood-de i ed SiC.22 The conduc i i ies o nano-amo phous bioca bon31 and he beech wood-de i ed SiC22 we e well desc ibed wi h ega d o he quan um co ec ions o conduc i i y. Since he inal ine mic os uc u e o biomo phic SiC depends on po e size o wood-de i ed cabon p ecu so s,14 we pe o med elec on mic oscopy s udies o mic os uc u e speci ici ies o MDF-SiC. Fig. 6 p esen s an elec on mic oscopy image o he ypical mic os uc u e o he MDF-SiC samples, which demons a es he p esence o mic o- and nanoc ys allineSiC and esidual amo phous ca bon. Es ima ion o he g ain size L yields L ≈ 10–70 nm o nano-SiC and L ≈ 1–10 μm o he mic o-SiC. The misc os uc u e o MDF-SiC is e y simila o ha o he na u al wood-de i ed SiC. 12, 14 and 22 Thus, elec ical cha ge anspo h ough he nanoc ys alline egions can esul in he con ibu ion o quan um e ec s o esis i i y ha equi es aking in o accoun quan um co ec ions o conduc i i y26 and explains non-classical (non-me allic) ρ(T) dependences o MDF-SiC wi h nea ly ‘me allic’ ca ie concen a ion. The esis ance o MDF-SiC can be also media ed by cha ge anspo h ough esidual amo phous ca bon laye s which we e shown 32 and 33 o con ain g aphi e-like nanoc ys alli es. 3.2. Ele a ed- empe a u e s udies 6 The s udies o elec ical p ope ies a ele a ed empe a u es om 300 o 1300 K we e ca ied ou on he samples ob ained on he base o he ini ial MDF boa ds wi h di e en densi ies om 0.6 o 0.8 g/cm3. The con en o Si he MDF-SiC/Si samples a ies in he limi s 30–50 ol. %. We measu ed he empe a u e dependences o esis i i y o he MDF-SiC/Si composi es and app op ia e po ous MDF-SiC samples in he empe a u e ange 300–1300 K. Typical ρ(T) dependences o MDF-SiC/Si composi es and po ous MDF-SiC a e shown in Fig. 7a and b, espec i ely. The alues o oom empe a u e esis i i y a y in a wide ange om 0.08 o 0.2 Ωcm o he MDF-SiC samples and om 0.003 o 0.015 Ωcm o he MDF-SiC/Si samples. Such a he high a ia ion in he ρ300 alues o di e en samples o he same ma e ial is caused by di e ence in densi y (po osi y) o he ini ial MDF p ecu so s and in he amoun o un eac ed esidual ca bon and he Si con en ( o he MDF-SiC/Si composi es). Despi e his di e ence all he ρ(T) dependences o MDF-SiC samples ollow semiconduc ing pa e ns in he whole empe a u e ange 300–1300 K as well as a low empe a u es (5–300 K). Howe e , he ρ(T) dependences o he composi e changes me allic pa e n o he semiconduc ing pa e n beginning a T = 500–600 K as he empe a u e inc eases. Such change in he conduc i i y ype o he MDF-SiC/Si composi e wi h inc easing T mos likely occu s because he con ibu ions o bo h componen s (MDF-SiC and Si) become compa able nea 500 K and hen MDF-SiC ma ix can de e mine he cha ac e o conduc i i y o he composi e. Thus, conduc i i y o MDF-SiC/Si composi e is de e mined mainly by Si componen a low empe a u es, whe eas MDF-SiC ma ix is mo e esponsible o he composi e conduc i i y a high empe a u es (>500 K). 4. Conclusions The p esen in es iga ion has shown ha empe a u e dependence o esis i i y o MDF-SiC/Si composi e has signa u e o me allic cha ac e jus as o biomo phic SiC/Si composi es de i ed om na u al wood wi h open po osi y (eucalyp us, beech, sapele). The conduc i i y o SiC/Si in he empe a u e ange 5–500 K o igina es p ima ily om silicon esiding in cellula po es o he MDF-SiC ma ix. Such po es illed by Si seem o o m in e connec ing ne wo k. Because o i s dependence on esidual po osi y and he Si con en , he esis i i y o MDF-SiC/Si composi e a oom empe a u e a ies in a ange om 0.003 o 0.02 Ω cm. Simila o na u al wood-de i ed bio-SiC ma e ials, he ρ(T) dependence o po ous biomo phic MDF-SiC has semiconduc ing cha ac e , i.e. he elec ical esis i i y g ows wi h dec easing empe a u e in he whole empe a u e ange 5–1300 K. The main cha ge ans e pa ame e s in MDF-SiC ha e been de e mined by Hall me hod: he ca ie s a e n- ype wi h concen a ion ∼1020 cm−3 and e y low mobili y ∼0.4 cm2 V−1 s−1. In MDF-SiC, he con adic ion be ween he high bulk ca ie concen a ion app oaching he me allic one and semiconduc ing ype o he ρ(T) dependence is explained by quan um e ec s which a ise in diso de ed conduc ing sys ems and con ibu e o he esis i i y. Ou TEM s udies did e eal he p esence o diso de ing elemen s in he s uc u e o MDF-SiC, speci ically nanoc ys allineSiC egions, cha ge ans e h ough which can esul in he necessi y o accoun ing quan um co ec ions o he conduc i i y. The anspo in MDF-SiC occu s simila ly o ha in he bio-SiC de i ed om eucalyp us21 and beech.22 7 A T > 500 K, he cha ac e o he ρ(T) dependence o MDF-SiC/Si composi e changes om me allic o semiconduc ing ype. This seems o occu because a high empe a u es he con ibu ion o MDF-SiC ma ix o he conduc i i y o MDF-SiC/Si composi e becomes subs an ial and e en mo e decisi e compa ed wi h he con ibu ion o Si componen . Acknowledgemen s This s udy was suppo ed by he Russian Basic Resea ch Founda ion (p ojec no. 7-03- 91353NNF_a), he P esidium o he Russian Academy o Sciences (p og am no. P-03), and he Spanish P ojec s MAT 2007-30141-E and PET 2006-0658. 8 Re e ences 1.H. Siebe , C. Ho mann, A. Kaindl, P. G eil Biomo phic cellula ce amics Ad Eng Ma e , 2 (2000), pp. 105–109 2. T. Kosolapo a Ca bides p ope ies, p oduc ion and applica ions Plenum P ess, New Yo k, London (1971) 3. C. Fo es , P. Kenedy, J. Shennam The ab ica ions and p ope ies o sel -bonded silicon ca bide Special Ce am, 5 (1972), pp. 99–123 4. O. Chak aba i, S. Ghosh, J. Muke ji In luence o g ain size, ee silicon con en and empe a u e in he s eng h and houghness o eac ion bonded silicon ca bide Ce am In , 5 (2) (1991), pp. 118–123 5. S. P ochazka The ole o bo on and ca bon in he sin e ing o silicon ca bide Special Ce am B Ce am Res Assoc, 6 (1975), pp. 171–181 6. C. G esko ic, J.H. Rosolowski Sin e ing o co alen solids J Am Ce am Soc, 59 (7–8) (1976), pp. 336–343 7. T. O a, M. Takahashi, T. Hibi, M. Ozawa, S. Suzuki, Y. Hikichi Biomime ic p ocess o p oducing SiC “Wood” J Am Ce am Soc, 78 (12) (1995), pp. 3409–3411 8. P. G eil, T. Li ka, A. Kaindl Biomo phic cellula silicon ca bide ce amics om wood. I. P ocessing and mic os uc u e J Eu Ce am Soc, 18 (1998), pp. 1961–1973 9. P. G eil, T. Li ka, A. Kaindl Biomo phic cellula silicon ca bide ce amics om wood: II. Mechanical p ope ies 9 Figu e 2 16 Figu e 3 17 Figu e 4 18 Figu e 5 19 Figu e 6 20 Figu e 7 21