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ISSN 1063-7834, Physics of the Solid State, 2006, Vol. 48, No. 9, pp. 1711–1715. © Pleiades Publishing, Inc., 2006.Original Russian Text © B.K. Kardashev, A.S. Nefagin, B.I. Smirnov, A.R. de Arellano-Lopez, J. Martinez-Fernandez, R. Sepulveda, 2006, published in Fizika Tverdogo Tela,2006, Vol. 48, No. 9, pp. 1617–1621.
1711
1. INTRODUCTION
This study is a continuation of the systematic exper-imental investigations [1–3] into the elastic and inelas-tic characteristics of biomorphic materials preparedfrom different kinds of wood. Of most interest are pres-ently biomorphic SiC/Si composites (called also ecoce-ramics, a term coined from environment consciousceramics), which are prepared by pyrolysis (carboniza-tion) of a chosen wood with subsequent infiltration ofmolten silicon into a carbon matrix [4]. The chemicalreaction with the carbon matrix produces cubic siliconcarbide (3
C
-SiC) and, in the presence of excess silicon,the SiC/Si composite. The composites form cellularchannel structures characteristic of each kind of woodin which silicon carbide, empty channels, and channelsfilled with excess silicon are arranged along the direc-tion of tree growth.
In [1], the elastic properties of biomorphic SiC/Sicomposites prepared from oak and eucalyptus woodwere studied at elevated temperatures (up to 1300 K).The effect of vibrational strain amplitude on theYoung’s modulus
E
and the logarithmic decrement
δ
ofthe SiC/Si composite samples prepared from whiteeucalyptus were studied at moderate temperatures(116–296 K) [2]. It was shown that, although this com-posite is destroyed without a noticeable residual strain,it is subjected to a considerable nonlinear inelasticmicroplastic strain at large amplitudes. It was also dem-onstrated that, because the SiC/Si composite containspores and residual carbon, adsorption and desorption of
the environmental (air) molecules affect both the elasticmodulus and ultrasonic damping. The conclusionsdrawn from an analysis of the data obtained for theSiC/Si composite samples were confirmed in our previ-ous study [3], in which similar effects were observed insamples of carbon biomatrix also prepared from whiteeucalyptus.
At present, there is a set of experimental data on theelastic and inelastic properties of the biomorphic car-bon matrix prepared from white eucalyptus and theeucalyptus-based SiC/Si composite. Similar studies ofbiomorphic SiC, which can be prepared from theSiC/Si composite by chemically removing excess sili-con [5], have become a topic of considerable currentinterest.
We report here on a comparison of the resultsobtained in acoustic studies of the elastic and inelasticproperties of biomorphic SiC/Si composites preparedfrom oak and eucalyptus with experimental data on bio-morphic SiC, which does not contain excess silicon(after the removal of excess silicon from the abovecomposites).
2. SAMPLE PREPARATION AND EXPERIMENTAL TECHNIQUES
Biomorphic SiC/Si ceramic samples were preparedby vacuum infiltration of molten silicon into a porouscarbonized wood (Spanish oak haya and white eucalyp-
DEFECTS AND IMPURITY CENTERS, DISLOCATIONS, AND PHYSICS OF STRENGTH
Elastic and Inelastic Properties of SiC/Si Biomorphic Composites and Biomorphic SiC Based on Oak and Eucalyptus
B. K. Kardashev
a
, A. S. Nefagin
a
, B. I. Smirnov
a
, A. R. de Arellano-Lopez
b
, J. Martinez-Fernandez
b
, and R. Sepulveda
b
a
Ioffe Physicotechnical Institute, Russian Academy of Sciences, Politekhnicheskaya ul. 26, St. Petersburg, 194021 Russiae-mail: [email protected]
b
Universidad de Sevilla, Sevilla, 41080 Spain
Received December 2, 2005
Abstract
—This paper reports on the results of a comparative investigation into the elastic and microplasticproperties of biomorphic SiC/Si composites and biomorphic SiC prepared by pyrolysis of oak and eucalyptuswith subsequent infiltration of molten silicon into a carbon matrix and additional chemical treatment to removeexcess silicon. The acoustic studies were performed by the composite oscillator technique using resonant lon-gitudinal vibrations at frequencies of about 100 kHz. It is shown that, in biomorphic SiC (as in biomorphicSiC/Si) at small-amplitude strains
ε
, adsorption and desorption of the environmental (air) molecules determineto a considerable extent the Young’s modulus
E
and the internal friction (decrement of acoustic vibrations
δ
)and that the changes in
E
and
δ
at these amplitudes are irreversible. The stress–microplastic strain curves areconstructed from the acoustic data for the materials under study at temperatures of 100 and 290 K.
PACS numbers: 62.20.Fe, 62.20.Dc, 81.40.Jj
DOI:
10.1134/S1063783406090150
1712
PHYSICS OF THE SOLID STATE
Vol. 48
No. 9
2006
KARDASHEV et
al.
tus tree) after pyrolysis in an argon atmosphere at1000
°
C [6].
The highly specific, biologically complex cellularstructure of the wood containing channels arranged pre-dominantly along the direction of tree growth persistsin the biocarbon matrix. This matrix has empty chan-nels of two types [7], namely, with large diameters(average size of ~30
µ
m for oak and ~60
µ
m for euca-lyptus) and small diameters (~4
µ
m in both cases). Theoverall volumetric fraction of these pores for oakamounts to ~36.8%, and that for eucalyptus, to ~43.3%.The chemical reaction of silicon infiltrating into chan-nels of the biocarbon matrix produces cubic silicon car-bide 3
C
-SiC. The SiC/Si biocomposite is formed from3
C
-SiC, excess silicon loaded in the channels of thematrix, and a small amount of carbon unreacted withsilicon [4].
We prepared biomorphic SiC by chemically remov-ing excess silicon [5] from SiC/Si biocomposites.
The samples intended for acoustic studies were pre-pared in the form of 45- to 50-mm-long rectangularrods ~(4
×
4) mm in cross section oriented along thedirection of tree growth. The measurements were con-ducted using the composite oscillator technique [8]. Alongitudinal standing wave was excited in the sampleattached to a quartz transducer. The lengths of the half-wave rod samples and of the quartz transducer werechosen such that the resonant vibration frequency was
approximately equal to 100 kHz. The measurements ofthe Young’s modulus
E
and the decrement
δ
were con-ducted consecutively under an increasing and decreas-ing vibrational strain amplitude
ε
in the range from~10
–7
to ~(1–3)
×
10
–4
.The acoustic measurements were performed, as in
[2, 3], according to the following procedure. First, theamplitude dependences
E
(
ε
) and
δ
(
ε
) were measuredusing an as-prepared sample stored for a long time afterpreparation under atmospheric pressure at room tem-perature in air. After this, the acoustic system (i.e., thesample attached to the transducer) was placed in a vac-uum chamber. All subsequent measurements of thedependences
E
(
ε
) and
δ
(
ε
) were conducted under vac-uum (~10
–3
mm Hg) both at room (~290 K) and low(100 K) temperatures. The dependences
E
(
ε
) were usedto construct stress–microplastic strain curves by thetechnique proposed in [9, 10]. These graphs offer a pic-torial comparison of the microplastic properties of var-ious materials (and also of the macroplastic propertiesin some cases).
3. EXPERIMENTAL RESULTS AND DISCUSSION
Figures 1 and 2 show the amplitude dependences ofthe Young’s modulus
E
and the logarithmic decrement
δ
of biomorphic SiC and biomorphic SiC/Si compositesamples prepared from oak. In these experiments, theas-prepared samples were subjected to large-amplitudestrains at room temperature (290 K). We readily seethat, as the vibrational load on the sample graduallyincreases, the elastic modulus increases noticeably inboth cases, whereas the damping decreases (except fora few of the last points at large values of
ε
, where thedecrement may increase weakly and the modulusdecrease). The changes in
E
and
δ
observed in the first
ε
-increasing run are irreversible. Indeed, neither themodulus nor the decrement recovers its original valuewith decreasing
ε
. Subsequent measurements of
E
(
ε
)and
δ
(
ε
) practically coincide with the curves obtained inthe first
ε
-decreasing run. Further tests performed undervacuum revealed a similar behavior of
E
(
ε
) and
δ
(
ε
).Figure 3 presents the data obtained at a low (100 K)
temperature for the same biomorphic SiC sample pre-pared from oak. Note that, at moderate amplitudes (upto
ε
~ 10
–4
), the
E
modulus measured in the first strain-increasing run also decreases insignificantly. In thisexperiment, the increase observed in
E
at large-ampli-tude strains is associated with the values of
ε
exceedingits maximum value in Fig. 1 by about a factor of 3.Obviously, the large-amplitude strains applied to thegiven sample for the first time gave rise to the desorp-tion of air molecules, an effect which adds to the onethat had been observed at room temperature.
It is appropriate to note that, for both biomorphicSiC/Si and SiC ceramics prepared from oak, the depen-dences
E
(
ε
) and
δ
(
ε
) resemble qualitatively similar
60100
δ
, 10
–5
ε
, 10
–7
1
2
E
, GPa
80
100
120
143.8
143.9
144.0
100010
T
= 290 K
1
1
1
2
Fig. 1.
Amplitude dependences of the Young’s modulus
E
and the decrement
δ
for an oak-based biomorphic SiC sam-ple measured twice consecutively in the (
1
) first and (
2
) sec-ond runs at an interval of about 1 min. The measurementswere performed under atmospheric pressure in air. Arrowsindicate the directions of variation in
ε
.
T
= 290 K.
PHYSICS OF THE SOLID STATE
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No. 9
2006
ELASTIC AND INELASTIC PROPERTIES 1713
21
δ
, 10
–5
ε
, 10
–7
1
2
E
, GPa
23
25
27
216.94
216.96
216.98
100
T
= 290 K
100010
1
2
1
,
2
Fig. 2.
Amplitude dependences of the Young’s modulus
E
and the decrement
δ
for an oak-based biomorphic SiC/Sisample measured twice consecutively in the (
1
) first and(
2
) second runs at an interval of about 1 min. The measure-ments were performed under atmospheric pressure in air.Arrows indicate the directions of variation in
ε
.
T
= 290 K.
δ
, 10
–5
ε
, 10
–7
1
2
E
, GPa
10
15
20
144.6
144.7
144.8
1000
T
= 100 K
100
144.9
1
1
1
1
1
2
Fig. 3.
Amplitude dependences of the Young’s modulus
E
and the decrement
δ
for an oak-based biomorphic SiC sam-ple preliminarily subjected to large-amplitude strains at290 K and measured twice consecutively in the (
1) first and(2) second runs at an interval of about 1 min under vacuum.Arrows indicate the directions of variation in ε. T = 100 K.
1
0
σ, M
Pa
εd, 10–8
1234
1 2 3 4
1
2
3
4
0.1
10
100
T = 290 K
SiC/Si (eucalyptus)SiC (eucalyptus)SiC/Si (oak)SiC (oak)
Fig. 4. Stress–microplastic strain curves obtained fromacoustic measurements for different samples of the biomor-phic SiC ceramics and SiC/Si composites at 290 K.
0
σ, M
Pa
εd, 10–8
1234
1 2
1
2
3
4
10
T = 100 K
SiC/Si (eucalyptus)SiC (eucalyptus)SiC/Si (oak)SiC (oak)
100
1
Fig. 5. Stress–microplastic strain curves obtained fromacoustic measurements for different samples of the biomor-phic SiC ceramics and SiC/Si composites at 100 K.
1714
PHYSICS OF THE SOLID STATE Vol. 48 No. 9 2006
KARDASHEV et al.
curves obtained earlier on eucalyptus-based biomor-phic SiC/Si [2]. The same applies to the eucalyptus-based biomorphic SiC ceramics.
Thus, our results suggest that, both in the biomor-phic SiC freed of excess silicon and in the biomorphicSiC/Si composite, adsorption and desorption of airmolecules exerts a considerable effect on the Young’smodulus and the decrement of the elastic vibrations.The desorption initiated by large-amplitude strainbrings about irreversible consequences, such as anincrease in the E modulus and a decrease in the decre-ment δ. Similar effects (an increase in E and a decreasein δ) also occur in biomorphic SiC under evacuation.
At the same time, at large amplitudes, the reaction ofthe material turns out to be related not only to mole-cules of adsorbed gases. The large-amplitude rangewhere one observes an increase in the damping and adecrease in the modulus with increasing strain ampli-tude is characteristic of many materials exhibiting plas-ticity [8]. This behavior of E and δ in the biomorphicSiC/Si and SiC samples under study, which is observed,as a rule, in repeated measurements of E(ε) and δ(ε),suggests that these materials involve structural ele-ments that act as mobile dislocations in crystals, thusproviding noticeable microplastic deformation underultrasonic loading. The acoustic measurements per-formed here on the biomorphic SiC/Si and SiC samplespermit one to construct stress–microplastic strain (σ–εd) diagrams. The procedure used to construct thesediagrams from the dependences E(ε) was described in[9, 10].
The mechanical (elastic and microplastic) charac-teristics of the materials studied here are presented inthe table. The microplastic properties are displayed inmore detail in Fig. 4 (at room temperature) and Fig. 5(at 100 K). The dependences σ(εd) shown in Figs. 4 and5 were constructed from data similar to curves 2 forE(ε) presented in Figs. 1–3. As is evident from the fig-ures and the table, the highest mechanical characteris-tics were found in the SiC/Si composite samples pre-pared from white eucalyptus (at 100 K, the microplasticstrain εd for this material becomes barely noticeable atthe largest vibrational stresses σ reached in the experi-ment). Similar samples prepared from oak are charac-terized by lower values of the Young’s modulus E andthe conventional microyield point σs. Removal of
excess silicon brings about a substantial decrease in Eand σs, both for white eucalyptus and for oak. Interest-ingly, this decrease in σs for white eucalyptus turns outto be more pronounced than that for oak. This becomesparticularly evident in low-temperature measurements(Fig. 5).
It does not presently appear possible to pinpoint thereasons for the observed dependence of the elastic andmicroplastic properties of the ceramics under study onthe species of wood. Obviously enough, the specificfeatures of the structure of white eucalyptus and oakshould manifest themselves somehow in the measuredvalues of E and σs. These features may be connectedboth with the different shapes and geometric sizes ofthe cells and pores and with the different elementalcompositions of different wood species. Answers tothese questions may be found in further studies.
4. CONCLUSIONS
Thus, it has been shown that the elastic and micro-plastic properties of SiC bioceramics depend substan-tially on the following factors: (i) the species of wood,from which the ceramics was prepared; and (ii) thepresence of excess silicon in it, which noticeablystrengthens the material. The results obtained areaccounted for by the different chemical compositionsof the original wood, as well as by the presence of poresand other structural defects, which can affect themechanical properties of the materials to a certainextent. Moreover, the adsorption and desorption ofmolecules of the ambient medium determine to a con-siderable extent the effective Young’s modulus and theacoustic vibrational decrement for all the materialsunder investigation.
ACKNOWLEDGMENTS
This study was supported by the Russian Founda-tion for Basic Research (project no. 04-03-33183), thePresidium of the Russian Academy of Sciences (pro-gram P-03), and the Ministry of Science and Technol-ogy of Spain (project no. MAT 2003-05202-CO2-01).
Density ρ, Young’s modulus E, and microyield point σs at the inelastic strain εd = 0.4 × 10–8 for different samples of siliconcarbide ceramics
MaterialT = 290 K T = 100 K
ρ, g/cm3 E, GPa σs, MPa E, GPa σs, MPa
Biomorphic SiC/Si (eucalyptus wood) 2.28 235.12 73 235.55 >90
Biomorphic SiC (eucalyptus wood) 2.00 182.75 5.7 183.27 7.8
Biomorphic SiC/Si (oak wood) 2.01 216.93 28 217.28 51
Biomorphic SiC (oak wood) 1.64 144.08 4.2 144.84 19
PHYSICS OF THE SOLID STATE Vol. 48 No. 9 2006
ELASTIC AND INELASTIC PROPERTIES 1715
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Translated by G. Skrebtsov
