Shah, B S

Vaidya, Nimisha

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Indian J. Phys. 77A (1). 59-61 (2003)U P  A— an international journalAbsolute hardness o f phenanthrene <h>ped anthracene single crystals by Knoopindentatfon technique!Nimisha Vaiiya*^ and B S wShahSolid State Physics and Materials Science Laboratories,Department of Physics, Saurashira Ufljlversiiy, P.O., Rajkol-360 005, Gujarat, India  ^ Christ College, P. B No 5, V i^a Niketan, Saurashira University, P.O., Rajkot-360 |K)5, Guiarat, IndiaH-inail . NVi^IDYA^'yahoo.coniReceived 21 Novenibei 2000, accepted 23 Septembet 2002Abstract ' The absolute hardness value of anthracene doped with phenanthrene using Knoop hardness technique has been obtained The .ibsokite baldness is defined as the hardness which is not load dependent The results obtained by graphical analysis compare well with the calculated results, within experimental error The present note reports the results obtained, using the method utilizing the extrapolation function The higher absolute hardness of the phenanthrene doped anthracene is due to the molecular hindrance to slipKeywords Absolute hardness, Knoop indentation, doped anthracenePACS Nos. 62.20.Fe, 81.40 NpIt IS a well known fad that the physical and chemical properties of materials arc effected by the addition of impurities. This depends on how the impurities enter the lattice. It has been icportcd f l j  that the doping of anthracene by different compounds does not alter the slip systems. The addition of impurities changes the activencss of the slip systems [2]. The present communication compares the absolute hardness of phenanthrene doped anthracene crystals with that of pure anthracene crystals [3].Single crystal cleavages of phenanthrene doped anthracene were indented on the (001) planes using the Knoop indenter. A large number of indentations were made at each load and the hardness value obtained using the following relation :Knoop hardness = 0.14 x PIL^ (MPa), (1)where P is the applied load in g and L is the mean diagonal length in micron. The indentation time of 10 s was kept constant this time was adequate to minimize the vibration effects on the results. The crystal size was much larger than the indentation thus eliminating the boundary effects on the results. The Corresponding Authordistance between the indents was five times the size of the largest indentation mark. The crystal thickness was relatively large such that the indenter did not sense the lower surface [4]. A large number of crystals were indented.The reciprocal indentation length was plotted against hardness and a best fit straight line was drawn from the points using the Harvard graphics software [5J. This method utilizes an extrapolation function to determine the true hardness [6| and is as follows :LetP = applied load,= observed length of indentation,L, = true length of indentation under applied load,AL = Lq = length correction of indentation,C = a constant,Wq = observed hardness,H, = true hardness.©20031ACS60 Nimisha Vaidva and B S ShahThenHo =PI { CI ^ ~)  and H, = P / ( CL , ^ ) .Tlicrcforc, Hq = H, L,^ (2)But L,=/ SL+I^ ,  hence, = H, {AL+ L ^ f .Therefore, Hq = H, [\ + A U  L ^ ) '. (3)Tlic precision of the hardness measurements is not better than \% so the quadratic term [AL!  is neglected. If (/dL//-())< 0.1 then//o = / / , + 2 ^ / / ,  (4)The correction term AL has been interpreted as being due to cither elastic recovery [7] or optical resolution [81. In either case, if AL is a constant and if the true hardness is independent of indentation size, then according to eq. (4), a plot of Hq versus 1 /Lq gives a straight line with an ordinate interceptand a slope (2 AL H f).The relation (4) was used to calculate the absolute hardness where = true hardness values obtained from Figures (1-2).Phenanthrene doped anthraceneFigure 1. Reciprocal indentation diagonal length versus hardness for phenanthrene doped anthracene.Table I presents the graphical and calculated absolute hardness values.Table 1. The true hardness values obtained from graphical and mathematical analysis.Crystal Knoop hardness Knoop hardness (graphical) (calculated)(MPa) (MPa)Anthracene 50.10Phenanthrene doped anthracene 92.0049.5590.64Figure 1 depicts the plot of reciprocal indentation length versus hardness for phenanthrene doped anthracene single crystals.Figure 2 depicts the plot of reciprocal indentation length versus hardness for pure anthracene single crystals.AnthraceneFigure 2. Reciprocal indentation diagonal length vetsus hardnes.s loi pure anthracene.The absolute hardness values obtained from the graphs (Figures 1,2) are used to find the length correction tenns AI. The calculated true hardness values arc found using the relations;(Knoop) = 0.14 X (MPa). (5)Tabic 1 prc.scnls the graphical and calculated absolute hardness values for both crystals. These values compare well The absolute hardness value for pure anthracene is 49.55 MPa whereas that c>f phenanthrene doped anthracene is 90.64 MPa Phenanthrene goes in substitutionally into the anthracene lattice. The shape of the phenanthrene molecule is not similar to the anthracene molecule, both being made up of three benzene rings and having the formula The molecular shape causes ahindrance in the motion of the slip planes giving rise to a highci absolute hardness value for phenanthrene doped anthracene single crystals.The absolute hardness value of phenanthrene doped anthracene obtained by the utilization of the extrapolation function is 90.64 MPa and is higher than that of pure anthracene which is 49.55 MPa. This increase in absolute hardness value is due to the molecular shape of the phenanthrene molecule which goes in substitutionally into the anthracene lattice and causes a hindrance to the motion of the slip planes.References[ 1 ] Nimisha Vaidya, J H Yagnik and B S Shah fndian J. Phys. 72A 295 (1998)(21 J H Yagnik Ph.O, Thesis (Saurashtra University, India) (1996)(5) Nimisha Vaidya and B S Shah {To he published) (2002)Absolute hardness of phenanthrene doped anthracene single rr^’stals etc 61B W  M o lt  M irro in d e n ta lio n  H a rd n e ss  T estin g  (L o n d o n  Buiierw orths) (19 5 6 )Harvard G raph ics IBM  C o rp o ia iio n  V fr  3 .0  M  L  Tarkanian. J P Ne um ann and L  Raym o nd The Scien ce olHardness Tesun); an d  its Keseart'h Applications  cd J H  Westbrook and Conrad (O h io  A S M )  p. 187 (19 75 )1 7 ]  I) R  Tate Tiaiis. ASM  35 3 74  (19 4 5 )|8 ) L  P Tarasov and N  W  Thibault 'Duns ASM  38 331 (1 9 4 7 )

Absolute Hardness of Phenanthrene Doped Anthracene Single Crystals by Knoop Indentation Technique

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Absolute Hardness of Phenanthrene Doped Anthracene Single Crystals by Knoop Indentation Technique

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