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Ultrasonic pre-treatment for enhanced diamond nucleation
ELSEVIER
Diamond
and Related
Materials
5 (1996) 206-210
?llOND RELATED MATERIALS
Ultrasonic pre-treatment for enhanced diamond nucleation Kasper 0. Schweitz I, Ralf B. Schou-Jensen, Svend S. Eskildsen Danish Technological Institute, DTI Tribology Centre, Teknologiparken, DK-8000 Aarhus C, Denmark
Abstract The nucleation of diamond has been investigated as a function of surface pre-treatment by ultrasonic agitation of diamond powder in different liquids. The diamond nucleation density on silicon has been measured as a function of ultrasonic treatment time and liquid properties. A standard ultrasonic bath for cleaning has been used and diamond powder was tested in 11 liquids with densities between 0.63 and 2.89 g cmm3. Diamond was deposited in a microwave plasma CVD system. The influence of pre-treatment on nucleation density has been related to ultrasonic theory, and correlations have been found between the surface tension and the density of the liquid. The results indicate that by themselves, jet streams from collapsing cavities cannot form surface defects acting as nucleation sites, whereas particles accelerated by the jet streams can. The density of cavities close to the surface is important and the velocity of the diamond particles scales linearly with the diamond nucleation density. Keywords:
Ultrasonic agitation; Diamond nucleation; Ultrasonic cavitation
1. Introduction Enhancement of nucleation of diamond by surface pre-treatment is still not explained [ 11. An increase in the nucleation density occurs as a result of polishing, scratching, blasting or other types of mechanical actions on a surface by powders, preferably diamond, before diamond deposition [1,2]. Yugo et al. [3] found that ultrasonic agitation of diamond powder suspensions gave similar results. More recently, an initial in-situ step of biased enhanced nucleation [4] has also proved a very useful technique. Control of nucleation density and the following influence on diamond grain size of the coating is technologically important in many applications, and the ultrasonic pre-treatment method is potentially a way of treating three-dimensional objects. Only very few investigations of the method are found in diamond literature [ 51. The purpose of this work has been to investigate the influence of the dispersion medium, the diamond powders and the ultrasonic waves on the diamond nucleation density and especially to investigate a possible correlation to ultrasonic theory.
2. Ultrasonic theory Sound waves at frequencies above 16 kHz are called ultrasound. When these waves pass through a liquid, 1Present address: Aarhus, DK-8000
Institute of Physics and Astronomy, Aarhus C, Denmark.
University
0925-9635/96/$15.00 0 1996 Elsevier Science S.A. All rights reserved SSDI 0925-9635(95)00499-8
of
the longitudinal pressure waves alternate between expansion and compression waves [6,7]. If and when the expansion waves can pull the liquid apart and overcome the surface tension of the liquid, a bubble (cavitation) is formed. By different mechanisms the cavity will grow slowly and at a critical size (e.g. 170 pm diameter at 20 kHz) it will most efficiently absorb energy and grow rapidly in one expansion cycle. After this, it cannot absorb energy efficiently, and during the next compression cycle it will not continue to withstand the surface tension of the liquid. The liquid will rush in and the cavity implodes within less then 1 ps. Close to surfaces the implosion is asymmetrical and a fast liquid jet stream is directed towards the surface with typical velocities of 30-150 ms-l. The surface tension to be overcome by an expansion wave is reduced by the addition of particles to the liquid, since gas trapped in small cracks of the particles will expand and form the start of a cavity. The cavity activity increases with grain size, and the threshold is higher for smaller particles. For a given particle size the concentration of cavities is proportional to the particle concentration, but the influence from grain size is more profound [ 81. Cavities are also formed at microscopic cracks or scratches on the surface. The velocity of the jet stream can be calculated by [6]:
K. 0. Schweitz et al./Diamond and Related Materials 5 (19%) 206-210
where q-l = (Ro/R)3y, q:=O.O2, P= 1 atm, R cavity radius, R, cavity start radius, y ratio between the specific heat capacities at constant pressure and constant volume, p density of liquid, and p0 density of particle material. Particles exposed to jet streams will get a velocity of [9]: u = & (p -. pO) U/p (2), which can be more than the sound velocity in the liquid. From the ultrasonic waves particles only have velocities range [lo]. The pressure from the jet in themm-’ stream is normally a majjor cause of damage, and is called the water hammer pressure [ 111: Pwh = pcU, where c is the sound velocity in the liquid.
3. Experimental details Deposition was made on as-received, polished (111) Si wafers cut into 7 x 7 mm pieces. Before pre-treatment the silicon was ultrasonically cleaned in acetone and ethanol for 5 min. for each cleaning process, and after pre-treatment it was ultra.sonically cleaned for 5 min. in ethanol. The ultrasonic pre-treatment was performed in a 240 W, 35 kHz cleaning apparatus (Branson 8200) filled with approx. 20 1 water. 37.5 or 375 mg of dry diamond powder was poured into 15 ml of liquid in a 23 ml glass bottle with screw cap. To ensure maximum suspension of the powder ultrasonic agitation for 3 min. was followed by turning each bottle upside-down 10 times. Immediately after this and prior to the start of the ultrasonic pre-treatment, the Si was put into the bottle in a fixture made of Ta t.hread, placing the Si 15 mm above the bottom to avoid any influence from sediments and to ensure the same position independent of the density of the liquid. Four glass bottles were lowered simultaneously into the water bath at symmetrical positions giving about 105 mm from the bottom to the Si pieces. The water bath was tempered to about 30 “C at the start of the treatment and the temperature increased roughly 5 “C per 30 min. of ultrasonic agitation. The 11 liquids used are shown in Table 1 with some of their properties and some calculated values. Dry diamond powder De Beers Micron + MDA of sizes l-2 urn and 6-12 pm were used. Diamond was grown using an Astex HPMD microwave system with etching in hydrogen plasma for one minute followed by deposition in a hydrogen-methane plasma for 1 h with the following parameters: pressure 5.3 x lb Pa, power 1500 W, substrate holder temperature 800 “C, flow of 500 seem hydrogen and 3.5 seem methane. Nucleation density was evaluated by counting the number of diamond grains on a scanning electron microscope (SEM) picture at x 5000. The overall reproducibility of the nucleation density was found to be within 50%. Micro-Raman spectroscopy was done in selected cases to ensure the diamond quality.
4. Results and discussions 4.1. Nucleation density as a function of pre-treatment time
Hexane and 1,3-propanediol were used with 375 and 37.5 mg of 6-12 urn diamond powder, respectively. Sedimentation was observed for hexane and the suspension was unclear. For 1,3_propanediol no sedimentation was observed and the suspension was clear. The nucleation density results are plotted in Fig.1. The nucleation density for samples which have only been subjected to ultrasonic cleaning is 2-6 x lo7 cm-‘. An increase of the nucleation density as a function of pre-treatment time is seen, and it is observed that treatment in hexane results in higher nucleation density than in 1,3-propanediol, where 10 times less diamond powder is used. A linear fit shows the slope to differ by a factor of 9. Okubu et al. [12] found a nucleation density of approx. 10” cmv2 in hexane under similar conditions with two possible differences, one being lower power, and the other that only suspended particles affect the result in our case. The increase of the nucleation density as a function of pre-treatment time has been observed by Mehlmann et al. [ 131 for WC-6%Co for 3 different powder concentrations. This is also observed for powder polishing [14]. A saturation is seen in these cases, but does not occur in our case because of low ultrasonic power and lower diamond particle concentration at the sample position. The results from hexane can be fitted with an exponential function of the type N(t) = a - b x exp( - t/r), where a, b and z are constants. We find a saturation value of aprrox. 2 x lo9 cmm2 and a time constant of approx. 200 min. 4.2. Nucleation density as a function of dispersion medium All 11 liquids were tested in 2 series using 375 mg of l-2 urn and 6-12 urn of diamond powder, respectively. The pre-treatment time was 30 min. in all cases. Various degrees of sedimentation were observed in both series. For the l-2 pm series all results were at the level of non pre-treated silicon of 2-6 x lo7 cmp2, except for carbon tetrachloride which showed a nucleation density of 2 x 108cm- ‘. Consequently, no correlation between the density and any liquid properties could be seen. Table 2 and Figs. 2-4 show the nucleation density results from the 6-12 urn series. Despite a difference in suspension concentration a number of relations can be seen. The low value for pentane, with a boiling point of 36 “C, can possibly be ascribed to the fact that the liquid is close to boiling after 30 min. in the ultrasonic bath. For liquids with a low surface tension a high nucleation density is seen (Fig. 2). This observation corresponds with the fact that the formation of cavities is easier for liquids with low surface tension. No correlation can be
K 0. Schweitz et al. /Diamondand Related Materials 5 (1996) 206-210
208
Table 1 List of liquids used in this work as dispersion media for diamond powder and some relevant properties and calculated values. Not all property values were available. Velocities for 1,3-propanediol were calculated using the minimum and maximum y values from the other liquids as an estimate Jet stream velocity (m s-r)
Diamond particle velocity (m s-t)
Water hammer pressure (P& (MPa)
Surface tension ( 10e4 N m-r)
36
45.6
208
29.6
_
69 216
45.2 43.7
195 161
32.8 42.7
1.843 _
0.789 0.79 1.000 1.0597
78 56 100 214
39.3 39.6 32.8 36.8-31.9
135 137 82.4 73.7-85.0
35.7 36.6 48.7 -
2.189 2.370 7.305 _
cc14 BrCH,CH,Br, CH,Br,
1.594 2.17 2.4970
77 131 96
28.0 24.1 21.8
33.8 14.7 8.81
41.6 52.8 52.6
2.695 _ _
CHBq
2.8899
149
20.9
4.47
56.1
4.153
Liquid
Formula
Density (g cme3)
Pentane
CHs(CH,),CH,
0.632
Hexane Dodecane
CHs(CH,)&H, CH3(CH,),,CH3
0.659 0.750
Ethanol (ethyl alcohol) 2-Propanone (acetone) Water 1,3-Propanediol (trimethylene glycol)
C2HSOH CH,COCH, Hz0 HO(CH,),OH
Tetrachloromethane (carbon tetrachloride) 1,2_Dibromoethane (ethylene dibromide) Dibromomethane (methylene bromide) Tribomomethane (bromoform)
140 120
Boiling point (“C)
70 7
A 0
I
I
hexane 1,3-propanediol
I
I
I
,
60
A
t
l
0
20
40
60
Pre-treatment
80
100
120
time [min.]
0
1
2
3
4
5
6
7
8
Surface tension of dispersion medium [I O4 N/m]
Fig. 1. Dependence of diamond nucleation density on ultrasonic pretreatment time in suspensions of hexane and 1,3-propanediol. Linear fits are also shown.
Fig. 2. Dependence of diamond nucleation density on surface tension of dispersion medium for ultrasonic pre-treatment.
Table 2 Nucleation density of diamond on silicon for ultrasonic pre-treatment in 6-12 pm diamond powder suspensions
found with the water hammer pressure, indicating that by themselves the jet streams cannot produce defects on the surface which act as diamond nucleation sites. Very good correlation is observed between the calculated particle velocity and the nucleation density (Fig. 3). Comparison with the calculated particle velocity and with this velocity squared cannot differentiate whether the momentum or the energy of the particle is the decisive factor in the creation of nucleation sites. The correlation is seen as a strong indication of the correctness of the hypothesis that it is the impact of particles accelerated by jet streams from collapsing cavities that creates nucleation sites. If this is the case, there should also be a dependence of the nucleation density on the density of the liquid, which is a primary and well
Liquid
Nucleation density ( x lo7 cm?)
Pentane Hexane Dodecane Ethanol Acetone Water 1,3-Propanediol Carbon tetrachloride 1,2Dibromoethane Dibromomethane Bromoform
1.8 62 52 35 29 16 3.0 14 5.0 12 22
K. 0. Schweitz et al./Diamond and Related Materials 5 (1996) 206-210
‘Or-----l
1
l
60
0 0
0
100 Theoretical
200
diamond particle velocity [m/s]
Fig. 3. Dependence of diamond nucleation density on theoretical diamond particle velocities resulting from jet streams in various dispersion media.
7ol-T-------l
0
1
2
They find that large metal particles give rise to the highest nucleation densities, while the smallest particles give only a modest increase compared to a suspension of diamond only, which corresponds with the fact that grain size is more important to cavity formation than the number of particles, and that the threshold of cavity formation is higher for smaller particles. Also, all of the reported results from the variations of Fe/diamond suspensions are in accordance with the hypothesis that diamond particles accelerated by jet streams onto the surface are important to the nucleation density. By this bombardment diamond residues are most probably left on the surface and act as nucleation sites, as has been claimed by several investigations (eg. [ 141). Anger et al. [ 161 have documented the increased roughness of silicon and the independence of the 45 pm powder used. They touch upon the sonochemical effects that may indeed be responsible for changing the surface chemical state and a part of the nucleation enhancement, but they also find that diamond residues are the most important cause of enhancement and therefore their findings are also in accordance with our hypothesis.
5. Conclusions
IO
0
209
3
Density of dispersion medium [g/cm31
Fig.4. Dependence of diamond nucleation density on density of dispersion medium for ultrasonic pre-treatment. A fit to the theoretical relationship is also shown.
determined property. Fig. 4 shows this dependence as well as the fitted curve describing the theoretical relationship from combining Eqns. 1 and 2. This correlation shows that liquid density is the most important property of the dispersion medium used. Recently, Chakk et al. [lS] have shown that the diamond nucleation density increases by adding metal particles to a diamond suspension. In view of our findings we believe that Chakk et al. observe the result of enhanced cavity formation and thereby enhanced bombardment of diamond particles onto the surface.
The first evidence has been presented relating the theories of ultrasonic cavitation to the diamond nucleation enhancement by pre-treatment in diamond powder suspensions under ultrasonic agitation. Direct damage from liquid jet streams is not responsible for the enhancement. A linear relationship between the nucleation density and the velocity or the velocity squared of diamond powders being accelerated by the jet stream is evidence of the importance of the diamond particle momentum or energy to the nucleation enhancement. A correlation between the nucleation density and the density of the dispersion liquid is found experimentally and is supported by ultrasonic theory, which supports the hypothesis that cavity formation during ultrasonic agitation is a necessary factor in obtaining nucleation enhancement.
Acknowledgements This work was supported in part by the EU BriteEuRam project COTE and by the Danish Framework Programme for Low Temperature Surface Technology and Associated Minicentre for Nanotribology.
References [l] R. Haubner and B. Lux, Proc. 3rd Int. Symp. on Diamond Materials, Honolulu, May, 1993, The Electrochemical Society, Pennington, 1993, p. 198.
210
K. 0. Schweitz et al./Diamond and Related Materials 5 (1996) 206-210
[Z] S.S. Eskildsen, Proc. 2nd Int. Conf on the Applications of Diamond Films and Related Materials, Omiya, August, 1993,
MYU, Tokyo, 1993, p. 291. [3] S. Yugo, T. Kimura and H. Kanai, Proc. 1st. Int. Conf of New Diamond Science and Technology, Tokyo, October, 1988, KTK Scientific, Tokyo, 1990, p. 119. [4] S. Yugo, T. Kanai, T. Kimura and T. Muto, Appl. Phys. Lett., 58 (1991) 1036. [ 53 S.S. Eskildsen,
DlAmond Database, Danish Technological Institute, Denmark, 19881995. [6] V.A. Shutilov, Fundamental Physics of Ultrasound, Gordon and Breach, New York, 1988. [7] K.S. Suslick, MRS Bulletin, 20, 4 (1995) 29. 181 S.I. Madanshetty and R.E. Apfel, J. Acoust. Sot. Am., 90 (1991) 1508.
[9] R.H. Cole, Underwater explosions, Princeton University Press, Princeton, 1948. [lo] Z. Mandralis, W. Bolek, W. Burger, E. Benes and D.L. Feke, Ultrasonics. 32 (1994) 113.
[ll]
S.A. Perusich
and R. C Alkire, J. Efectrochem.
Sot.,
138
(1991) 700. [ 121 T. Okubo, S. Nakata, H. Nagamoto, M. Ihara and H. Komiyama, Jpn. J. Appl. Phys., 32 (1993) L1767. [13] A.K. Mehlmann, A. Fayer, S.F. Dirnfeld, Y. Avigal, R. Porath and A. Kochman, Diam. Relat. Mater., 2 (1993) 317. [ 141 E.J. Bienk and S.S. Eskildsen, Diam. Relat. Mater., 2 (1993) 432. [15] Y. Chakk, R. Brener and A. Hoffman, Appl. Phys. Lett., 66 (1995) 2819. [ 161 E. Anger, A. Gicquel, Z.Z. Wang and M.F. Ravet, Diam. Relat. Mater., 4 (1995) 759.
Diamond
and Related
Materials
5 (1996) 206-210
?llOND RELATED MATERIALS
Ultrasonic pre-treatment for enhanced diamond nucleation Kasper 0. Schweitz I, Ralf B. Schou-Jensen, Svend S. Eskildsen Danish Technological Institute, DTI Tribology Centre, Teknologiparken, DK-8000 Aarhus C, Denmark
Abstract The nucleation of diamond has been investigated as a function of surface pre-treatment by ultrasonic agitation of diamond powder in different liquids. The diamond nucleation density on silicon has been measured as a function of ultrasonic treatment time and liquid properties. A standard ultrasonic bath for cleaning has been used and diamond powder was tested in 11 liquids with densities between 0.63 and 2.89 g cmm3. Diamond was deposited in a microwave plasma CVD system. The influence of pre-treatment on nucleation density has been related to ultrasonic theory, and correlations have been found between the surface tension and the density of the liquid. The results indicate that by themselves, jet streams from collapsing cavities cannot form surface defects acting as nucleation sites, whereas particles accelerated by the jet streams can. The density of cavities close to the surface is important and the velocity of the diamond particles scales linearly with the diamond nucleation density. Keywords:
Ultrasonic agitation; Diamond nucleation; Ultrasonic cavitation
1. Introduction Enhancement of nucleation of diamond by surface pre-treatment is still not explained [ 11. An increase in the nucleation density occurs as a result of polishing, scratching, blasting or other types of mechanical actions on a surface by powders, preferably diamond, before diamond deposition [1,2]. Yugo et al. [3] found that ultrasonic agitation of diamond powder suspensions gave similar results. More recently, an initial in-situ step of biased enhanced nucleation [4] has also proved a very useful technique. Control of nucleation density and the following influence on diamond grain size of the coating is technologically important in many applications, and the ultrasonic pre-treatment method is potentially a way of treating three-dimensional objects. Only very few investigations of the method are found in diamond literature [ 51. The purpose of this work has been to investigate the influence of the dispersion medium, the diamond powders and the ultrasonic waves on the diamond nucleation density and especially to investigate a possible correlation to ultrasonic theory.
2. Ultrasonic theory Sound waves at frequencies above 16 kHz are called ultrasound. When these waves pass through a liquid, 1Present address: Aarhus, DK-8000
Institute of Physics and Astronomy, Aarhus C, Denmark.
University
0925-9635/96/$15.00 0 1996 Elsevier Science S.A. All rights reserved SSDI 0925-9635(95)00499-8
of
the longitudinal pressure waves alternate between expansion and compression waves [6,7]. If and when the expansion waves can pull the liquid apart and overcome the surface tension of the liquid, a bubble (cavitation) is formed. By different mechanisms the cavity will grow slowly and at a critical size (e.g. 170 pm diameter at 20 kHz) it will most efficiently absorb energy and grow rapidly in one expansion cycle. After this, it cannot absorb energy efficiently, and during the next compression cycle it will not continue to withstand the surface tension of the liquid. The liquid will rush in and the cavity implodes within less then 1 ps. Close to surfaces the implosion is asymmetrical and a fast liquid jet stream is directed towards the surface with typical velocities of 30-150 ms-l. The surface tension to be overcome by an expansion wave is reduced by the addition of particles to the liquid, since gas trapped in small cracks of the particles will expand and form the start of a cavity. The cavity activity increases with grain size, and the threshold is higher for smaller particles. For a given particle size the concentration of cavities is proportional to the particle concentration, but the influence from grain size is more profound [ 81. Cavities are also formed at microscopic cracks or scratches on the surface. The velocity of the jet stream can be calculated by [6]:
K. 0. Schweitz et al./Diamond and Related Materials 5 (19%) 206-210
where q-l = (Ro/R)3y, q:=O.O2, P= 1 atm, R cavity radius, R, cavity start radius, y ratio between the specific heat capacities at constant pressure and constant volume, p density of liquid, and p0 density of particle material. Particles exposed to jet streams will get a velocity of [9]: u = & (p -. pO) U/p (2), which can be more than the sound velocity in the liquid. From the ultrasonic waves particles only have velocities range [lo]. The pressure from the jet in themm-’ stream is normally a majjor cause of damage, and is called the water hammer pressure [ 111: Pwh = pcU, where c is the sound velocity in the liquid.
3. Experimental details Deposition was made on as-received, polished (111) Si wafers cut into 7 x 7 mm pieces. Before pre-treatment the silicon was ultrasonically cleaned in acetone and ethanol for 5 min. for each cleaning process, and after pre-treatment it was ultra.sonically cleaned for 5 min. in ethanol. The ultrasonic pre-treatment was performed in a 240 W, 35 kHz cleaning apparatus (Branson 8200) filled with approx. 20 1 water. 37.5 or 375 mg of dry diamond powder was poured into 15 ml of liquid in a 23 ml glass bottle with screw cap. To ensure maximum suspension of the powder ultrasonic agitation for 3 min. was followed by turning each bottle upside-down 10 times. Immediately after this and prior to the start of the ultrasonic pre-treatment, the Si was put into the bottle in a fixture made of Ta t.hread, placing the Si 15 mm above the bottom to avoid any influence from sediments and to ensure the same position independent of the density of the liquid. Four glass bottles were lowered simultaneously into the water bath at symmetrical positions giving about 105 mm from the bottom to the Si pieces. The water bath was tempered to about 30 “C at the start of the treatment and the temperature increased roughly 5 “C per 30 min. of ultrasonic agitation. The 11 liquids used are shown in Table 1 with some of their properties and some calculated values. Dry diamond powder De Beers Micron + MDA of sizes l-2 urn and 6-12 pm were used. Diamond was grown using an Astex HPMD microwave system with etching in hydrogen plasma for one minute followed by deposition in a hydrogen-methane plasma for 1 h with the following parameters: pressure 5.3 x lb Pa, power 1500 W, substrate holder temperature 800 “C, flow of 500 seem hydrogen and 3.5 seem methane. Nucleation density was evaluated by counting the number of diamond grains on a scanning electron microscope (SEM) picture at x 5000. The overall reproducibility of the nucleation density was found to be within 50%. Micro-Raman spectroscopy was done in selected cases to ensure the diamond quality.
4. Results and discussions 4.1. Nucleation density as a function of pre-treatment time
Hexane and 1,3-propanediol were used with 375 and 37.5 mg of 6-12 urn diamond powder, respectively. Sedimentation was observed for hexane and the suspension was unclear. For 1,3_propanediol no sedimentation was observed and the suspension was clear. The nucleation density results are plotted in Fig.1. The nucleation density for samples which have only been subjected to ultrasonic cleaning is 2-6 x lo7 cm-‘. An increase of the nucleation density as a function of pre-treatment time is seen, and it is observed that treatment in hexane results in higher nucleation density than in 1,3-propanediol, where 10 times less diamond powder is used. A linear fit shows the slope to differ by a factor of 9. Okubu et al. [12] found a nucleation density of approx. 10” cmv2 in hexane under similar conditions with two possible differences, one being lower power, and the other that only suspended particles affect the result in our case. The increase of the nucleation density as a function of pre-treatment time has been observed by Mehlmann et al. [ 131 for WC-6%Co for 3 different powder concentrations. This is also observed for powder polishing [14]. A saturation is seen in these cases, but does not occur in our case because of low ultrasonic power and lower diamond particle concentration at the sample position. The results from hexane can be fitted with an exponential function of the type N(t) = a - b x exp( - t/r), where a, b and z are constants. We find a saturation value of aprrox. 2 x lo9 cmm2 and a time constant of approx. 200 min. 4.2. Nucleation density as a function of dispersion medium All 11 liquids were tested in 2 series using 375 mg of l-2 urn and 6-12 urn of diamond powder, respectively. The pre-treatment time was 30 min. in all cases. Various degrees of sedimentation were observed in both series. For the l-2 pm series all results were at the level of non pre-treated silicon of 2-6 x lo7 cmp2, except for carbon tetrachloride which showed a nucleation density of 2 x 108cm- ‘. Consequently, no correlation between the density and any liquid properties could be seen. Table 2 and Figs. 2-4 show the nucleation density results from the 6-12 urn series. Despite a difference in suspension concentration a number of relations can be seen. The low value for pentane, with a boiling point of 36 “C, can possibly be ascribed to the fact that the liquid is close to boiling after 30 min. in the ultrasonic bath. For liquids with a low surface tension a high nucleation density is seen (Fig. 2). This observation corresponds with the fact that the formation of cavities is easier for liquids with low surface tension. No correlation can be
K 0. Schweitz et al. /Diamondand Related Materials 5 (1996) 206-210
208
Table 1 List of liquids used in this work as dispersion media for diamond powder and some relevant properties and calculated values. Not all property values were available. Velocities for 1,3-propanediol were calculated using the minimum and maximum y values from the other liquids as an estimate Jet stream velocity (m s-r)
Diamond particle velocity (m s-t)
Water hammer pressure (P& (MPa)
Surface tension ( 10e4 N m-r)
36
45.6
208
29.6
_
69 216
45.2 43.7
195 161
32.8 42.7
1.843 _
0.789 0.79 1.000 1.0597
78 56 100 214
39.3 39.6 32.8 36.8-31.9
135 137 82.4 73.7-85.0
35.7 36.6 48.7 -
2.189 2.370 7.305 _
cc14 BrCH,CH,Br, CH,Br,
1.594 2.17 2.4970
77 131 96
28.0 24.1 21.8
33.8 14.7 8.81
41.6 52.8 52.6
2.695 _ _
CHBq
2.8899
149
20.9
4.47
56.1
4.153
Liquid
Formula
Density (g cme3)
Pentane
CHs(CH,),CH,
0.632
Hexane Dodecane
CHs(CH,)&H, CH3(CH,),,CH3
0.659 0.750
Ethanol (ethyl alcohol) 2-Propanone (acetone) Water 1,3-Propanediol (trimethylene glycol)
C2HSOH CH,COCH, Hz0 HO(CH,),OH
Tetrachloromethane (carbon tetrachloride) 1,2_Dibromoethane (ethylene dibromide) Dibromomethane (methylene bromide) Tribomomethane (bromoform)
140 120
Boiling point (“C)
70 7
A 0
I
I
hexane 1,3-propanediol
I
I
I
,
60
A
t
l
0
20
40
60
Pre-treatment
80
100
120
time [min.]
0
1
2
3
4
5
6
7
8
Surface tension of dispersion medium [I O4 N/m]
Fig. 1. Dependence of diamond nucleation density on ultrasonic pretreatment time in suspensions of hexane and 1,3-propanediol. Linear fits are also shown.
Fig. 2. Dependence of diamond nucleation density on surface tension of dispersion medium for ultrasonic pre-treatment.
Table 2 Nucleation density of diamond on silicon for ultrasonic pre-treatment in 6-12 pm diamond powder suspensions
found with the water hammer pressure, indicating that by themselves the jet streams cannot produce defects on the surface which act as diamond nucleation sites. Very good correlation is observed between the calculated particle velocity and the nucleation density (Fig. 3). Comparison with the calculated particle velocity and with this velocity squared cannot differentiate whether the momentum or the energy of the particle is the decisive factor in the creation of nucleation sites. The correlation is seen as a strong indication of the correctness of the hypothesis that it is the impact of particles accelerated by jet streams from collapsing cavities that creates nucleation sites. If this is the case, there should also be a dependence of the nucleation density on the density of the liquid, which is a primary and well
Liquid
Nucleation density ( x lo7 cm?)
Pentane Hexane Dodecane Ethanol Acetone Water 1,3-Propanediol Carbon tetrachloride 1,2Dibromoethane Dibromomethane Bromoform
1.8 62 52 35 29 16 3.0 14 5.0 12 22
K. 0. Schweitz et al./Diamond and Related Materials 5 (1996) 206-210
‘Or-----l
1
l
60
0 0
0
100 Theoretical
200
diamond particle velocity [m/s]
Fig. 3. Dependence of diamond nucleation density on theoretical diamond particle velocities resulting from jet streams in various dispersion media.
7ol-T-------l
0
1
2
They find that large metal particles give rise to the highest nucleation densities, while the smallest particles give only a modest increase compared to a suspension of diamond only, which corresponds with the fact that grain size is more important to cavity formation than the number of particles, and that the threshold of cavity formation is higher for smaller particles. Also, all of the reported results from the variations of Fe/diamond suspensions are in accordance with the hypothesis that diamond particles accelerated by jet streams onto the surface are important to the nucleation density. By this bombardment diamond residues are most probably left on the surface and act as nucleation sites, as has been claimed by several investigations (eg. [ 141). Anger et al. [ 161 have documented the increased roughness of silicon and the independence of the 45 pm powder used. They touch upon the sonochemical effects that may indeed be responsible for changing the surface chemical state and a part of the nucleation enhancement, but they also find that diamond residues are the most important cause of enhancement and therefore their findings are also in accordance with our hypothesis.
5. Conclusions
IO
0
209
3
Density of dispersion medium [g/cm31
Fig.4. Dependence of diamond nucleation density on density of dispersion medium for ultrasonic pre-treatment. A fit to the theoretical relationship is also shown.
determined property. Fig. 4 shows this dependence as well as the fitted curve describing the theoretical relationship from combining Eqns. 1 and 2. This correlation shows that liquid density is the most important property of the dispersion medium used. Recently, Chakk et al. [lS] have shown that the diamond nucleation density increases by adding metal particles to a diamond suspension. In view of our findings we believe that Chakk et al. observe the result of enhanced cavity formation and thereby enhanced bombardment of diamond particles onto the surface.
The first evidence has been presented relating the theories of ultrasonic cavitation to the diamond nucleation enhancement by pre-treatment in diamond powder suspensions under ultrasonic agitation. Direct damage from liquid jet streams is not responsible for the enhancement. A linear relationship between the nucleation density and the velocity or the velocity squared of diamond powders being accelerated by the jet stream is evidence of the importance of the diamond particle momentum or energy to the nucleation enhancement. A correlation between the nucleation density and the density of the dispersion liquid is found experimentally and is supported by ultrasonic theory, which supports the hypothesis that cavity formation during ultrasonic agitation is a necessary factor in obtaining nucleation enhancement.
Acknowledgements This work was supported in part by the EU BriteEuRam project COTE and by the Danish Framework Programme for Low Temperature Surface Technology and Associated Minicentre for Nanotribology.
References [l] R. Haubner and B. Lux, Proc. 3rd Int. Symp. on Diamond Materials, Honolulu, May, 1993, The Electrochemical Society, Pennington, 1993, p. 198.
210
K. 0. Schweitz et al./Diamond and Related Materials 5 (1996) 206-210
[Z] S.S. Eskildsen, Proc. 2nd Int. Conf on the Applications of Diamond Films and Related Materials, Omiya, August, 1993,
MYU, Tokyo, 1993, p. 291. [3] S. Yugo, T. Kimura and H. Kanai, Proc. 1st. Int. Conf of New Diamond Science and Technology, Tokyo, October, 1988, KTK Scientific, Tokyo, 1990, p. 119. [4] S. Yugo, T. Kanai, T. Kimura and T. Muto, Appl. Phys. Lett., 58 (1991) 1036. [ 53 S.S. Eskildsen,
DlAmond Database, Danish Technological Institute, Denmark, 19881995. [6] V.A. Shutilov, Fundamental Physics of Ultrasound, Gordon and Breach, New York, 1988. [7] K.S. Suslick, MRS Bulletin, 20, 4 (1995) 29. 181 S.I. Madanshetty and R.E. Apfel, J. Acoust. Sot. Am., 90 (1991) 1508.
[9] R.H. Cole, Underwater explosions, Princeton University Press, Princeton, 1948. [lo] Z. Mandralis, W. Bolek, W. Burger, E. Benes and D.L. Feke, Ultrasonics. 32 (1994) 113.
[ll]
S.A. Perusich
and R. C Alkire, J. Efectrochem.
Sot.,
138
(1991) 700. [ 121 T. Okubo, S. Nakata, H. Nagamoto, M. Ihara and H. Komiyama, Jpn. J. Appl. Phys., 32 (1993) L1767. [13] A.K. Mehlmann, A. Fayer, S.F. Dirnfeld, Y. Avigal, R. Porath and A. Kochman, Diam. Relat. Mater., 2 (1993) 317. [ 141 E.J. Bienk and S.S. Eskildsen, Diam. Relat. Mater., 2 (1993) 432. [15] Y. Chakk, R. Brener and A. Hoffman, Appl. Phys. Lett., 66 (1995) 2819. [ 161 E. Anger, A. Gicquel, Z.Z. Wang and M.F. Ravet, Diam. Relat. Mater., 4 (1995) 759.