Nucleation enhanced

The dispersant property of the CPI is impeded by the nucleation enhancement performance of a pour point improver. In other words, the CPI functions to inhibit crystal growth while the pour point improver functions to maintain and control crystal growth. These two performance properties result in an antagonism between... 

Therefore, the heat transfer coefficients given in this table are only for very rou estimating purposes and assume the use of plain or low-Tinned tubes without special nucleation enhancement. th maximum allowable temperature difference between... 

The most common technique involves abrading the substrate surface with hard powders (preferably diamond), or ultrasonic treatment in a slurry of an abrasive powder (preferably diamond grit) in an organic liquid for a prolonged period. This type of pretreatment leads to embedding of the abrasive particulates into the surface as well as mechanical damage to the substrate, both of which are believed to enhance the nucleation density. The maximum nucleation enhancement is observed when diamond grit is used, and this has been attributed to implantation of diamond particles... 

In an effort to enhance diamond nucleation and to control film morphology, extensive work on the nucleation and early growth stages has been performed. As a result, technology problems associated with the nucleation of polycrystalline diamond films have been adequately addressed. A number of nucleation enhancement methods have been developed that enable the control of nucleation density over several orders of magnitude. Nucleation density has been increased from < 10 cm on untreated substrates up to 10 cm on scratched or biased substrates. The effects of surface conditions on nucleation processes have been investigated to provide the guideline for the selection of optimum surface pretreatment methods. In this chapter, substrate materials, surface pretreatment methods and their influences on diamond nucleation are discussed. 

SURFACE PRETREATMENT METHODS AND NUCLEATION ENHANCEMENT MECHANISMS... 

In the following sections, the methods which can modify substrate surface conditions and enhance diamond nucleation are reviewed and the corresponding nucleation enhancement mechanisms are discussed. 

Scratching substrates with diamond powder or other abrasive materials significantly enhances surface nucleation density and rate compared to untreated substrates.t hi70][i79] different suggestions have been made to account for the mechanisms of the nucleation enhancement by scratching (Fig. 2). The first of these is the seeding effect, i.e., diamond, DLC,... 

Figure 2. Schematic diagram of mechanisms for diamond nucleation enhancement on scratched substrates. l (Reproduced with permission.)...

Yugo et al.P employed a negative bias and high CH4 concentrations during pretreatment to generate diamond nuclei on a Si mirror surface in PACVD. The several-minute pretreatment resulted in an enormous nucleation enhancement. Diamond nucleation densities as high as 10 cm were achieved. For the onset of diamond nucleation, a minimum voltage of -70 V and a minimum concentration of 5 vol.% CH4 in H2 were necessary. 

The mechanisms of the nucleation enhancement by biasing have been addressed in several studies, as depicted in Fig. 8. 

Figure 8. Schematic diagram showing the mechanisms of diamond nucleation enhancement on biased substrates, (a) Negative biasing carbon-containing cations are accelerated toward the substrate surface, (b) Positive biasing electrons are accelerated toward the substrate surface and bombard carbon-containing molecules adsorbed on the surface. (Reproduced with permission.)...

Carbon ion implantation on a single crystal copper surface at a temperature of 820°C, an ion dose of 10 ions cm and a beam energy of 65 to 120 keV was found to result in an enhancement of diamond nucleation. The nucleation enhancement was postulated to be due to the formation of a graphite film on the copper sur ce, with subsequent diamond nucleation occurring preferentially on the edges of the graphite lattice. 

Nucleation enhancement on precarburized W and Mo substrates has been observed. These metals form carbides suchas W2CandWC,aswell... 

Yehoda et al. ° presented a method to catalyze the nucleation and growth of diamond films in MW PACVD without seeding substrates. A thin film of Fe, Cu, Ti, Nb, Mo, or Ni was abraded or deposited onto SiC-coated substrate surfaces. The metal films resulted in varying degrees of diamond nucleation enhancement. A qualitative ordering of the best to the worst nucleating metals was established to be Fe, Cu, Ti, Ni, Mo to Nb at the substrate center (hot area), and Fe, Nb, Cu, Mo, Ti, to Ni away fi om the center (cold Fe exhibiting the most pronoimced effect on diamond... 

Carburization of substrates (Mo, W, Si, Fe/Si) leads to diamond nucleation enhancement due to the formation of carbides and the saturation of carbon at the substrate surface. 

Compared to the significant development in nucleation enhancement methods, fundamental scientific issues related to diamond nucleation processes remain less well addressed. In this chapter, the theoretical and modeling studies on surface nucleation of diamond are reviewed on the basis of available literature. 

As discussed in Ch. 6, diamond nucleation enhancement may be achieved by, for example, predeposition of a thin DLC layer in combustion flame or DC glow-discharge plasmas, or, by carburization/biasing to develop a complete SiC layer on the substrate surface in MW PACVD. 

K. V. Ravi, and C. A. Koch, Nucleation enhancement of diamond synthesized by combustion flame techniques, Appl Phys. Lett., 57(4) 348-50 (1990)... 

---