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HIGH-FIELD CRYSTAL GROWTH
F. Okuyama
To cite this version:
F. Okuyama. HIGH-FIELD CRYSTAL GROWTH. Journal de Physique Colloques, 1986, 47 (C7), pp.C7-75-C7-80. �10.1051/jphyscol:1986714�. �jpa-00225904�
HIGH-FIELD CRYSTAL GROWTH*
Department of Systems Engineering. Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya, Japan
Abstract
The high-field crystal growth, which refers to crystal growth events occurring on metallic substrates placed in an intense electric field, differs essentially from conventional crystal growths, in that its particle supply process is governed by electrostatic forces. This paper reviews related topics including a recent finding by an indus- trial research group. The underlying physical processes are also discussed briefly.
Introduction
The "high-field crystal growth" is a general term given to crystal growth phenomena taking place on field electrodes interacting with low-vacuum ambients. The phenomena are classified into two cate- gories: anodic fiber growth and cathodic fiber growth. The former, discovered by Beckey and his associates, refers to the growth of non-metallic fibers on field anodes. In the latter, metallic fibers grow on electron emitters reacting with plasmas of metal-containing inorganic compounds, typically metal carbonyls. In the following, selected topics on these phenomena are described, placing a stress on their physical aspects, primarily growth process.
Anodic Fiber Growth
The discovery of anodic fiber growth dates back to 1970, when ~b'llgen and ~ e c k e ~ ' noticed a gradual increase of acetone molecular ions field-emitted from a filamentary anode of Pt. Needle-like micro-
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*Dedicated to Professor Hans D. Beckey on the occasion of his 65th birthday
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986714
C7-76 JOURNAL DE PHYSIQUE
projections covering the anode surface were detected by electron microscopy, which led them to conclude that a field enhancement due to the growth of microprojections was responsible for the ion current increase. The discovery of this "anode activation" was followed by efforts to find out materials which effectively activate field anodes, and benzonitrile was finally determined to be the best one.
When interacting with a heated field anode, benzonitrile vapor gener- ates carbon fibers in an unusually high density (Fig. 1) 2. The tech- nological significance of this so-called "high-temperature" (H-T) activation is enormous, since it paved the way to field desorption mass spectrometry of organic solids 3
.
The structural determination of carbon fibers grown by the H-T acti- vation technique were attempted by the groups of Beckey and Neumann.
Based on detailed electron dif- fraction analysis, Ajeian et al. 4 concluded that carbon fibers assume a graphite structure attainable by a pyrolytic process. Neumann et al. 5
,
on the other hand, found by X-ray diffraction that it does not match Fig. 1. Carbon fibers grown by any of the known graphite struc- the H-T activation technique. tures. According to their hypoth- Substrate: 10-)m W filament. esis, the structure that they found
is a "modified" graphite structure, which may be ascribable to strong electric fields acting on growing fiber tips. Since X-ray diffraction is superior to electron dif- fraction in accuracy, the view of Neumann et a1 seems to be more reasonable. The growth mechanism of carbon fibers was theoretical- ly surveyed by the two groups 5 f 6 . Both of the proposed models presume the polarization of benzonitrile molecules in the vicinity of fiber growth sites. These models, however, do not hold for the H-T acti- vation mode, because, at such a high temperature as used in the H-T activation (1000-1500 K)
,
benzonitrile molecules will thermally de- compose inthe space close to the growth sites. In addition, the car- bon fiber growth from non-polar substances like indene and naphtha- lene7 is also difficult to explain by these models. Thus, the growth mechanism of carbon fibers still remains to be an open question.In Xerography, a well-known copying technique, positive ions created by a positive corona discharge are used to form latent image. The
of Fuji-Xerox discovered that the surface of the corotron is covered with Si02 fibers after a prolonged operation, thereby deteriorating the print contrast 8. Indisputably, this phenomenon is an anodic
fiber growth, for the corotron is biased at a high positive potential.
(The origin of Si is supposed to be air dusts.) At the present time, physical and chemical processes underlying the fiber growth are en- tirely unknown. To eliminate this technologically undesirable effect, a scientific approach is indispensable.
Cathodic Fiber Growth
Systematic investigations on cathodic fiber growth have been done in our laboratory since 1974. The first paper9 dealt with the whisker growth of W from W03 vapor. The growth took place during an electri- cal discharge which was induced by W03 vapor thermally evaporated from the substrate. A hypothesis was then proposed that W vapor originating from a discharge-induced decomposition of WOj, molecules drove the whisker growth. Unfortunately, the experiments lacked data reproducibility, owing to thedifficulty of controlling the pressure of W03 vapor. In 1978, the use of metal carbonyls was proposed by
Linden et a1 .I who successfully grew dendritic W fibers on filament- ary field cathodes from W(C0I6. This finding triggered a dramatic development in experimental research on this subject.
In our experiments, Cr(CO)6 and M O ( C O ) ~ were mainly employed 11-20
I
because of their high vapor pressure facilitating the pressure con- trol. The experimental principle of cathodic fiber growth is very simple: biasing the substrate, opposed to a grounded plane electrode, at a negative potential in carbonyl vapor of 1 0 - ~ - 1 0 - ~ Torr induces an electrical discharge, thereby growing metallic fibers at the electron-emitting area. Quite different growth morphologies are ob- served depending on the mode of cathode operation. With Cr(CO)6 as the parent substance, for example, a cathode operation at 0.5-1 kV
and maintaining the temperature at 600-700 K generates a corona plasma enveloping the substrate, and leads to the formation of densely populated Cr fibers (Fig. 2(a)) 14. Biasing the cathode above 1 kV at a temperature of 300-1300 K, on the other hand, causes an arc dis- charge, and the products are sparsely distributed polycrystalline
C7-78 JOURNAL DE PHYSIQUE
Fig. 2. Corona, (a), and arc, (b)
,
fibers of Cr.fibers of Cr (Fig. 2 (b)) 14. Particularly strange is the morphology of fibers grown at a corona discharge mode: the fibers extend radial- ly from a point source to construct a structure like a chestnut shell, or a "corona figure" 19. Such a growth morphology cannot be explained in terms of the existing concepts of crystal growth. Generally, co- rona fibers consist of two phases: extremely thin, amorphous cores and fcc crystallites deposited on the cores (Fig. 3). This means that their axial and lateral growths proceed via different mechanisms.
Fig. 3. (a) Electron microscope image of a Cr corona fiber
.
(b)Correspond- ing diffraction pattern and (c) dark field image produced by the arrowed spot, revealing crystallites deposited on the thread-like core.Fig. 4 illustrates the expected corona growth process. In brief, field electrons emitted from the cathode dissociatively ionize car- bony1 molecules through an electron impact process, forming a plasma
(Fig. 4(a)). Positive ions thus produced, most of which may be metal
the discharge, and positive ions are left behind in the path of elec- trons. These positive ions lengthen the fibers by impinging on their tips. This process repeats itself to maintain the discharge and, at the same time, drives the axial growth (Figs. 4(b) and 4(c)). This ionic particle supply is unlikely to occur at the sides of growing fibers, because of rather low electric fields there. It is currently speculated that the lateral growth is promoted by a plasmarinduced chemical vapor depostion of neutral particles. According to the above model, the fiber distribution in a corona figure represents the motion of electrons in the relevant corona plasma. Thus, the corona fiber growth has a close relation to the mechanism of sustaining the corona discharge.
Cr and Mo fibers obtainable at the arc discharge mode are thought to be formed by metal ions supplied from arc streamers 16. Their con- figuration may therefore reflect the streamer propagation in a low- pressure arc discharge. To confirm this point, further experiments are required.
Neutral Particles
Fig. 4. Expected mechanism of the corona fiber growth. (a) re- presents the inception of discharge.
C7-80 J O U R N A L DE PHYSIQUE
References
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3 H.D. Beckey: Principles of Field Ionization and Field Desorption Mass Spectrometry, Pergamon, Oxford 1977.
4 B. Ajeian, H.D. Beckey, A. Maas and U. Nitschke: Appl. Phys.
6 (1975) 1 1 .
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10 H.B. Linden, E. Hilt and H.D. Beckey: J. Phys. E11(1978)1033.
1 1 F. Okuyama: Appl. Phys. Lett. 36(1980)46.
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13 H.B. Linden, H.D. Beckey and F. Okuyama: Appl. Phys. 22(1980)83.
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