an experiment with dielectrics

Hello all!

I am new on this list, so I though I would start with a quick
intro:  I am a PhD student in computer science who has been
fascinated with high voltages ever since I can remember.  I
started with auto ignition coils and relays in the fourth or
fifth grade and eventually worked up to tesla coils, neon sign
transformers, pulse generators, and other such stuff.  Currently,
I am attempting to build small (in physical size), large-capacity
capacitors - which leads me to the main body of my message...

According to a physics book I have laying around, doubling the
dielectric constant of a capacitor doubles the capacitor's value
(capacity).  Since the intensity of electric fields falls off
with the square of the distance to the plate, most of the energy
in a capacitor is stored very close to the plates.  (I remember a
college physics teacher explaining this to the class.)  This
means that the capacity of a standard aluminum-glass-aluminum
sandwich capacitor should be increased by putting a small layer
of a dielectric with a high dielectric constant between the
aluminum plates and the glass dielectric, resulting in a sandwich
configuration as follows:

         ***** Aluminum plate ******
       dielectric with a high constant 
       dielectric with a high constant
         ***** Aluminum plate ******

Although this increases the distance between the aluminum plates
(lowering the capacity), the high dielectric constant in close
proximity to the aluminum plates should more then make up the
difference, especially if dielectrics with a high permittivity,
such as distilled water (80), titanium dioxide (86-170), or
manganese dioxide (~10,000) are used.  (All permittivity values
came from the CRC handbook of physics.)

With this in mind, I designed an experiment to measure the
effectiveness of the various dielectrics, in the above sandwich
configuration, but using a single layer of 3 mil plastic sheet
instead of a glass plate.  Capacity was measured by using the
capacitor sandwich as the timing capacitor in a 555 timer circuit
and observing the resulting frequency on an oscilloscope.  A
standard formula provided in a linear IC databook was used to
relate the frequency of the oscillator to the capacity of the
timing capacitor.  The active area of the capacitor was about 3
inches by 3 inches.  The capacitor sandwich was compressed with
considerable force to eliminate air and to distribute the
dielectric evenly.  Sticking various dielectrics between the
aluminum plates and the plastic sheet resulted in the following

Capacitance       Dielectric used (in addition to the
in pF                      sheet of plastic)
1104           <No additional dielectric (besides the plastic)>
1368           corn oil  (it was handy in the fridge)
1440           corn oil / titanium dioxide slurry
2280           tap water
2160           distilled water                       {e=80}
 816           manganese dioxide (VERY fine powder)  {e=10,000}
1920           water / manganese dioxide paste
 480           titanium dioxide  (VERY fine powder)  {e=170}
1740           water / titanium dioxide paste

The question is, why didn't the titanium dioxide and the
manganese dioxide work?  Although there is plenty of room for
error in this simple experiment, the sheer magnitude of the
differences between the dielectric constants of plastic, water,
titanium dioxide, and manganese dioxide should result in hugely
different capacitance values.  For example, using distilled water
(e=80) as a baseline, the titanium dioxide (e=170) should have
given a capacitance of twice that of the pure water (4590pF). 
The manganese dioxide (e=10,000) should have given a capacitance
125 times that of the pure water (270,000pF).  Instead, the
titanium dioxide and the manganese dioxide (either mixed with
water or dry) resulted in considerably lower capacitances then
the distilled water.  What am I missing?

I know that there is *some* titanium-based chemical that can be
used to increase the capacitance of capacitors, due to the
existence of "titanium capacitors", which exhibit large
capacities in small packages.  What are the builders of these
caps doing that allows them to use the high dielectric constant
of whatever titanium chemical they are using?  (What titanium
chemical are they using anyway?)

Thanks in advance for any insight or explanations that you can
give me on this matter.

[ I've always heard that the chemical is barium titanate.  It is known
  in the tesla coiling community as a high RF loss dielectric -- Chip ]

                     Kevin D Christiansen
      User Interface Group - The University of Virginia
              "The best VR that pizza can buy" 
   kevin-at-virginia.edu    http://www.cs.virginia.edu/~kdc4n/

"The universe is composed of space, galaxies, and intergalactic
 dust.  Galaxies themselves are composed of space, stars, and
interstellar dust.  From the omnipresence of dust, we conclude
that nature abhors a vacuum and won't pick up a broom, either."