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Re: an experiment with dielectrics
Keven,
Interesting experiment! The high dielectric constant for magnesium
titanate and titanium dioxide may be related to their pre-treatment or
(proprietary?) treatment during firing into a ceramic by capacitor
vendors. My CRC Handbook of Chemistry and Physics show quite a range of
dielectric constants for these materials:
Titanates: (Ba, Sr, Ca, MG, and Pb): 15 - 12,000
Titanium Dioxide: 14 - 110
This would suggest that the physical structure or treatment plays an
important part in the actual value. Also, the above values are for solid
ceramics, not powders. A powder contains a substantial amount of free
space (air), and the particles are not physically "bound" to each other
as they would be in a piece of ceramic made from the material. It is also
very difficult to make electrical contact from the capacitor plate to
individual grains in the powdered material, and dielectric "stress" may
not be propagated through the material to the plates. The combination of
these factors may be the cause for the unexpectedly low values. It almost
looks like you are seeing the effect of simply increasing the
plate-to-plate spacing with a material having a low composite dielectric
constant (air??).
You may want to "fire" a batch of the above material and measure the
dielectric constant on the fired material. This may help "bound" the
dielectric constanct for YOUR particular material. If this shows a high
value, then the experiment could be re-run using thin sheets of fired
material...
-- Bert --
Kevin D Christiansen wrote:
>
> >From kdc4n-at-viper.cs.virginia.eduThu Jul 18 22:22:49 1996
> Date: Thu, 18 Jul 1996 12:32:23 -0400
> From: kdc4n-at-viper.cs.virginia.edu
<Snip>
> 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
> XXXXXXXXXXXXXXX GLASS PLATE XXXXXXXXXXXXXXXXXXXX
> 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
> values:
>
> 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."
> -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-