Dynamic solvent permittivity instrument

An instrument for measuring the dielectric constant of samples with complex or uncertain geometries that can include unusually shaped single sample pieces or collections of differently shaped sample pieces such as powders. The premise of the measurement technique is that a reference capacitor plate and a sample capacitor plate are exposed to a dynamically-changing solvent mixture. When the capacitance of these two fixtures become equal at the point at which a particular solvent mixture is reached, the dielectric constant of the sample is then known.

FIELD OF THE INVENTION 
An instrument for measuring the dielectric constant of samples with complex 
or uncertain geometries that can include unusually shaped single sample 
pieces or collections of differently shaped sample pieces such as powders. 
BACKGROUND OF THE INVENTION 
The dielectric constant of irregularly shaped samples, or collections of 
small pieces of a sample type, often cannot be measured. This is because 
the conversion of a capacitance measurement to a dielectric constant 
requires exact knowledge of sample dimensions. 
A number of cumbersome and notably inaccurate methods of estimating the 
dielectric constant of samples with complex geometry are currently used. 
But, their shortcomings are widely acknowledged and are often not utilized 
because of their questionable results. These methods require the 
application of assumptions concerning the additivity of dielectric 
constants of mixtures when the sample is mixed with materials such as 
epoxy resins in proportions which must be known with precision.

SUMMARY OF THE INVENTION 
A broad aspect of the present invention is an apparatus for measuring a 
dielectric constant of a sample containing: 
a first capacitor having a first space; a second capacitor having a second 
space to contain the sample; and a means for adding a material in the 
first space and to that portion of the second space not occupied by the 
sample until the dielectric constant of the first space is equal to the 
dielectric constant of the second space. 
Another broad aspect of the present invention is an apparatus for measuring 
the dielectric constant of a sample containing a first capacitor having a 
first space between a pair of plates within which the sample is disposed; 
a second capacitor having a second space between a pair of plates; a 
container; the first capacitor and said second capacitor are disposed in 
the container; 
a means for adding a solution having a first dielectric constant to the 
container so that the first space is filled with the medium and that part 
of the second space not occupied by the sample is filed as the solution; 
and a means for adding to said solution a second solution having a second 
dielectric constant to adjust the dielectric constant of said first space 
and said second space until said dielectric constant of said first space 
and said second space are equal. 
DETAILED DESCRIPTION 
The technique described here utilizes no assumptions but is a direct 
measurement technique, and requires no knowledge of sample size, weight, 
or volume. Thus, the technique described here provides values with greater 
accuracy and convenience than previous techniques. 
This invention allows measurement of complex samples by dynamically 
comparing the capacitance of two parallel plate fixtures. In the simplest 
case, the two fixtures share the same overall dimensions. One fixture 
contains the sample while the other remains empty. As the solvent mixture 
which surrounds both fixtures changes from one extreme of having a lower 
dielectric constant than the sample to one which a has higher dielectric 
constant, a point is reached where the capacitance of the sample and 
reference cells match, as shown in FIG. 1. The dielectric constant of this 
solvent mixture, which can be calculated because the dimensions of the 
empty reference cell are known exactly, is equal to the dielectric 
constant of the unknown sample. 
The sample arrangement consists of a fluid chamber common to both sample 
and reference capacitance fixtures, a solvent reservoir, a 
height-adjustable drain to adjust the fluid level, and electrical leads 
from each of the two sets of positive and negative terminals on the 
capacitance fixtures (FIG. 2). 
This technique can be applied to samples where the sizes do not lend 
themselves to traditional techniques--such as granular powders and to 
samples that cannot be machined into known dimensions such as very 
brittle, hard, or precious materials. 
While this technique was implemented using a slow drip addition method, as 
shown in FIG. 2, the solvent comparison concept can easily be extended to 
continuous flow techniques. These would lend themselves to a new 
analytical technique utilizing the dual programmable pumps as used High 
Performance Liquid Chromatography (HPLC) or Gel Permeation Chromatography 
(GPC). Such arrangements are shown in FIGS. 3 and 4. 
DESCRIPTION OF THE INVENTION: PREFERRED EMBODIMENTS 
An example experiment starts with loading the sample fixture with an 
unknown amount of sample. The fluid chamber is then filled to a level 
submersing the fixtures with a solvent known to have a dielectric constant 
which is below that of the sample. As long as the dielectric constant of 
this liquid is known to be lower than the sample, there is no need to know 
its exact dielectric constant value. The solvent reservoir is then loaded 
with a second solvent which has a dielectric constant known to be higher 
than that of the sample. This second solvent should preferably be miscible 
with the first solvent to ensure good mixing. Non miscible solvents may 
also be used in conjunction with surfactants to compatibalize otherwise 
phase separated solvent mixtures. 
The capacitance is measured and recorded for each of the fixtures 
separately. Then, the solvent reservoir is opened and a slow drip started. 
A stirring mechanism is preferably engaged to ensure good uniformity of 
mixing of the solvents. The capacitance of the reference and sample 
fixtures are measured at time intervals. The dielectric constant of the 
solvent mixture gradually changes as more of solvent 2 from the reservoir 
is mixed with solvent 1. A point is reached where the dielectric constant 
of the solvent mixture is equal to that of the sample. At this point the 
capacitance of the sample and reference fixtures will be equal, as shown 
schematically in FIG. 5. Then, using this capacitance value, the 
dielectric constant for the geometry of the reference cell is calculated. 
This is also then the dielectric constant of the unknown sample. 
In addition to the cell designs discussed thus far, many other designs 
based on the same principle can be envisaged by those skilled in the art. 
These include flow through cells where the sample and reference capacitor 
plates (or equivalent active components) are of different size and/or 
shape. This will allow greater flexibility in designing the cell. In order 
to facilitate comparison between the sample and reference cells, 
appropriate geometric factors would have to be incorporated into the 
comparison circuit or algorithm used to determine the point at which the 
sample and reference cell contents have dielectric constants which are 
equal. For example, if the sample capacitor plate were 2 cm in diameter, 
and the reference plate were 1 cm in diameter, and they had equal gaps, 
then the dielectric constants of the sample and reference would be equal 
when the capacitances (C) held the relationship that 
C(samp)=4.times.C(ref). This follows from the relationship of capacitance 
to dielectric constant. E'=(Capacitance.times.gap width)/(Area of plate) 
Further, there is no requirement that the sample and reference capacitance 
plates have any particular placement relationship to one another within 
the cell as long as the solvent mixture uniformity was great enough to 
allow the assumption that the solvent mixture was essentially the same at 
each at a particular point in time. An example is shown in FIG. 6. In 
addition, FIG. 7 of a flow through cell type shows a second reference 
electrode by which the assumption of solvent uniformity within the cell 
could be tested. Further, the use of the second reference capacitor in 
this flow-through cell design would allow averaging the capacitances of 
the two references. This averaged result could then be used to compare to 
the sample thus eliminating any error associated with solvent gradient in 
the direction of flow. 
Further, this technique lends itself to being carried out with the use of 
other commonly available instruments which can provide programmable 
control over solvent mixing such as an HPLC (High Performance Liquid 
Chromatograph) or GPC (Gel Permeation Chromatograph). If these instruments 
were available, only the measurement cell and the comparison algorithm 
would need to be provided to allow the dynamic solvent permittivity 
measurement to be completed. Thus, the most difficult experimental portion 
of a DSP measurement, the solvent mixing, could be handled by commercially 
available instrumentation. 
This technique could be extended to permittivity measurements in the 
microwave frequency range using waveguides and resonance perturbation 
techniques and sequentially measuring sample and reference for each of a 
range of solvent mixtures. Again the match point would provide the unknown 
dielectric constant. 
This could be accomplished using two side by side waveguide resonance 
chambers separated by a screen which isolates each chamber to microwave 
radiation but allows solvent to flow freely ensuring uniform solvent 
composition. The dielectric constant of each chamber can be periodically 
measured and compared. When equal for both chambers then the dielectric 
constant of the sample is then known. 
The microwave apparatus in FIG. 8 is operated in a manner similar to the 
parallel plate apparatus described earlier. A microwave switch is used to 
switch between 2 different waveguide sample chambers. The input impedance 
of each chamber is measured and compared to that of the other chamber. A 
screen or fine mesh is used to allow the dielectric fluid into both 
chambers, thus when the dielectric constant of the surrounding fluid is 
equal to that of the test sample, the input impedance of both waveguides 
will be identical and the dielectric constant of the sample can be 
determined. 
This technique can be used with any type of waveguide structure. This 
includes but is not limited to rectangular waveguide, circular waveguide, 
elliptical waveguide, coaxial waveguide, and ridged waveguide. In 
addition, any structure that allows the controlled and repeatable 
measurement of microwave impedance can be used with this technique. 
This kind of structure and technique allows measurements at high microwave 
and millimeter wave frequencies. Normally these measurements are extremely 
difficult because the sample is required to completely fill the waveguide 
structure or you must account for interaction between the microwave or 
millimeter wave and the physical shape and size of the structure. Using 
this technique, measurements from 300 Mhz to 1 Thz (1000 Ghz) should be 
possible. 
The total frequency range measurable using the Dynamic Solvent Permittivity 
technique by either the capacitance or microwave impedance method is 10 Hz 
to 1000 GHz. 
One particular experiment used Alumina (Al.sub.2 O.sub.3) shards as the 
sample, ethyl ether as solvent 1 and ethyl alcohol as solvent 2. 
Capacitance measurements of both reference and sample cells were made at 5 
minute intervals. The progression of the dielectric constants of the 
reference and sample cell contents can be seen in FIG. 1. The crossover 
point occurs at a dielectric constant of 9.644, which is thus the measured 
dielectric constant of the unknown. This value is in excellent agreement 
with literature values of the dielectric constant of Alumina (Al.sub.2 
O.sub.3). 
Description of FIG. 1--Dynamic Solvent Permittivity experimental data 
FIG. 1 shows a plot of the dielectric constant vs. time of both the sample 
and reference of an experiment run using the prototype instrument shown 
schematically in FIG. 2. This trial used crushed Al203 pieces as the 
sample, ethyl ether as the initial solvent and ethyl alcohol as the second 
solvent. As the second solvent was slowly added to the experimental 
chamber containing the sample and reference electrode assemblies, the 
capacitance and the corresponding dielectric constant was followed and 
plotted. A crossover point is reached where the capacitance of the sample 
and reference are equal, which determines the dielectric constant of the 
unknown. 
Description of FIG. 2--Dynamic Solvent Permittivity prototype 
FIG. 2 shows the Dynamic Solvent Permittivity setup used in the laboratory 
trials. It also constitutes a prototype DSP system. A main solvent chamber 
(2) contains both a sample electrode (4) assembly and a reference 
electrode (6) assembly, each of which consists of two parallel conducting 
metallic plates. Within the sample electrode is inserted a sample material 
(8), which is the material piece or pieces for which a dielectric constant 
value is desired. Additionally, a stirring mechanism (24) may be in place. 
Initially, the sample chamber is filled with a solvent (22) which has a 
dielectric constant known to be lower than the sample. A second solvent 
(20) which is known to have a dielectric constant higher than the sample. 
(Alternatively, 22 could be higher and 20 could be lower). A mechanism for 
slowly introducing the second solvent 20 into the sample chamber 2 to mix 
with the first solvent 22 is an addition funnel (12). The sample electrode 
assembly 4 is connected to a capacitance meter (16) and likewise the 
reference electrode is connected to a capacitance meter (14). 
Description of FIG. 3--Schematic of DSP using programmed solvent sources 
and computer data collection and analysis 
FIG. 3 shows a dynamic solvent permittivity set up utilizing programmable 
solvent pump 8, first solvent 22, second solvent 20, test chamber 2 which 
contains both sample and reference electrode assemblies, signal wires 4, 
computer for analysis and program control 6, and control signal wires 12. 
Additionally, a single tube or pipe containing the solvent mixture 10 
delivers solvent from the programmable pump 8 to the test chamber 2. 
Description of FIG. 4--Schematic of DSP using gel permeation chromatograph 
(GPC) or high performance liquid chromatography HPLC instrument as solvent 
mixer and pump 
In this configuration, the programmable solvent mixing is carried out by 
commercially available solvent mixing systems (GPC or HPLC) 2, solvent 
mixture is delivered to the sample test chamber 6 by tube 4, and sample 
and reference capacitance signals conducted to the computer by wire set 8, 
to computer 10 for analysis and control of solvent pumps via control 
signal wires 12. 
This diagram essentially shows how the DSP instrument could be incorporated 
into GPC or HPLC instruments as an analytical add-on tool. 
Description of FIG. 5--Schematic of end-point condition 
A schematic diagram of the sample capacitance circuit 2 includes the sample 
electrode assembly 2 where the sample is placed and the capacitance bridge 
4 where the capacitance is measured. The diagram also shows the 
corresponding reference circuit 8 which included the empty electrode 
assembly 12 and the capacitance bridge 10 where the capacitance is 
measured. Also shown are the endpoint conditions where the capacitance of 
the sample electrode equals the capacitance of the reference 
electrode--for the case where the sample and reference electrode assembly 
geometries are the same 14. More generally, if the geometries of the 
sample and reference electrode assemblies differ, the endpoint can be 
specified as the point at which the total combined dielectric constant of 
the solvent plus sample within the sample electrodes is equal to the 
dielectric constant of the solvent within the reference electrode 16. 
Description of FIG. 6--Illustrating that the sample and reference 
capacitance plates need not be of identical geometry. 
The solvent container 2 includes the sample electrode assembly 4 and the 
reference electrode assembly 6 which is not necessarily of the same size 
or shape as that of the sample electrode assembly. Figure also points out 
electrical connectors 8 by which the electrodes of the sample and 
reference assemblies send electrical signals to the capacitance meter or 
other device intended to compare the end-points. 
Description of FIG. 7--Illustrating that sample and reference capacitance 
plates can be physically oriented wrt each other in ways other than 
side-by-side, and that a second reference can be utilized for maximum 
accuracy. 
There are circumstances where the sample and reference electrode assemblies 
would be more conveniently or more advantageously oriented differently 
than the side by side configuration previously shown. And in addition, 
there are circumstances where two reference electrode assemblies 
straddling the sample electrode assembly may prove advantageous to the 
accuracy of the measurement. When there is a solvent gradient from the 
solvent entry side of the cell to the solvent exit side, two references 
can be used to eliminate inaccuracies caused by uncertainties in the 
solvent gradient field. This figure shows a housing 2 which contains 
solvent and solvent mixtures, a reference electrode assembly 6, a sample 
electrode assembly 4, irregularly shaped sample particles 10, and a second 
reference electrode assembly 8. In addition it shows a port for solvent 
ingress 12, and a port for solvent egress 14. 
Description of FIG. 8--Shows sample cell 8 and reference cell 6 in 
microwave frequency waveguide assemblies 20 and 22. The sample 10 sits 
inside the sample waveguide cavity 22. Solvent and solvent mixtures are 
introduced through port 14 and exits through port 16. The screen 18 
creates an electrical separation between the sample waveguide assembly 22 
and the reference waveguide assembly while allowing unimpeded flow of 
solvent from one cell to the other. The waveguide switch allows sequential 
probing of the dielectric properties of the sample 22 and reference 20 
cells which is necessary for comparing the dielectric properties of the 
two cells in anticipation of the endpoint which occurs when the dielectric 
constant of the two cells is equal. More precisely, the endpoint in the 
case of a resonance perturbation type microwave frequency dielectric 
measurement is when the resonance frequency shift for the sample chamber 
is equal to the resonance frequency shift of the reference chamber--in the 
case of equal cell dimensions. 
While the present invention has been shown and described with respect to a 
preferred embodiment, it will be understood that numerous changes, 
modifications, and improvements will occur to those skilled in the art 
without departing from the spirit and scope of the invention.