NAT'L INST. OF STAND & TECH

AlllDt. 173STE

NIST Technical Note 1514

NIST

PUBLICATIONS

Metal Detector Studies:

Research Materials

James Baker-Jarvis

Raian Kaiser

Michael D. Janezic

N. G. Paulter

K. L. Stricklett

QC

100 >nal Institute of Standards and Technology

1)5753 ology Administration, U.S. Department of Commerce

#1514

2002

NIST Technical Note 1514

Metal Detector Studies:

Research Materials

James Baker-Jarvis

Raian Kaiser

Michael D. Janezic

N. G. Paulter

K. L. Stricklett

Radio-Frequency Technology Division

Pliysics Laboratory

National Institute of Standards and Technology

325 Broadway

Boulder, Colorado 80305

August 2002

U.S. Department of Commerce

Donald L. Evans. Secretary

Technology Administration

Phillip J. Bond, Under Secretary of Commerce for Technology

National Institute of Standards and Technology

Arden L. Bement, Jr., Director
Certain commercial entities, equipment, or materials may be identified in this document in order to describe an experimental procedure or concept

adequately. Such identification is not intended to imply recommendation or endorsement by the National Institute of Standards and Technology, nor is it intended to imply that the entities, materials, or equipment are necessarily the

best available for the purpose.

National Institute of Standards and Technology Technical Note 1514

Natl. Inst. Stand. Technol. Tech. Note 1514, 36 pages (August 2002)

CODEN: NTNOEF

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Contents

1 Introduction 2

2 Overview of the Problem 2

3 Applications of Phantom Materials 5

4 Conductivity of Materials 6

4.1 Electrical Properties of Conductive Materials 6

4.2 Conductivity Measurements 9

4.3 Variable-Temperature Measurements 9

4.4 Conductivity of Candidate Liquids 10

4.5 Conductivities of Carbon-Black-Silicone Materials 11

4.6 Glycine-Based Phantom Material 14

5 Discussion 18

6 References 22

7 Appendix A: How Carbon Black is Produced 25

8 Appendix B: Percolation Threshold

for Conductivity 27

9 Appendix C: Shielded Open-Circuited Holder 27

9.1 Measurements with the Shielded Open-Circuited Holder 27

9.2 Theoretical Formulation 27

9.3 Matching of Tangential-Field Components 29

9.4 Solving for the Coefficients 30

9.5 Integrals 31

10 Appendix D: Mixture Theories for

Conductive Materials 32

ni

Metal Detector Studies: Research Materials

James Baker-Jarvis* Raian Kaiser*

Michael D. Janezic* N. G. PaulterJ K. L. Stricklett^

The purpose of the work described in this report was to develop and characterize

phantom materials that mimic the electromagnetic properties of the human body. The

materials were characterized from 100 Hz to 10 MHz and over a temperature range

of 15 °C to 40 "C. Such phantoms will aid in the evaluation and assessment of the

interaction of medical devices and metal objects or weapons with metal detectors. The

materials studied were: (1) a mixture of potassium chloride in water, (2) a mixture

of propylene carbonate, ethylene carbonate, and salts, (3) a semi-solid nano-composite

material consisting of semi-solid silicone filled with carbon black particles, (4) a mixture of glycine, carrageenan, and potassium chloride, in water. The conductivities of all the materials were analyzed for stability over time and temperature dependence by use

of both a conductivity meter and an open-ended coaxial line. We found that the conductivity of the carbon-black-silicone composite exhibited a percolation threshold as a

function of carbon-black concentration. We also found that to obtain reproducibility,

the carbon black mixture must be temperature annealed and consistently mixed. The

silicone composite has the advantage of being more rugged than the liquid mixtures.

Key words: Conductivity; dielectric; frequency; nano-composite; permittivity; phantom.

*National Institute of Standards and Technology, Radio-Frequency Technology Division, MS 813.01,

Boulder, CO e-rnaiI:jjarvis@boulder. nist.gov

^National Institute of Standards and Technology, Electricity Division, MS 811, Gaithersburg MD
1. Introduction

The purpose of this report is to disseminate information on the development and characterization of materials that simulate the relevant electromagnetic (EM) properties of the

human body. These materials when used as phantoms will allow evaluation and assessment

of the interaction of personal medical electronic devices (PMEDs) and metal weapons with

magnetic fields generated by hand-held (HH) and walk-through (WT) metal detectors. The

goal was to study various mixtures of materials that simulate the relevant EM properties of

human body tissue over the frequency range of 100 Hz to 10 MHz and a temperature range

of 15 °C to 40 °C.

2. Overview of the Problem

The dielectric properties of human tissues have been studied extensively over the years. The

range of conductivities that occur in the human body are well known. Therefore phantom

materials that simulate the human body's electrical properties play a crucial role. In the

past, phantoms have been used for: (a) studies on the low-frequency electromagnetic interaction with the human body, (b) health effects of microwaves, and (c) interaction of wireless

transmitters with human tissue [1]

The effects of electromagnetic waves on biological tissues are related to the conductivity

and dielectric properties of tissues. Interactions of magnetic fields with materials are related

to the permeability, ^* = iiq{^'^ — JMr)' where /zq = 47r x 10~^ (H/m), and conductivity a

(S/m) of the object in a metal detector. Time varying magnetic fields of a metal detector

interact with metallic objects by inducing eddy currents ( J = V x H) in objects that modify

the incident magnetic field (see figures 1 and 2). Although the permittivity e* = eo(e(. — je^'),

where cq = 8.85 x 10~^^ (F/m), does not directly enter the equations for the surface resistance,

the permittivity of the material determines the impedance and thereby affects the reflectivity

of an incident wave from the interface of the object and air. A magnetic field incident on the human body also generates eddy currents in the tissues,

depending on their conductivity, but of much smaller magnitude than in metals. Due to the

conductivities of the materials in the body, the incident magnetic fields attenuate as they

pass into tissues. In order to model the response of a metal detector to a PMED or weapon,

we need to understand the conductivity of the medium in which the device is immersed and

also the permeability and conductivity of the PMED or weapon. Potential electromagnetic

interference with implant devices relates to how deeply the fields penetrate the human body

and the mismatch of impedance between the air and body. At low frequencies the fields
~-- —-^H (variable)

Figure 1. Eddy currents in a metal object subjected to time varying magnetic fields.

FTimary magnetic field

Figure 2. Approximate eddy currents on a handgun due to an incident \'ariahle

field.

magnetic
penetrate through the body.

The body consists of muscle, fat, bone, organs of various types of tissue, and cavities.

All these materials can be treated as nonmagnetic. Since the dielectric properties of human

tissues are heterogeneous and vary from tissue to tissue, we need to correctly simulate the

electrical response of the particular area of the human body where the PMED is implanted.

For metal detector research, we need to emphasize tissue conductivity, rather than permittivity, since skin depth and plane-wave impedance, in nonmagnetic conductive materials at

low frequency, depend more strongly on the conductivity. At low frequencies the real part of

the permittivity has little effect on the impedance. Gabriel and Gestblom have compiled ex- tensive data on various human tissues and developed a database which reports measurements

of various human tissues such as head, brain, muscle, fat, and bone [2]. Phantom materials studied by previous researchers are summarized in table 1 [3-11].

Previously developed phantoms have been based on liquids and salts, gelling agent and

salts, carbon black mixtures with polymer resins. For example, Hagmann et al. used glycine,

salt, and the gelling agent, agarose or carrageenan [5]. With this mixture, they found they

could match permittivity and conductivities of the human body. Kato [8] used a mixture

of agar, water, and salts that could be heated to a gel state. Kato found that the semisolid material could be sculptured to produce a body shape, but degraded in time. Chou

et al. [6] used a mixture of water, salts, and a gelling agent to form a semi-solid material.

Andreuccetti [7] used a mixture of water, polyacrylamide, and salts to produce a semi-solid

phantom. Marchal [10] used a mixture of water, salts, and gelatine to produce a stable

phantom jelly-like material. The glycine- agarose-based materials need refrigeration. Most

of the studies that dealt with low frequencies (1 kHz to 1 MHz) used liquid materials, while

the studies that dealt with megahertz to gigahertz frequencies used semi-solid materials

[9]. Liquids work well at low frequencies because the mobile ions from the salts mimic the

tissue's dielectric-loss response. The standard method for Specific Absorption Rate (SAR)

for wireless communication devices uses a liquid mixture of water, NaCl, alcohol, sugar, and

other additives, depending on the frequency [11]. The IEEE Standard uses a mixture of

water, NaCl, sugar, and couple other compounds (see the new IEEE Standard Method 1528-

200x, in review at time of publication). These methods were developed to mimic human

body properties in the low microwave range.

In our study we considered four candidate materials. These were potassium chloride

solution (KCI), a liquid composed of a mixture of ethylene carbonate, propylene carbonate,

and tetraethyleammonium tetrafluoroborate (TEATFB) [3], carbon black mixed into silicone

rubber (CBS), and glycine, carrageenan, KCI, and water. All of these composites can be

mixed to match the average conductivity of the human body tissues. There are advantages
Table 1. Previous work on human phantom materials.

Frequency Materials Citation and Date

1 Hz to 1 GHz TEATFB Broadhurst et al. 1987 [3] 900 MHz graphite and resin Kobayashi et al. 1993 [4] 10 to 100 MHz glycine, NaCl Hagmann et al. 1992 [5] 500 MHz to 3 GHz silicone rubber, carbon black Nikawa et al. 1996 [9] 200 MHz to 2.45 GHz gel, aluminum powder, NaCl Chou et al. 1984 [6] 750 MHz to 5 GHz polyacrylamide, NaCl Andreuccetti et al. 1988 [7] 5 to 40 MHz agar, sodium azide, PVC powder Kato et al. 1987 [8] 10 to 50 MHz gelatine and NaCl Marchal et al. 1989 [10]

300 MHz to 1 GHz liquid CENELEC Standard [11]

and disadvantages for each of these phantom materials for medical detectors (PMMDs). We

concentrated the majority of our effort on understanding a semi-solid, CBS PMMD (for

background on CBS see Appendices A and B). Semi-solid materials have several advantages

over liquids. For example, CBS phantom composites are rubbery materials that allow test objects to be permanently embedded or encapsulated at a particular position.

3. Applications of Phantom Materials

The draft revisions of the National Institute of Justice (NIJ) standards for HH and WT metal

detectors contain a body-cavity concealment test, which is intended to detect metal objects

concealed within the human body cavities by use of human subjects. What is desired is a phantom material that can correctly simulate the EM properties of human body tissues over the frequency range at which metal detectors operate so that the body-cavity concealment

test can be reproduced.

Electrical interference in medical implant devices is another area of concern in metal

detectors. These devices are often studied by testing them when encapsulated in phantom

materials. The types of medical electronic devices used to assist or supplant poor or absent

physiological functions include, but are not limited to, cardiac defibrillators, hearing aids,

cardiac pacemakers, infusion pumps, and spinal cord stimulators. PMEDs may be implanted

within the body or located on the surface. Some PMEDs are programmed magnetically, and

all may be susceptible to external electromagnetic energy radiated by other electronic or

electrical devices. PMEDs are frequently exposed to electromagnetic interference (EMI)
emitted from other electronic devices. The interaction between PMEDs and intentional

radiators is a major concern. Commonly encountered radiators include HH and WT metal

detectors typically used for screening applications. These detectors emit frequencies close

to those used by PMEDs, that could cause interference problems. Unfortunately, there is

little information, other than anecdotal, about the interaction of PMEDs with HH and WT

metal detectors. Since testing of PMEDs is often accomplished by encapsulating the device

in phantom materials, we must characterize materials that have conductivities that are close

to those of human tissues.

4. Conductivity of Materials

4.1 Electrical Properties of Conductive Materials

Electric and magnetic fields are attenuated as they travel through lossy materials. The skin

depth 5 is the distance a plane wave travels, where it is decreased in magnitude by 1/e:

^= / / , (1)

In eq. (1), (j is the dc conductivity, /^^ is the real part of the relative permeability, and /

is the frequency. We see that the frequency, conductivity, and permeability of the material

determine the skin depth. The higher the frequency, permeability, and conductivity, the

smaller the skin depth [12].

The wavelength of an electromagnetic wave in a material is influenced by the permittivity

and the wavelength in the material is A = c^ad \f^r^ • where c^ac is the speed of light in

vacuum. The complex permittivity including the conductivity is defined as

e* = 4eo-i(e^'eo + -). (2)

The presence of dc conduction produces a low-frequency loss a/a;, in addition to polarization

loss (e^'). When electromagnetic fields impinge on a metal surface, the wave is strongly

attenuated and the surface resistance is

fi, = i. = ^ihf.. (3)

The surface resistance increases in proportion to the square root of the frequency. The

impedance is a strong function of o at low frequencies. U Z = Jn* /e* = Jiu^'/{jue' + a)

and Zq = JfiQ/eo are plane-wave impedances, then the plane-wave refiection coeflacient is Z-Zq

' Z + Zo- ^'
1.2

blood T!r lung inflated bone cortical -e- muscle

grey matter -f- skin dry

heart -e- skin wet ^ liver ^

stomach

-B- lung deflated

Frequency (Hz)

Figure 3. Measurements of the ac conductivity (S/m) on various body tissues by Gabriel [2] (no uncertainties assigned).
Frequency (Hz)

Figure 4. Measurements of relative permittivity (S/m) on various body tissues by Gabriel [2] (no uncertainties assigned).
^r2i Mr2

sample

3

2

Cl.^^*rl 1 bead

z=L

P

z=0

- calibration plane

Figure 5. The shielded open-circuited sample holder for measurements of conductivity. The

regions 1 through 3 are referred to in the full-mode model developed in the Appendix C.

4.2 Conductivity Measurements

The conductivities of CBS PMMDs were measured using an open-circuited coaxial holder

depicted in figure 5. This fixture consists of an open-circuited 14 mm coaxial hue with the

center conductor shorter than the outer-conductor shield. We developed a full-mode model

for the open-circuit termination (see Appendix C). This model includes a rigorous treatment

of the fringing capacitance at the open-circuit termination. The measurement data are the

reflection coefficient, F, from a network analyzer or C and G from a LCR meter. As a check

of our model we measured liquid KCl reference standards.

4.3 Variable-Temperature Measurements

The temperature system we used was an environmental chamber that operates from -150 °C

to 150 °C [13]. The chamber has feed-through bulkhead adapters on the sides for passage of

coaxial feeds for cavities or transmission lines. The chamber has purging ports for applying

nitrogen gas to reduce oxidation and content of water vapor.

9
KCI Solution

1.2

£

'>oac:oO

O

Q

KCI added to 500 ml deionized water

O a = -0.002 + 1.322 X (grams of KCL)] (20.5 - 21 .7 C) O CT= 0.011 + (.363 X(grams of KCL)] (22.0- 22.3 C) C a = 0.034 -I. [364 X (grams of KCL)] (22.6 - 22.8 C)

_l I I I I 1 I I L 1.5 2.0

KCI (grams)

Figure 6. Conductivity of potassium chloride solution. Also shown are regression fits for the

conductivity as a function of concentration of KCI and for different temperatures.

4.4 Conductivity of Candidate Liquids

In this section we summarize the measurements on liquid PMMDs. We will present results on

the concentration and temperature dependence of the conductivity. For the measurements on

liquids we used both a dc conductivity probe and a shielded open-circuited holder. In figure

6, the dc conductivity of KCI in 500 ml deionized water is plotted versus KCI concentration

for minor variations in temperature. The uncertainties in the liquid measurement are due

to uncertainties in the measurement device and uncertainties in the chamber temperature.

Our lab has a temperature stability of ±2 "C. For measured conductivities of cr = 0.5 (S/m),

the Type B expanded relative uncertainty was U = kuc = 0.02 (k-2).

For the KCI solution the dependence of conductivity on KCI concentration is nearly linear.

We see that we can obtain the required conductivities using the appropriate concentration of

KCL We also studied the temperature dependence and time stability of these solutions and

the results are shown in figures 7 and 8 for temperature of 22 "C ± 0.5 °C. The fluctuations

in figure 8 are due to temperature variations when calibrating the conductivity probe. For

comparison, typical conductivities and permittivities of the human body were shown in

figures 3 and 4. There is no percolation threshold for these materials.

The following formula fits the conductivity of a mixture in (S/m) of deionized water as a function of KCI content at 22 °C, where rukci is the mass per millihter of the potassium

10
10 20 30 40 50 60

Tenperature (Q

Figure 7. Temperature dependence of conductivity of the potassium chloride solution.

chloride,

a = 0.011 + 181.5 mfcc^ (5)

We also measured another liquid mixture that was studied by Broadhurst [3] . The results are shown in figure 9. This liquid contains a 50-50 mix of ethylene carbonate and propylene

carbonate and the salt tetraethyleammonium tetrafluoroborate (TEATFB). We found that

by varying the salt concentration we could vary the conductivity in the desired range. A

measurement of the mixture given in figure 9. The following formula fits the conductivity

data as a function grams of TEATFB {rritea) per 100 g of propylene carbonate

a = 0.0095 + m25mtea - 0.00112mL- (6)

We also studied the temperature and time stability of these solutions and the results are shown in figures 10 and 11.

4.5 Conductivities of Carbon-Black-Silicone Materials

We studied two types of CBS (a silicone rubber composite) mixtures. These were 'XC-72"

and 'Black Pearls', both from Cabot Inc.^ The conductivity of CBS is not a linear function

^Products or companies named here are cited only in the interest of complete scientific description, and

neither constitute nor imply endorsement by the National Institute of Standards and Technology or by the

U.S. Government. Other products may \^c found to servo just as well.

11
u

0.320

0.290

one g KCl in 500 mL deionized water ''' I ' 'I 'I 'I I I I I I I I I I I I I I I I I I I I I I I I I I ( I I I I I I 100 200 300 400 500

Hours

Figure 8. Time stability of the potassium-chloride solution conductivity normalized to the

measurement at 21 °C. Note: Fluctuations are due primarily to temperature variations.

en

3

-oB

OO

10 12 14

Grams TEMFB

Figure 9. TEATFB solution with 100 ml propylene carbonate and 125 g ethylene carbonate.

12
s

on

>

o

O

10 20 30 40 50 60

Tenperature (Q

Figure 10. Temperature dependence of the conductivity for various concentrations of the salt

for TEATFB with 100 ml propylene carbonate and 125 g ethylene carbonate and varying

salt concentrations.

u

o

-a

SI

B

O

0.330

0.300

6 grams TEATFB in mixture of 50% Propylene Carbonate and 50% Ethylene Carbonate by weigiit

' ' ' I ' '

I 'I I '

I I I I I I I I I I I I I I I I I I 100 200 300 400 500 600 700

Hours

Figure 11. Time stability of the conductivity of 6 g TEATFB with 100 ml propylene car- bonate and 125 g ethylene carbonate. The temperature uncertainty over the measurement

was ±1°C.

13
of concentration, as was the case with KCl. The CBS conductivity goes through a percolation threshold where it abruptly increases. This percolation phenomena has been studied

previously [14-16]. In figures 12 through 22 we display conductivities of CBS composites.

The measurements of the composite, as a function of carbon-black-silicone weight fractions,

showed that we could also simulate human-body conductivity with carbon-black, 'XC-72'

and 'Black Pearls'. As the concentration increased to 20 % for 'XC-72' carbon black and to

10 % for the 'Black Pearls', the CBS became very diflicult to mix. The percolation threshold occurs between 10 % to 15 % carbon black by weight for the 'XC-72' material and between

6 % to 8 % by weight for the 'Black Pearls' mixture. We determined that with CBS conductivity of (7 = 0.001 (S/m), the Type B expanded relative uncertainty was U = kuc — 0.001

(k-2); with a = 0.1 (S/m), the Type B expanded relative uncertainty was U = kUc = 0.02 (k2); and with a = 0.56(5/m), the Type B expanded relative uncertainty was U = kUc = 0.02

(k-2). The density of carbon black is about 1.8 g/ml. Therefore conversion between weight

fraction and volume fraction is accomplished by multiplying by 1.8. We performed tests that study the concentration dependence and reproducibility of the

mixtures. In figure 12, we plot the conductivity versus frequency as a function of the weight

fraction of carbon black 'XC-72' in silicone. The conductivity increases abruptly near 12

% carbon-black concentration for the 'XC-72' mixture. This conductivity increase is due

to a percolation phenomenon, where electronic tunneling transport connect the conducting

paths throughout the sample [16]. The measurements on different batches as a function

of curing time indicates the degree of reproducibility with the mixing process we used as shown in figures 12 through 16. We see that the average conductivity varies exponentially

with concentration of 'XC-72' carbon black as shown in figure (12) for 'XC-72' and in figure

(16) for 'Black Pearls' (see Appendix D). Above the percolation threshold it varies nearly

linearly with concentration. The conductivity stabilized after temperature annealing at a temperature of 60 °C for 2 h, as shown in figures 19 and 20. See Appendix D for a review of

conductivity of mixtures.

The time drift in the measurement is given in figures 19 and 20 by the "box" symbols.

The temperature dependence is given figures 21 and 22. The initial increase in conductivity

as temperature increases we attribute to relaxation and curing of the composite.

4.6 Glycine-Based Phantom Material

We also studied a mixture of glycine, carrageenan, KCl, and water. The results are shown

in figures 23 and 24. The carrageenan causes the glycine solution to gel to a jelly-like consistency, depending on water content. The resultant mixture requires refrigeration. Most

of the measurements on this solution were made with a dc-conductivity meter. However, a

14
E

|>

o

o

oO

1.0

0.8

0.6

0.4

0.2

-e- Sil ^Sil

-a- Sill --Sill

--Sill

cone + 10% Carbon Black by weight cone + 12% Carbon Black : 2 measurements of different materials cone + 12.5% Carbon Black cone + 13% Carbon Black : 2 measurements of different rraterials icone + 13.5% Carbon Black : 3 measurements of different materials icone + 15% Carbon Black

10^

? ?? ? ? ? w^ ' f f Iff ffff Iff fff ffff f ffy:f=|df

D D DDD DDDD D DD D DD DDDD D D DDD D[ ] D D DDD DDDD DDD D DD DDDD D D DDD D[ ]

QQ(j) i...ji ^ j) ij^ i (i)

i (j)(j) (j> i j) 6j) (j)

M

i (i)i (})(!)

6

' (!) d) (!) (j) j) i (iii (!) 23456 23456 23456

10 10

Frequency (Hz)

10^

Figure 12. CBS using 'XC-72'.

CO

-T3(3O

0.26

0.24

V 9—9 99W f yy f 999 ^ff Iff W 9—i i f ^ ^

<'^ 00000000000000000 ooc^f^ '

If A A AAA AAAA AAA AAA AAAA A A A AfAr-^ ^ 0.22

Hr;min from first meas = 0:0 030

1:00

1:30 2:30

19:15

19:30

Q 18 I i I il I

' ' ' I III . ..I I

I . . . .1 I I 2 3 4 5 6 2 3 4 56 23456

10' 10' io' 10'

Frequency (Hz)

Figure 13. Conductivity changes in time due to curing and relajcation for 13.5 % CBS 'XC-72'

mixture, sample 1.

15
0,32

on

•X3a

o

o

0.30

0.28^

0.26

0.24

0.22

0.20

0.18

51 9

y iff iff i iff iff fiWi i—i i 1 1 i "

H 1 I I I I I I I III I I I II

-- Hr:min from first meas= 0:0 ^0:15

--0:45

-t- 1:00 -e- 1:30 -e- 16:30 -f- 17:30 J I I I I I II ... .1 I

I I I I Ml . ..I I

I I I I II 2 3 4 56 23456 23456

3 4 5 6 10 10 10 10

Frequency (Hz)

Figure 14. Conductivity changes with time due to curing and relaxation in the 13.5 % CBS

'XC-72' mixture, sample 2.

16
0.28

0.26

• 1—1 >

lb

0.24 ir

^ 0.22 c

o

Hr:minfrom nrblmeas = UO

30

30

30

30

30

4 15

H 1 III I I I I I I I III I

Q -IQ I ' I

I I I Ml ..I I II ..I I I I I Mil

23456 23456 23456

10^ 10'^ 10^ 10^

Frequency (Hz)

Figure 15. Conductivity changes in time due to curing and relaxation for 13.5 % CBS

'XC-72', sample 3.

1.2-

1.0 - ^ 0.8

>-.

*-* >

o 0.6 3

-aB

OU 0.4

0.2 -

0.0

Percent Carbon Black Mb<ed with Silicone (by weight)

Figure 16. Conductivity as a function of carbon-black concentration for 'Black Pearls' mixture.

17
oo

-ao

Percent Carbon Black mbied with Silicone (by weight)

Figure 17. Conductivity as a function of carbon-black concentration for 'XC-72' mixture.

test was also performed to compare the dc-conductivity results to those from the frequencydependent shielded open-circuit holder. The measurements at 100 kHz are shown in figure 24.

5. Discussion

Conductivities close to those of tissues in the human body can be reproduced by either

carbon-black composites, glycine composites, or various salty solutions. The liquids can be

easily mixed to obtain the desired response. These hquids also show good time stability

in the conductivity and a predictable temperature response. The CBS shows less time

stability. To obtain time stability in the CBS the sample must be temperature annealed.

The conductivity of CBS exhibits a percolation threshold at which conductivity increases

abruptly. The temperature dependence in the CBS is more complicated than that of liquids

and depends on whether the sample's carbon-black concentration is above or below the

percolation threshold. The silicone material has the advantage of being more rugged than

the glycine and liquid mixtures. CBS has the disadvantage that its curve for conductivity

versus concentration is very steep around the percolation threshold. Liquids are not as

18
a

on

>

=3O

-•- Silicone + 5% Carbon Black by weight -9- Silicone + 6% Carbon Black by weight -k- Silicone + 7% Carbon Black: measurements of 6 different samples -B- Silicone + 8% Carbon Black -- Silicone + 10% Carbon Black

* D D DDD DDDD DDD DDD DDDD D DDDDDfl]

1.2

1.0

0.8

0.6

0.4

0.2

Q Q^ '

' 'i § i if '§ 4 i %

' i 9i i ii i§ 4 i ^ 'i 8 i ii 'i'il

2 3 4 5 6 2 3 4 5 6 2 3 4 5 6

k k kkk kkkk kkk kkk kkkk k kkkkki i i= k k kkk kkkk kkk kkk kkkk k kkkkki i timttititHMHttntuti

10^ 10^ 10= 10"

Frequency (Hz)

Figure 18. Conductivity as a function of carbon-black concentration for 'Black Pearls' mixture.

c?5

3

-aco

20x10

-.9)

15

10

DD

D after annealing test

Odd DDDD DD C

Q

II I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I 200 400 600 800

Hours

Figure 19. Change in conductivity due to relaxation in 6 % 'Black Pearls' CBS.

19
on

>

o

(^

0.34

0.32

0.30

0.28

0.26

0.24

0.221-

0.20

(5)

D after annealing test

®

[P] %] DO

D

J I I I i I L J I I I I I \IM I I 500 1000

Hours

1500 2000

Figure 20. Change in conductivity due to relaxation in 7 % 'Black Pearls' CBS.

5.0x10'

S

on

3

o

4.5

4.0

3.5 -

3.0

average for frequency range 1 KHz - 1 MHz

I I I

' ' ' '

I

'

I

' '

I

' ' ' '

I

'

I I 'I 'I'll'

10 20 30 40 50 60

Temperature (Q

Figure 21. Dependence on temperature of 6 % by weight 'Black Pearls' CBS.

20
0.28

0.26

>• 0.24

oO

0.22

0.20 -

Silicone + 7% Carbon Black by weight O average for frequency range 1 KHz - 1 MHz

I " 'I " I " 'I I I " " I " " I " I 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

1

10 20 30 40

Tennperature (C)

50 60

OLESmBasured 10/2 and 10/10 2001 rik

Figure 22. Dependence on temperature of 7 % by weight CBS 'Black Pearls' CBS.

•

1.0 : y^

0.8 L M^

e Jf ^ Jr

>-, ^/

0.6 — Jf > m—

o y^

3 Jm

-o ^r c 0.4 — Jf o ^r U Jf

0.2

ri 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1. i.-i 1 1 1 1 1 1

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

Ka(g)

Figure 23. The conductivity of 6 % by weight glycine in water, with carrageenan as a function

of KCl concentration.

21
C/3

o

3

-ac

o

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.1 gKCl

0.2gKCi

0.4 g KCl

0.6 g KCl

0.6 g KCL measured with LCR meter

6% glycine and 2% carageenan by weight

I I I I I I I I I M 1 I I I I I I I I I r I I I I I I I I I I I I I I I I I I 1 I I r I I I I I I I I I I I I I 10 20 30 40 50 60

Tenperature (Q

Figure 24. The conductivity of 6 % by weight glycine in water, with carrageenan as a function

of temperature and KCl concentration.

convenient as solids for research materials for use with metal detectors.

We acknowledge the support of the National Institute of Justice and Dr. George Lieberman

and Kathy Higgins of the Office of Law Enforcement Standards of NIST. We acknowledge

Les Burk of Ranger Security Detectors for helpful conversations and permission to reproduce

figures of metal detectors and magnetic fields found on their web site.

6. References

[1] K. L. Stricklett and J. Baker-Jarvis, "Electrical properties of biological materials: A

bibliographic survey," NISTIR 6564, Natl. Inst. Stand. TechnoL, 2001.

[2] C. Gabriel and B. Gestblom, "Microwave absorption in aqueous solutions," Nature,

vol. 328, pp. 145-146, 1987.

[3] M. G. Broadhurst, C. K. Chiang, and G. T. Davis, "Dielectric phantoms for electromagnetic radiation," J. Mole. Solids, vol. 36, pp. 47-64, 1987.

22
[4] T. Kobayashi, T. Nojima, K. Yamada, and S. Uebayashi, "Dry phantom composed

of ceramics and its application to SAR estimation," IEEE Trans. Microwave Theory

Techn., vol. 41, pp. 136-140, 1993.

[5] M. J. Hagmann, R. Calloway, A. Osborn, , and K. Foster, "Muscle equivalent phantom materials for 10-100 MHz," IEEE Trans. Microwave Theory Techniques, vol. 40,

pp. 760-762, 1992.

[6] C. Chou, G. Chen, A. Guy, and K. Luk, "Formulas for preparing phantom muscle tissue

for various radiofrequencies," Bioelectromagn. , vol. 5, pp. 435-441, 1984.

[7] D. Andreuccetti, M. Bini, and A. Ignesti, "Use of polyacrylamide as a tissue-equivalent

material in the microwave range," IEEE Trans. Biomed. Eng., vol. 35, pp. 275-277,

1988.

[8] H. Kato and T. Ishida, "Development of an agar phantom adaptable for simulation of

various tissues in the range 5-40 MHz," Phys. Med. Bio., vol. 32, pp. 221-226, 1987.

[9] Y. Nikawa, M. Chino, and K. Kikuchi, "Soft and dry phantom modeling material using

silicone rubber with carbon fiber," IEEE Trans. Microwave Theory Techn., vol. 44,

pp. 1949-1953, 1996.

[10] C. Marchal, M. Nadi, A. J. Tosser, C. Roussey, and M. L. Gaulard, "Dielectric properties

of gelatine phantoms used for simulations of biological tissues between 10 and 50 MHz,"

Int. J. Hyperthermia, vol. 5, no. 6, pp. 725-732, 1989.

[11] "CENELEC Standard 50361: Basic standard for the measurement of the Specific Absorption Rate related to human exposure to electromagnetic fields from mobile phones

(300 MHz - 3 GHz)" CENELEC, 2000.

[12] M. J. Peters, J. G. Stinstra, and M. Hendriks, "Estimation of the electrical conductivity

of human tissue," Electromagn. , vol. 21, pp. 545-557, 2001.

[13] J. Baker-Jarvis and J. H. Grosvenor, "Dielectric and magnetic measurements from -50c

to 200 c and in the frequency band 50 MHz to 2 GHz," Tech. Rep. NISTIR 5045, Natl.

Inst. Stand. Technol., 1996.

[14] E. Tuncer and S. M. Gubanski, "Electrical properties of filled silicone rubber," J. Phys.

Condens. Matter, vol. 12, pp. 1873-1897, 2000.

23
[15] A. Nakayama, M. Inagaki, M. Miyauchi, and N. Itoh, "Dielectric dispersion of polycrystalline aluminum nitride at microwave frequencies," J. Am. Ceram. Soc, vol. 79,

pp. 1453-1456, 1996.

[16] S. Nakamura, T. Tomimura, and G. Sawa, "Electrical conduction mechanism of polymer

carbon black composites below and above the percolation threshold," in Proc. Electrical

Insulation and Dielectric Phenomena, vol. 1, pp. 265-268, IEEE, 1998.

[17] L. Hartshorn and W. H. Ward, "The measurement of the permittivity and power factor

of dielectrics at frequencies 10'* to 10* cycles per second," J. Inst. Elec. Eng., vol. 79,

pp. 597-609, 1936.

[18] A. von Hippel, Dielectric Materials and Applications. Cambridge, MA: M.I.T. Press,

1954.

[19] H. E. Bussey, "Dielectric measurements in a shielded open circuit coaxial line," IEEE

Trans. Instrum. Meas., vol. IM-29, pp. 120-124, June 1980.

[20] J. Baker-Jarvis, M. D. Janezic, and C. A. Jones, "Shielded open-circuited sample holder

for dielectric measurements of solids and liquids," IEEE Trans. Instrum. Meas., vol. 47,

pp. 338-344, April 1998.

[21] S. Jenkins, T. E. Hodgetts, R. N. Clarke, and A. W. Preece, "Dielectric measurements on

reference liquids using automatic network analyzers and calculable geometries," Meas.

Sci. Tech., vol. 1, pp. 691-702, 1990.

[22] P. I. Somlo, "The discontinuity capacitance and the effective position of a shielded

open circuit in a coaxial line," Proc. Inst. Radio Elec. Eng. Australia, vol. 28, pp. 7-9,

January 1967.

[23] W. R. Scott and G. S. Smith, "Error analysis for dielectric spectroscopy using shielded

open-circuited coaxial lines of general length," IEEE Trans. Instrum. Meas., vol. IM-35,

pp. 130-137, June 1986.

[24] P. N. Hill and H. E. Green, "/n situ measurement of soil permittivity and permeability,"

I. Electrical Electronics Eng., Australia, vol. 2, pp. 205-209, December 1982.

[25] R. L. Jesch, "Dielectric measurements of oil shales as functions of temperature and

frequency," IEEE Trans. Geosci. Remote Sensing, vol. GE-22, pp. 99-105, March 1984.

24
[26] M. A. Stiichly and S. S. Stuchly, "Coaxial line reflection methods for measuring dielectric

properties of biological substances at radio and microwave frequencies-A review," IEEE

Trans. lustrum. Meas., vol. IM-29, pp. 176-183, 1980.

[27] J. R. Mosig, J.-C. E. Besson, M. GE-Fabry, and F. E. Gardiol, "Reflection of an openended coaxial line and application to non-destructive measurement of materials," IEEE

Trans. Instrum. Meas., vol. 30, no. 1, pp. 46-51, 1981.

[28] H. Levine and C. H. Papas, "Theory of the circular diffraction antenna," J. Appl. Phys.,

vol. 22, pp. 29-43, January 1951.

[29] D. S. McLachlan, "Analytical functions for the dc and ac conductivity of conductorinsulation composites," J. Electroceramics, vol. 52, pp. 93-110, 2000.

7. Appendix A: How Carbon Black is Produced

Carbon black is formed from heating hydrocarbons in a furnace as shown in flgiure 25. The

reaction follows

C^Hy + 02^C + CH^ + CO + H2 + CO2 + H2O. (7)

Carbon black is a particulate form of elemental carbon. In the carbon formation process,

the liquid feedstock decomposes to form carbon radicals and condenses into spherical particles. The average diameter of the primary particles is between 12 nm to 75 nm. Therefore

these are nanomaterials. The surface areas are between 25 m^/g to 500 m^/g. The primary

particles coUide in the reactor to form aggregates. The aggregates coalesce into larger particles caUed agglomerates, as shown in figure 26. The aggregates are held together by weak

intermolecular forces. Carbon black has a density of approximately 0.16 g/cc. The smaUer

the size of the carbon-black particles, the higher the conductivity. The higher electrical

conductivity is due to the interconnectedness of the aggregates. The size distribution of the

carbon black changes when mixed, due to break up of conglomerates. The degree of breakup

slightly changes the conductivity from batch to batch. In order to enhance reproducibility

of the electrical conductivity, the materials must be mixed in a reproducible fashion.

Carbon-black polymer composites have a resistive relaxation due to motion of the polymer

chains, which allows some reorganization of the carbon black with time. This manifests itself

in a small drift in conductivity until the cure is complete. Literature sources recommend

heating the cured sample (or annealing) to minimize resistive relaxation.

25
Refractory

Figure 25. Carbon black manufacturing process.

Particle

Aggregate

Agglomerate

Figure 26. Carbon-black formation.

26
8. Appendix B: Percolation Threshold

for Conductivity

When carbon black is added in a matrix material, a transition occurs from an insulating

to conductive material as the volume fraction of carbon black increases. Percolation oc- curs when conducting paths are formed across the sample. Near the percolation threshold,

the conductivity increases rapidly. The relaxation processes are dominated by tunneling

processes that are strongly temperature dependent. Resistive relaxation is influenced by the

application of static bias electric fields. This field-dependent eflPect is related to local dielectric breakdown and charge hopping over reduced potential barriers. Above the percolation

threshold, the conduction is dominated by charge hopping.

9. Appendix C: Shielded Open-Circuited Holder

9.1 Measurements with the Shielded Open-Circuited Holder

The shielded open-circuited holder is very useful for measuring the dielectric properties of

powders, liquids, and semisolids because of the ease of sample insertion. Previous work

performed using the shielded open circuit at microwave frequencies has been performed

by Hartshorn and Ward [17], Von Hippel [18], Bussey [19], Baker-Jarvis [20], and others

[21-23]. Also Hill and Green [24] studied in situ measurements of soils using open-circuited

transmission lines. Jesch [25] used the shielded open-circuited holder for measurements on

oil shale. Biological tissues have been measured using the shielded open-circuited line, for

example, by Stuchly and Stuchly [26].

9.2 Theoretical Formulation

Permittivity measurements are most accurate when the sample is located in a region of

strong electric field. A strong electric field in a coaxial line is obtained most easily by use

of a shielded open circuit. The shield on the open circuit allows an accurate analytical field

model to be developed since the fields can be expanded into a series of discrete eigenmodes.

Consider a sample in the transmission line shown in figure 5. The shielded open-circuited

holder consists of three sections. Region 1 is the bead in the airline, region 2 is the sample,

region 3 is the shield region. The problem is to accurately characterize the shielded open

circuit. We will develop a compact expression for the refiection coefficient in terms of air

line and sample parameters.

27
We assume a time dependence oiexp{jut) in the coaxial line. Due to azimuthal symmetry,

we need to consider only TEM and TMomn modes. We also assume that an incident TEM

wave travels from the input port in the direction of the sample. At the sample discontinuity

the TMomn modes are reflected, and a TEM wave and evanescent TMomn waves travel into

the material. The problem we wish to solve is to relate the reflection coefficient to the

complex permittivity.

The TEM mode admittance and reflection coefficient are related by

where Yq is the admittance of the air-filled line and Y is the admittance with the sample in

the line.

The radial component of the electric fields in regions 1 through 3 are

Ep{i) = Ro{p) exp (-7102) + J2 ^nRnip) exp(7in2), (9)

n=0

00 00 ^P(2) = 5Z ^nRnip) exp (-72n2) + J] B^Rnip) exp(72n2;), (10)

n=0 n=0

oo ^P(3) = J2 QnGnip) exp (-73n(2 - L)), (11)

n=l

where

1.0 = :^^, (12)

C

for n = 0, for z = 1, 2, and if n >

A

a;2

^lab

for z = 1,2 and for all modes in region 3. If the argument is negative (evanescent wave) then

Iv

i

a;2 kin - eriPri^-- (14)

^lab

Here Gn are normalized eiegnfunctions in the cylindrical waveguide and Rn are the normalized eigenfunctions in the coaxial line, where

Gn{p) = SnJ\{hnP)- (15)

The eigenvalues are found from the vanishing of the tangential electric field on the side wall

Mhnh) = 0. (16)

28
Therefore ks„ — Pn/b, where Pn is the nth root of Jo{x) — 0, for n = 0, 1, 2... . The azunuthal magnetic fields in regions 1 through 3 are

^<^(i) = ^-Ro(p) exp (-7102) - Yl r„ -Ra{p) exp(7in2), (17)

7lO ^0 7ln

00 • CX3 ^^{^) = Jl -AnRn{p) exp (-72n2) - J^ -BnRn{p) eX-p{j2nZ), (18)

71=0 '^2n n=0 ')'2n

oo

^</>(3) = E ^^^QnG'„(p) exp (-73n(2 - L))

.

(19)

71=0 73n

9.3 Matching of Tangential-Field Components

According to Maxv^-ell's field equations, the tangential components of Ep and H^ must be

continuous across interfaces. If we match eqs. (9) and (10) at 2 = we obtain the following

equations

Am + Bjn = 5mQ + ^m-, (20)

where m = 0, 1, 2 If we match eq. (10) and eq. (11) at z = L we obtain

oo Qm=Yl ['^ri exp {-^2nL) + B„ exp (727xi^)] < GmRn >, (21)

77 =

or Q = M2[M6A + MrB], (22)

where

-^(2)mn =< GmRn >, (23)

^(6)mn = exp {-j2nL), (24)

M^7)mn = exp (72n^)- (25)

If we match eqs. (17) and (18) at 2 = we obtain

Am — Bm = [6mO " ^m] , (26)

where m = 0, 1, 2. If we match eqs. (18) and (19) at z = L we find

—^ E Qrn < GmRn >= An CXp (-727Ti^) - Bn CXp (72n-^), (27)

^2 771=0 73m

or MiQ = MeA-M-B. (28)

29
where

M,(2)r -< GmRn >

lZm^r2

9.4 Solving for the Coefficients

We can solve for the coefficients in eqs (20) and (26) using

and

Let

-^n —

n

2

C>n0(l + ) + i n(l - ) er27ln er27lr

< /, Crl72nx I r> /I I ^rl72nx

(^n0(l j + i n(H ) er-27ln l=C + M3f,

B = D + M4f,

er27ln

where

M(3)^^ = o P-

€rl72n

er27ln,

erl72n

M(4)mn = X (l +

2 \ er2lln

<^n0(l H ) ej.27in

^n0(l ) er27ln

(29)

(30)

(31)

(32)

(33)

(34)

(35)

(36)

(37)

We obtain the following equation for the permittivity given measured F

[MiM2(M6M3 + M7M4) - M3 + M4)]r = [I - MiMaMgJC - [I + MiMsMr]^, (38)

or in terms of matrices and vectors

p-r = r. (39)

Given measured results of the TEM mode Fq, we use Cramer's rule in eq. (39) to get an

equation for e* of the form Fq = /(esr)-

For measurements, the reference plane must be transformed through the bead (6) and

air (a) sections. The transformed reflection coefficient for the TEM mode is

Ttrans = Toexp (-2(7aLi + 76-^2)), (40)

30
where Li and L2 are the effective axial lengths of the air and bead sections of the connector.

The bead is usually nonuniform, so an effective length needs to be determined from a standard

measurement.

9.5 Integrals

also

< GnRo >—

and

Gn{p) = SnJl{hnP), (41)

S ^ I = ^

(42)

^/IS pJKhnP)dp bMksnby

f Mk3nP)dp=-—-, (43)

Jo ksn

/

1 Sn /ff> J\{hnp)dp= I 11

=Sn- Joik^na)

,

(44)

^\n{b/a) J- y/ln(6/a) ^3n

< GnKn >= Sn j pJi{k3nP)Rm{p)dp = SnDmn, (45)

Ja

where the radial eigenfunctions are

Rn{p) = Co/P

for n=o (TEM mode) = Cn[Mk2np)NQ{k2na) - Ni{k2nP)Mk2nCL)], (46)

^^ V

' n>o(TMo„ modes)

where Ni, i = 0, 1, are the Bessel functions of the second kind and the constants Cn are obtained by requiring orthogonality [27], [28]:

/ CRmibQRniaOdC = (5^n m, n = 0, 1, 2 . . . , (47)

Ja

Co = , ^

, (48)

/ln(6/a)

(49)

and

1 7r/C2n ^n —

\IapRlip)dp V2 / 4ik,.a) _

^-

The coefficients DmniC) = Iq pJ\{k2,mp)Rn{p)dp can be found analytically. For n =

Joik^mo) 1 DmoiQ = I / Jl{k3mP)dp = . -

Y' In 6/a •^'i Jin b/a /^ (50)

3m

31
and otherwise

rb Dmnihn) = / pRn{p)Jl{hmP)dp = ZTTTJi TjrMhma). (51)

10. Appendix D: Mixture Theories for

Conductive Materials

Consider the effective conductivity am from a material consisting of high a^ and low conductivity (7;. The volume fraction of the less conductive phase {ai) is 9 for the various models

we will review [29] (see figure 27).

The Maxwell-Wagner model can be envisioned as the effective conductivity for a space

filled with coated spheres. The Maxwell-Wagner model for a high-conductivity coating on

low-conductivity spheres is <T„-<T. ^g^i^^ (52)

(Tm + 2(7/1 CT/i + 2(7; The Maxwell-Wagner model for a low-conductivity coating on highly conducting spheres is ^-"^' =(1-^)J^^L_^. (53)

drn + 2(7; (T; + 2(Jh

Bruggeman's equation for asymmetric media for high-conductive coating on low conductive spheres is (^^-<^>f ^^^_gf (^H-C,f

^ (5^j

and Bruggeman's equation for asymmetric media for low-conductive coating on highly con- ductive spheres is (^m - Oh? ^ ^3 {(^l - (^hf ,^^.

Bruggeman's equation for symmetric media filled with high and low conducting ellipsoids is

^ /^^~;-\ + (1 - ^) /^^~;-\ ^ 0, (56)

where A depends on the demagnetization coefficients of the ellipsoids and is 2 for spheres.

None of the above described mixture equations model percolation behavior.

Particles in fluids are often modeled by Archie's law. If the particles occupy a volume

fraction ^, then the effective conductivity is

Oefr^^jp^l^ishL^^'^ ^i_e, (57)

O'solv ~ (^part \ (^eff J

32
o

a) MaxweU-Gamet, b) Symmetric Bmggeman

Asymmetric Bmggeman

Figure 27. The models used in the various mixture theories, with black denoting lowconductivity regions, and white the high-conductivity regions: (a) Maxwell-Garnet and

asymmetric Bruggeman models, and (b) symmetric Bruggeman model.

where La is the effective depolarization factor. In the special case where apart = 0, we have

Archie's law,

(Teff ^ (JsoUl - 0Y/^'-'^'^\ (58)

In highly conductive materials, the frequency dependence in ac conductivity can sometimes be modeled as a power-law:

(Jm oc (-)^ (59)

em oc (-)--, (60)

where m + n = I.

33

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