Reusable fast opening switch

A reusable fast opening switch for transferring energy, in the form of a high power pulse, from an electromagnetic storage device such as an inductor into a load. The switch is efficient, compact, fast and reusable. The switch comprises a ferromagnetic semiconductor which undergoes a fast transition between conductive and insulating states at a critical temperature and which undergoes the transition without a phase change in its crystal structure. A semiconductor such as europium rich europhous oxide, which undergoes a conductor to insulator transition when it is joule heated from its conductor state, can be used to form the switch.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a switching device and more particularly 
to a reusable, fast opening switch for producing electrical power pulses. 
2. Background of the Invention 
Electrical power pulses have been produced by charging an electrostatic 
energy store, such as a capacitor, and then discharging it, by means of a 
closing switch, into a load. Because known electrostatic energy stores 
have a very low energy density, they are generally of large physical 
dimensions. 
Electromagnetic energy storage systems, having very high energy densities 
are also known. However, when used in connection with short pulse duration 
(&lt;&lt;1 microsecond), high power, pulse forming networks prior art closing 
switch technology limits the power gain per stage of the electromagnetic 
energy storage systems to about a factor of 3. This is because prior art 
closing switch technology is basically directed to fuse type devices, 
which, as they get larger, require so much of the energy in the storage 
device to "switch" it, that very little power gain can be realized at the 
load. 
For a typical pulse forming switching system, the power to the switch, 
P.sub.in, is equal to the energy supplied from the source (W.sub.o) 
divided by the time it takes to charge up the storage device (t.sub.o). 
##EQU1## 
The power delivered to the load is equal to the efficiency of the switching 
multiplied by the energy supplied (W.sub.o) divided by the time it takes 
to deliver the energy from the storage device to the load (t.sub.i). 
##EQU2## 
Therefore the power gain, A, which is the ratio of P.sub.out over 
P.sub.in, can be described as follows: 
##EQU3## 
When the power gain of such systems is about 3, the efficiency drops off 
as t.sub.o goes up. Therefore, for high power requirements many large 
stages are required. A fast opening switch would permit the use of 
magnetic energy storage which may typically have up to one thousand times 
the energy density of an electrostatic system. However, for such an energy 
storage system a switch having a large impedance change from a low 
resistance to a high resistance resulting in a correspondingly large power 
gain is required. 
Moreover, fused wire type switches generally have a relatively low 
breakdown electric field (about 20 kilovolts per centimeter) and have to 
be replaced after each "shot". Since many "fuses" are usually involved in 
such switching systems, there are significant economic and technical 
disadvantages to the fuse switch approach. In addition, most fuse switches 
have switching times well in excess of 10.sup.-8 seconds and, as alluded 
to above, are relatively inefficient, absorbing about 30 percent of the 
energy available in the energy storage inductor. 
Therefore, a need exists for a fast opening (on the order of 1 nanosecond) 
switch, capable of switching between a highly conductive and highly 
resistive state and able to handle large current densities. To be useful 
for pulsed power source application, the switch must be able to switch 
back and forth repeatedly without damage. 
Vance, in U.S. Pat. No. 3,399,330, discloses a solid state switch which 
changes from a high resistivity to a low resistivity as a current pulse is 
applied. Return to high resistivity is accomplished by reversing the 
polarity of current flow. While such a device may be useful for computer 
memories or the like, it is not applicable to a pulse power generator 
application where a shift from low resistivity to high resistivity is 
required in order to produce the desired high power, short duration pulse. 
Geishecker, in U.S. Pat. No. 3,955,170, discloses a thermally activated 
opening switch. In accordance with the device of Geishecker, as the 
temperature of the device increases, the resistivity of an associated 
semiconductor also increases. Thus, the device of Geischecker displays a 
positive temperature coefficient (PTC). Geishecker is representative of 
many PTC devices presently available. In a 1967 article in Ceramic Age 
(Vol. 83, pages 44-47, May 1967) there are described many such devices. 
These PTC devices are used as switches to perform control, detection and 
regulatory functions. 
Barium titanate, the semiconductor material mentioned by Geishecker at 
column 1, line 42, is not metallic in its lower resistivity state and its 
low resistance value is relatively high as compared to metallic conductor 
materials, about 30 ohm-cm. Such a high impedance renders the material 
unsuitable for use in a pulse power application. While there is no hard 
barrier in terms of initial resistivity, for practical and efficient 
operation the initial resistivity of a switch for high power applications 
should be less than about 10.sup.-2 ohm-cm. As will be further developed 
below, an additional disadvantage attendant to high initial resistivity is 
that a large shock wave, proportional to the resistance, is created during 
switching. If high current densities are to be switched, the resistance 
must be kept low to prevent the creation of a shock wave which will be 
larger than the yield strength of the switch material. 
In addition, pulse power applications require that a very large increase in 
resistivity take place at the switch transition from low to high 
resistivity. In order to be effective the ratio of high to low resistivity 
must be at least a factor of 400. Higher ratios will yield cleaner 
switching with diminishing returns as the ratio increases. While in barium 
titanate, the high/low resistivity ratio is a factor of about 10.sup.4, 
the high initial resistivity and slow switching time make this material 
impractical for high power pulse generation. 
Moreover, the PTC-semiconductor transitions in material such as barium 
titanate involve changes of crystal symmetry. These structural changes 
seriously impair the usefulness of such materials when large current 
densities are involved since such materials tend to crack when subjected 
to large current densities due to the large and rapid changes in crystal 
structure which occur during transitions. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a reusable, 
fast opening switch. 
It is a further object of the present invention to provide a fast opening 
switch capable of passing high current densities. 
It is a still further object of the present invention to provide a switch 
which is metallic in its low resistivity state and operable to rapidly 
switch in response to the passage of a high current density, into a high 
resistivity or insulating state. 
It is a still further object of the invention to provide a switch which can 
switch back and forth repeatedly between a metallic and insulating state 
without sustaining damage. 
In accordance with the invention, an energy transfer circuit is provided 
which comprises an energy source, and an inductor connected to the energy 
source for receiving energy from the source, and a load switchably 
connected across the inductor. A reusable, high current, fast opening, 
semiconductor switch is connected across the load and operable to divert 
current flowing through the switch to the load as the switch is heated by 
charging current from the energy source. 
Preferably, the energy source can comprise a capacitor bank, a homopolar 
generator, a battery or the like and a crowbar switch can be used to 
protect the energy source. 
Advantageously, the switch material may comprise a ferromagnetic 
semiconductor such as oxygen deficient europhous oxide (EuO.sub.1-x). The 
switch of the present invention is thermally activated and operative to 
switch energy stored in the inductor to the load in less than 10 
nanoseconds, and preferably in less than 1 nanosecond, in order that high 
peak powers in the load can be obtained. In addition, the switch of the 
present invention is very efficient, absorbing less than 10 percent, and 
in some applications only about 2 percent, of the energy delivered to the 
load. 
Preferably, the semiconductor switch material functions as a metallic 
conductor with a resistivity of on the order of about 10.sup.-3 to 
10.sup.-6 ohm-cm in its low resistivity state and undergoes a resistivity 
change greater than a factor of about 400 and preferably greater than 
10.sup.10 when heated. The resistivity change preferably occurs over a 
temperature range of about 50.degree. C. and more preferably over a range 
of about 20.degree. C. Preferably, means are provided to maintain the 
switch below a critical temperature T.sub.c in which the semiconductor 
functions as a metallic conductor. 
The present invention is also directed to a method of producing electrical 
power pulses which comprises charging a magnetic energy store and then 
discharging the magnetic energy store into a load using a reusable, fast 
opening switch to divert current from the switch into the load, the switch 
being a thermally activated switch which exhibits a large impedance change 
to thereby generate a high voltage, short duration, power pulse. 
Preferably, the magnetic energy store is charged in about 5 microseconds 
and discharged in about the range of 1-10 nanoseconds, resulting in a very 
large power gain. 
As used herein, the term magnetic semi-conductor relates to a semiconductor 
having local magnetic moments. Such semiconductors have electronic and 
optical properties which alter upon the application of internal or 
external magnetic fields. Such materials also have magnetic properties 
which alter upon electronic or optical stimulation. Magnetic 
semiconductors can be tailored to specific uses by doping or alloying as 
further developed hereinbelow. 
Additional objects, advantages and novel features of the invention will be 
set forth in part in the description which follows and in part will become 
apparent to those skilled in the art upon examination of the following or 
may be learned by practice of the invention. The objects and advantages of 
the invention may be realized and attained by means of the 
instrumentalities in combinations particularly pointed out in the appended 
claims.

DETAILED DESCRIPTION OF THE INVENTION 
Reference will now be made in detail to the present preferred embodiment of 
the invention, an example of which is illustrated in the accompanying 
drawings. 
Turning first to FIG. 1, there is illustrated a circuit for producing a 
power pulse at a load D from a magnetic energy storage device, L.sub.L, in 
a time period of on the order of 10 nanoseconds or less. It should be 
understood that the switching speed will, to some extent, be dependent 
upon the load application involved and can be as fast as 1 nanosecond or 
less, or as slow as desired. 
The switch R, which forms the heart of the present invention, transfers 
magnetic energy stored in the energy storage inductor L.sub.L into the 
load D which may be resistive, reactive, time varying, distributed, etc. 
The load of FIG. 1 is an E beam diode which can be used as a source of 
electrons as developed below with regard to FIG. 4. 
The energy required to open the switch R of the present invention is only 
about 2 percent of the energy delivered to the load. Thus, the energy 
transfer is very efficient. The switch R is operable to pass a current 
density of on the order of 300 kiloamps per square centimeter as the 
inductor charging current in the circuit of FIG. 1 rises to its peak value 
in approximately 5 microseconds and operable switch from low to high 
resistance in approximately 1-10 nanoseconds as a result of joule heating 
from the charging current. The switch R is very compact, very fast, and 
overcomes the major disadvantages of the prior art devices discussed 
above. 
A high voltage DC source, such as a capacitor bank, is represented in FIG. 
1 by a series circuit consisting of R.sub.B, L.sub.B and C.sub.B. The 
capacitor bank is preferable for large, stationary applications. For 
mobile applications a battery or the like can be used. For other remote or 
mobile applications, a homopolar generator may be preferable. In any 
event, the source is only used to energize the load inductor L.sub.L 
which, in the embodiment of FIG. 1 is connected in series with the switch 
R. A crowbar switch S is connected across the source C.sub.B, L.sub.B, 
R.sub.B to short the source out of the circuit after the conductor L.sub.L 
is charged to thereby protect the source from over voltage. 
L.sub.s and C.sub.s represent any stray inductance and stray capacitance 
respectively on the load side of the switch R. In the embodiment 
illustrated, an E-beam diode D represents the load into which a power 
pulse, from the energy stored in the inductor L.sub.L, is transferred. 
It should be appreciated that the stray inductance L.sub.s, if large, will 
resist the buildup of current in the load since current through an 
inductor cannot change instantaneously. Therefore, if the inductance of 
L.sub.s is too large, a large voltage spike may appear across it. This, of 
course, should be carefully avoided when practicing the invention. 
The stray capacitor C.sub.s, can actually supply a transmission line effect 
in the circuit. This effect prevents any voltage from appearing across the 
inductor L.sub.s until the capacitor C.sub.s has charged. Since at very 
high frequencies the capacitor C.sub.s has a very low impedance, it can 
help to minimize the sum of the voltages V.sub.Ls +V.sub.Cs for a very 
short time. The operation of FIG. 1 will now be described. 
Assume first that the switch R is initially maintained at a temperature of 
about 4.degree. K., well below its critical transition temperature T.sub.c 
as indicated in FIG. 3. As the DC source (R.sub.B, C.sub.B, L.sub.B) 
supplies charging current to the load inductor L.sub.L, the inductor is 
energized. The energy dissipated in the switch R warms the switch material 
from its initial temperature of about 4.degree. K. to about 50.degree. K. 
by joule heating of the switch material. This occurs at the same time as 
the charging current in the inductor L.sub.L rises to near its maximum 
value. If the switch is an appropriate magnetic semiconductor of the 
construction detailed below, it is highly conductive below the critical 
temperature T.sub.c and undergoes a rapid transition to become an 
insulator above T.sub.c. For the circuit of FIG. 1, T.sub.c is taken to be 
50.degree. K. The cross section of the switch should be determined so as 
to ensure that the current, I, is near its maximum when the switch 
temperature approaches the critical temperature 50.degree. K. This should 
preferably occur in a time frame of on the order of 5 microseconds. The 
bridge wire or crowbar switch S shorts out the DC source to protect it 
from damage which could be caused by high currents. 
Additional heating of the switch R rapidly and dramatically increases its 
resistance to where it becomes an insulator. This diverts the current into 
the load. With a properly designed switch, the resistivity change may be a 
factor of 10.sup.13 or more and occur over a range only about 20.degree. 
K. While such dramatic resistivity changes are achievable with the present 
invention, it will be understood by the artisan that high/low resistivity 
ratios of 400 will provide adequate switching and that changes in 
resistivity of an order of magnitude for every 10.degree. C. will provide 
a useful switch. Since the resistivity change is very large and fast (on 
the order of about 1-10 nanoseconds), pulse power production is very 
efficient. 
In accordance with the present invention, high peak power at the load, in 
the range of 10.sup.10 through 10.sup.14 watts can be obtained utilizing 
reasonably sized circuit components. For instance, a relatively low 
voltage switch R for handling up to 3 kilovolts at about 200 amperes would 
be on the order of 1 cubic centimeter. For higher voltages (in the 1 to 10 
megavolt range) and currents of up to 1 megamp, the switch R might 
comprise a series of segmented switch sections of the type illustrated in 
FIG. 2 of about 30 centimeters in length and in the range of 1 to 2 
centimeters in diameter. The switch R is reusable and able to be used for 
many shots without sustaining damage. 
The switch R comprises a semiconductor material which does not undergo a 
phase change in its crystal structure during transitions between its 
conductive and non-conductive phases. Instead, the switch material 
undergoes an electronic transition at a critical temperature, T.sub.c, 
which does not affect the lattice symmetry. Moreover, the lattice 
parameters vary relatively smoothly throughout the transition from the 
metallic to insulative states. The breakdown voltage of the switch R is 
very high, and depending upon the switch geometry can be on the order of 
500 kilovolts per centimeter for even a relatively small switch. 
Therefore, large output voltages can be generated utilizing reasonably 
sized magnetic energy storage elements. 
It has now been found that certain doped magnetic semiconductor materials 
will exhibit the desired switching properties discussed above. Since 
material doping is a highly developed technology, very precise and 
reproducible switch materials can be tailored for various applications. 
An example of an appropriate magnetic semiconductor material is EuO.sub.1-x 
which is highly conductive below a critical temperature and which becomes 
an insulator above that temperature. See J. B. Torrance, M. W. Shafer, F. 
R. McGuire, Physical Review Letters 29, 7 pp. 1168-1171 (1972) hereby 
incorporated by reference. FIG. 3, which is a plot of the conductivity 
behavior of Eu rich EuO, is taken from that paper. Referring now to FIG. 
3, it can be seen that as the temperature of the material is lowered below 
300.degree. K., the conductivity decreases with an activation energy, 
E.sub.a, of about 0.3 eV which is similar to an ordinary semiconductor. 
Between 120 and 70.degree. K., the conductivity is too low to be 
accurately measured, i.e. is a non-conductor. Below the ferro-magnetic 
ordering temperature of 69.degree. K., the conductivity dramatically 
increases by over 13 orders of magnitude. The material is non-actuated or 
metallic below 50.degree. C. This behavior is known as an insulatormetal 
transition. 
Other references which provide background material regarding EuO behavior 
are A. Kornblit, G. Ahlers, and E. Buehler, Heat Capacity of RbMnF.sub.3 
and EuO near the Magnetic Phase Transitions, Phys. Letters, 43A, 531 
(1973), and M. B. Salamon, Dipolar-Dominated Critical Behavior of EuO, 
Solid State Comm. 13, pp. 1741-1745 (1973) and M. W. Shafer et al, 
Relationship of Crystal Growth Parameters To The Stoichiometry of EuO, J. 
Phys. Chem. Solids, 33, pp. 2251-2266 (1972), all of which are hereby 
incorporated by reference. 
Eu rich europium oxide (EuO.sub.1-x) is only an example of a ferromagnetic 
semiconductor which undergoes an electronic transition in which the 
material switches sharply with increasing temperature from being metallic 
to being a good insulator. It is noted that EuO.sub.1-x has a resistivity 
of less than 10.sup.-3 ohm-cm (i.e. is metallic) at low temperatures and 
transitions electronically, i.e. without changes or damage to its crystal 
structure, at temperatures above T.sub.c and therefore can be recycled 
back and forth without damage. The semiconductor may also comprise other 
ferromagnetic rare earth chalcogenide alloys with rare earth elements such 
as Eu, Gd or the like and group six elements such as O, S, Se, Te or the 
like. 
Not all ferromagnetic semiconductor materials can be used to make a switch 
according to the present invention. To be useful as a switch according to 
the present invention, the material must (a) exhibit the large 
conductor/insulator transition described above and (b) not experience a 
phase change in its crystal structure at the transition. Such phase 
changes can lead to the destruction of the switch material from internal 
stresses. In fact, some materials when forced to undergo very rapid 
crystal structure changes will literally blow themselves into powder due 
to the creation of large stresses in a small, constant volume. Also, 
switches having a large initial resistance, for example, greater than 
about 10 ohm-cm, will experience large shock waves upon switching since 
the shock power is directly proportional to the resistance. 
The physical structure of an exemplary switch will now be described with 
reference to FIG. 2. Switch material according to the present invention 
should have a low temperature resistivity (.rho..sub.o) of between about 
10.sup.-6 to 10.sup.-2 ohms-cm and a ratio of high to low temperature 
resistivity (.rho..sub.s) of at least about 400 and preferably in the 
range of 10.sup.3 to 10.sup.13. The switch should have a critical or 
transition temperature of on the order of about 20.degree. K. and an 
intrinsic switch time of less than 10 nanoseconds, preferably less than 
one nanosecond. For most applications, the switch should be able to 
survive a current density that deposits 100 J/gram in the material within 
50 ns and should not suffer damage from repeated, rapid switching or from 
handling large current densities. The switch material also has excellent 
thermal conductivity. 
Ferromagnetic semiconductors such as EuO and some other rare earth 
chalcogenide alloys are suitable for switch use. (EuS, NiO and UO.sub.2 
are anti-ferromagnetic and are not usable.) In these materials the 
electronic properties can be altered by changes in the internal or 
external magnetic field. The changes of the internal magnetic field are 
produced by changing the temperature of the material. These materials can 
also be tailored to specific applications by doping or alloying. For 
example, EuO.sub.1-x N.sub.x and EuNd.sub.x O.sub.1-x N.sub.x exhibit 
similar metal/insulator behavior as Eu rich EuO. 
Preferably, the switch material should have a uniform cross section A so as 
to have a uniform current density throughout. Otherwise, the switch will 
experience non-uniform joule heating and some regions of the switch will 
enter into conductor/insulator transition before others thus making 
inefficient use of the switch material. 
The plates 10 and 12 (FIG. 2) are mechanically or metallurgically bonded to 
the EuO material. The plates may be made of any good conductor such as 
steel, copper, aluminum or the like. For relatively low power 
applications, the switch and its associated components can be constructed 
on a printed circuit board using standard micro-circuit techniques. 
As alluded to above, during high power operation, the switch material will 
experience a shock wave which travels generally radially through the 
switch material (for a cylindrical geometry as shown in FIG. 2) and which 
can actually cause the switch to expand. As the shock wave reaches the 
outer periphery of the switch material, a relief wave is created which 
travels radially inward toward the center. The shock and relief waves will 
oscillate and quickly dampen out but the pressure difference across the 
initial waves can be very high. In very large power applications, the 
pressure difference may actually pull the EuO crystal apart. 
In order to attenuate the amplitude of the relief wave and to stretch out 
its duration, a shock absorber may be provided for the crystal. Such a 
shock absorber can be a simple casing, such as the coaxial sleeve 14 in 
FIG. 2, made of a resilient material which will operate to absorb or 
dissipate the initial shock so as to attenuate the relief wave. 
An explanation of the switch operation, based on the use of Eu rich 
europhous oxide (EuO) switch material follows with reference to FIGS. 2 
and 3. 
The resistivity of Eu-doped EuO increases from 10.sup.-5 .OMEGA.-m at 
T.ltoreq.50.degree. K. to 10.sup.10 .OMEGA.-m at 70.degree. K. as shown in 
the graph of FIG. 3, i.e., 
EQU .rho..sub.e =10.sup.-5 .OMEGA.-m, T&lt;50.degree. K. (4) 
EQU .rho..sub.e =10.sup.-5 exp(1.73[T-50]), 50.degree. K.&lt;T&lt;70.degree. K. 
where .rho..sub.e is the resistivity of the switch material. Therefore, the 
switch material is conductive below 50.degree. K. and its resistivity 
increases exponentially between 50.degree. and 70.degree. K. 
The resistance R of the switch of length and cross section A, as shown in 
FIG. 2, is 
##EQU4## 
The temperature T of the switch is given by 
##EQU5## 
where T.sub.o is 50.degree. K. A representative value of (C.sub.p 
m).sup.-1 is 1.12.times.10.sup.-6 m.sup.3 .degree.K./joule. Thus, 
##EQU6## 
Equations (5), (7) and (3) can be solved numerically for a given current 
density j(t) to analyze the behavior of the switch. 
By utilizing the switch of FIG. 2 in a circuit such as that of FIG. 1, the 
energy in the inductor L.sub.L may be switched to a load requiring a high 
energy pulse. 
A computer analysis of FIG. 1 indicates that the circuit ideally yields an 
output energy of 315 kilojoules, in a 1.1 MV pulse with a peak current of 
6.1 MA and a 10 to 100 percent current risetime of 2 ns. The parameters of 
the analysis included a standard half megajoule energy source such as a 
capacitor bank with 
C.sub.B =868 .mu.F, V.sub.o =33 kV, L.sub.B =4 nH, R.sub.B 
=4.times.10.sup.-4 ohms, L.sub.L =16 nH, R.sub.S =7.5.times.10.sup.-5 
ohm-cm initially, 
##EQU7## 
During the first 4.8 microseconds of analysis, the capacitor C.sub.B 
discharges 320 kJ into the load inductor L.sub.L and switch R. The current 
reaches 90 percent of its maximum value at that time. Then the switch R 
"opens" to divert the current into the load (L.sub.S,C.sub.S,D). The total 
energy expended in the switch (which weighed only 415 grams) was 6.3 
kilojoules. Energy per unit mass of 15 joules/gram will not damage the 
switch. 
The load inductance L.sub.S should be kept low to maintain the circuit in a 
stable condition. For the parameters indicated above, a load inductance 
L.sub.S of less than 1 nH will not give rise to excessive switch voltage. 
In the same analysis using a switch comprising 415 grams of EuO.sub.1-x, a 
standard half megajoule capacitor bank was used to charge an inductor 
L.sub.L in 4.8 microseconds with a peak power of 60 gigawatts. The opening 
of the switch R produced an output pulse of 6 terrawatts into a resistive 
load for a power gain of 100 in a single switching stage. The overall 
efficiency including losses in the capacitor bank an from stay load 
reactance was 64%. Only 15 joules was deposited in the switch for a switch 
efficiency of over 98%. The switch is survivable and reusable. 
As shown in FIG. 4, the switch may be initially maintained at 4.degree. K. 
or a sufficiently low temperature by an active cooling system F utilizing 
liquid helium or other suitable coolant and coolant supply and return 
lines 30 and 32 respectively. In order to prevent an electrical breakdown, 
in the form of a discharge along the surface of the switch between the 
plates 10 and 12 during high power operation, the switch (and the load in 
the case of FIG. 4) may be maintained in a vacuum chamber E. For very high 
power applications, a vacuum chamber may not be sufficient and a bath of 
liquid helium may be used to insulate the plates from each other. A helium 
bath is especially useful for those applications requiring full switch 
cooling. For most applications, however, the switch material can be 
adequately cooled by conduction through the plates 10 and 12. 
In the application illustrated in FIG. 4, the load consists of spaced 
plates 26 across which an electron beam is created when the switch R 
"opens" to divert the energy from the inductor L.sub.L into the load D. 
Field graded electrodes are used at the edges of the load to prevent the 
electron beam from breaking down along the edges of the plates 26. 
The foregoing description of a preferred embodiment of the invention has 
been presented for purposes of illustration and description. It is not 
intended to be exhaustive or to limit the invention to the precise form 
disclosed, and obviously many modifications and variations are possible in 
light of the above teaching. For example, the switch geometry, current 
carrying capacity, critical temperature, impedance transition magnitude, 
power handling capacity, switching time, etc. are all dependent on 
material selections and load applications. The energy source and load 
configurations illustrated are exemplary only and should not be construed 
to be limiting. The embodiment was chosen and described in order to best 
explain the principles of the invention and its practical application to 
thereby enable others skilled in the art to best utilize the invention in 
various embodiments and with various modifications as are suited to the 
particular use contemplated. It is intended that the scope of the 
invention be defined by the claims appended hereto.