US3371309A - Thermo-mechanical transducer - Google Patents

Description

Feb. 27, 1968 A. H. RICH 3,371,309

THERMO-MECHANICAL TRANSDUCER Filed June 10, 1965 2 sheetssheet 1 INVENTOR ALAN H. RICH BY Q48 ATTORNEY Feb. 27, 1968 A. H. RICH THERMO-MECHANICAL TRANSDUCER 2 Sheets-Sheet 2 Filed June 10, 1965 0 0 mu 0 a w m cry wmpzmumzmh zortmzeih INVENT OR 1 CHROMIUM CONCENTRATION ATOMS (X) ALA/V H RICH ATTORNEY United States Patent 3,371,39 Patented Feb. 27, 1968 3,371,309 THERMO-MEiIHANiCAL TRANSDUCER Alan H. Rich, Washington, D.C., assignor to the United States of America as represented by the Secretary of the Navy Filed June 10, 1965, Ser. No. 463,058 3 Claims. (Cl. 340-8) ABSTRACT OF THE DISCLGSURE A thermo-mechanical transducer utilizing chromium modified manganese antimonide as the driver element wherein the chromium modified manganese antimonide is characterized by the property of expanding in a sharp, step-like manner as the temperature of the material passes upward through an interval of a very few degrees about a given temperature and of contracting in a similar manner as the temperature passes downward through this interval so as to create mechanical motion which can be used to create compressional waves.

The invention described herein may be manufactured and used by or for the Government of the United States of America for governm ntal purposes without the payment of any royalties thereon or therefor.

The present invention relates to a transducer and more particularly to a low frequency acoustic transducer which is capable of producing large amplitude compressional waves.

In recent years, low frequency acoustic transducers have become increasingly important for both military and civilian purposes. Much effort has been expended on developing magnetostrictive and piezoelectric drivers suitable for such transducers with only limited success.

Magnetostrictive drivers for low frequency transducers are expensive and ineflicient. When it is desired to obtain a large amplitude, low frequency, compressional wave output, the magnetostrictive driving source and, hence the transducer, must be physically large. Furthermore, large currents are necessary to create the requisite magnetic field.

Piezoelectric drivers for low frequency transducers are expensive, diificult to fabricate and also inetficient. Moreover, these disadvantages are all aggravated when the transducer is designed to produce not only a low freuency but also a large amplitude compressional wave.

The present invention overcomes these disadvantages by approaching the problem of drivers for low frequency transducers in a novel manner. The driver of the instant invention does not employ conventional magnetostrictive or piezoelectric principles but, rather, converts heat into mechanical motion which motion can be used to produce compressional waves. Chromium modified manganese antimonide having the general formula Mn Cr Sb, where x equals the concentration of chromium in atoms and 0.025 0.20, can be used as the driver. Crystals composed of this compound and similar compounds have theproperty of expanding in a sharp, step-like manner as the temperature of the crystal passes upward through an interval of a very few degrees about a given temperature and of contracting in a similar manner as the temperature of the crystal passes downward through this interval. In chromium modified manganese antimonide, this given temperature is the temperature at which the chromium modified manganese antirnonide goes through a transition from a weakly magnetic to a ferrimagnetic state. This property is used to create mechanical motion which can be used in an acoustic transducer to create compressional waves. Inasmuch as the amount of me chanical motion created by the expansion and contraction of the crystal is dependent on the size of the crystal, a crystal can be chosen to produce compressional waves of any amplitude desired.

An object of the present invention is the provision of a simple, inexpensive, and efficient low frequency transducer.

Another object is to provide a compact low frequency transducer which is capable of producing large amplitude compressional waves and which is easy to fabricate.

A further object of the invention is the provision of a transducer which converts thermal energy into mechanical energy.

Still another object is to provide a compact underwater acoustic transducer which is simple to construct and capable of producing large amplitude compressional waves and is driven by a crystal which converts energy in the form of heat into mechanical motion.

Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings in which like reference numerals designate like parts throughout the figures thereof and wherein:

FIG. 1 shows a longitudinal cross-section of a cylindrical transducer incorporating the invention;

FIG. 2 illustrates a transverse section of the transducer taken along the line 22 of FIG. 1; and

FIG. 3 is a graph showing how the transition temperature varies with the amount of chromium present in the compound.

Referring now to FIG. 1, cylindrical crystal 11 forms the driver for cylindrical piston transducer 12. Crystal 11 is composed of a compound such as chromium modified manganese antimonide having the general formula IvIi1 Cr Sb, where x equals the atoms of chromium and 0.025 0.20. The ends of crystals 11 are butted against pistons 13 which may be formed from stainless steel. Preferably, pistons 13 have shank portions 14 which have relatively small diameters and have bores 28 formed therein to receive the ends of crystal 11 and face portions 15 which have relatively large diameters and are as thin as possible giving due consideration to the hydrostatic pressure that will be bearing against the transducer during operation. This construction has the advantages of holding crystal 11 firmly in place, having a large effective transmitting area 15, and having as little frictional contact between pistons 13 and sleeve 16 as possible.

Sleeve 16 which may also be formed from stainless steel, makes slideable contact with the peripheries of piston faces 15 and thereby prevents motion of the pistons transverse to the longitudinal axis of the cylinder. The surface of sleeve 16 which contacts faces 15 may be coated with a low coefficient of friction material such as Teflon to prevent seizure between the sleeve and the piston face. Pistons 13 are retained within sleeve 16 by retaining members 17 which may be snap rings. Flexible, fluid-impermeable sealing member 18, which may be rubber, encloses piston faces 15 and sleeve 15 thereby preventing flooding of the transducer.

Heating coil 19 is wound about the longitudinal axis of crystal 11, preferably throughout the full length of the crystal, on an electrical insulating, thermal conducting member 20 which may be ceramic. The amount of heat supplied by heating coii 19 is controlled by the amount of current flowing therethrough. This current may comprise an A.C. component supplied by power oscillator 21 superimposed on a DC. component supplied by DC. source 22.

When crystal 11 is chromium modified manganese antimonide, there is a change in length of crystal 11 of approximately 0.20% that occurs in a step-like manner as the temperature of the crystal passes through an interval of approximately 3 K. about the transition temperature. It is this step-like change in length that is used to drive piston 15. Clearly, the magnitude of the compressional waves that will be produced by the transducer is dependent on the length of crystal 11. The approximate 0.20% change in length of crystal 11 that occurs over the temperature interval means that the length of crystal 11 can be significantly less than the length of a magnetostrictive driver used to produce compressional waves of the same amplitude.

In operation, D.C. source 22 is adjusted to supply suflicient current to heating coil 19 to cause it to generate suflicient thermal energy to hold the temperature of crystal 11 just below the lowest temperature of the aforementioned temperature interval. Oscillator 21, which is coupled to heating coil 19 through a transformer 23 to prevent interaction between oscillator 21 and source 22, supplies an AC. current component which causes heating coil 19 to generate sufiicient additional thermal energy to cause the temperature of crystal 11 to pass through this temperature interval. Pistons 13 and sleeve 16 act as a heat sink for crystal 11 and thermally couple the crystal to the environment in which the transducer is used. Thus, if the temperature of the environment is below the lowest temperature of the temperature interval, the temperature of crystal 11 will be returned to a temperature below the lowest temperature of this interval after being increased through the interval by the heat transfer between the crystal and the environment.

The upper frequency of transducer 12 is limited by the rapidity of this recovery. When the transition temperature of crystal 11 is as high as possible, approximately 400 K., transducer 12 can operate at a maximum frequency of approximately c.p.s. if no cooling provisions are made other than the heat transfer between the crystal and the environment. This means that the oscillator 21 can have a frequency up to 10 c.p.s. If it is desired to operate at a lower frequency, the transition temperature can be controlled by varying the amount of chromium as appears from FIG. 3.

Of course, if separate cooling provisions were made, such as if the crystal were exposed to a refrigerant during the recovery period, transducer 12 could operate at a higher frequency.

It should be understood that the current source for heating coil 19 could be contained within transducer 12. In such an embodiment, the DC. component could be provided by a battery and the AC. component could be supplied by a second battery coupled to the heating coil through a chopper that makes and breaks contact at the desired frequency of operation.

Battery 24 furnishes current for a monitoring circuit that includes thermistor 25 and current sensitive meter 26. Thermis-tor 25 is enclosed in a thermally nonconductive envelope 27 on all sides except for the side disposed toward crystal 11 and is thermally coupled to the crystal so that its resistance will vary solely as a function of the variations in the temperature of the crystal. Current sensitive meter 26 is calibrated in terms of temperature so that it will provide a direct reading of the temperature of crystal 11. Using this circuit, one may monitor the temperature of crystal 11 and adjust the magnitude of the current supplied by DC. source 22 so that the AC. signal supplied by oscillator 21 will always cause the temperature of the crystal to vary through the aforementioned temperature interval. Of course, the monitoring and adjusting could be done automatically.

In practice, the leads from heating coil 18 and thermistor 23 pass through sleeve 16 and seal 18 in a fluid tight fitting (not shown).

FIG. 2 shows a transverse section of transducer 12 4 taken along line 22 in FIG. 1. It will be observed that in a cylindrical transducer, such as illustrated, the crystal 11, piston 13, electrical insulating, thermal conducting member 20, heating coil 19, sleeve 16, and seal 18 are all concentric.

It should be noted that compounds such as chromium modified manganese antimonide could be used as the driver in other than cylindrical transducers. The only limitation that exists on the type of transducer they could be the driver for is that it is necessary that a source of heat be present which can vary the temperature of the crystal through the interval about its transition temperature and that there be means to dissipate heat away from the crystal.

As can be seen from FIG. 3, the transition temperature for a crystal composed of chromium modified manganese antimonide having the general formula Where x equals the atoms of chromium, which is a typical compound for the driver according to this invention, varies from a little above K. to a little below 400 K. as X varies from 0.025 to 0.20 atoms.

One possible use of transducers with drivers according to the present invention is as underwater acoustic transducers, which are typically used in an environment having an ambient temperature of approximately 280 K. In the case where the driver is composed of chromium modified manganese antimonide and there is to be no separate provision made for cooling the crystal, the chromium concentration in atoms will have to be greater than approximately 0.10. In this way the environment will be able to provide the cooling necessary to bring the temperature of the crystal below the lowest temperature of the approximate 3 K. interval about the transition temperature after it is raised above the highest temperature of the interval to produce the desired step-like expansion. As the atoms of chromium are increased up to 0.20, it is possible to obtain a larger difference between the transition temperature of the compound and the temperature of the environment. The larger this difference, the higher the frequency at which the transducer being driven by a crystal composed of the compound can operate. As mentioned before, the maximum frequency of operation is approximately 10 c.p.s. unless separate cooling provisions are made.

It should now be clear that the present invention provides a simple, compact, low frequency transducer capable of producing large amplitude compressional waves.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood, that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

What is claimed and desired to be secured by Letters Patent of the United States is: j

1. A thermo-mechanical sonic transducer comprising:

a driver crystal composed of chromium modified manganese antimonide having the general formula Mn Cr Sb Where x equals the concentration of chromium in atoms and 0.0255X5020 and wherein said chromium modified manganese antimonide has the property of expanding in .a step-like manner as the temperature thereof is increased from a first temperature to a second temperature and of contracting in a step-like manner when the temperature thereof is decreased from said second temperature to said first temperature;

an electrical heater thermally coupled to said driver crystal;

a direct current heating signal source electrically coupled to said heater, and adapted to pass a first 'sig nal through said heater of such a magnitude that the thermal energy generated thereby heats the crystal to a temperature just below said first temperature;

means electrically coupled to said heater for passing a second heating signal therethrough, the magnitude of said second signal periodically varying at sonic frequencies from a first value to a second value wherein said first value is below the signal magnitude necessary to create sufficient additional thermal energy to increase the temperature of said crysml to said second temperature, and said second value is at least equal to the signal magnitude necessary to create suflicient additional thermal energy to increase the temperature of said crystal to said second temperature;

means thermally coupled to said crystal for monitoring the temperature thereof;

means for adjusting the magnitude of said first signal in response to the temperature of said crystal that is sensed by said monitoring means so that the temperature of said crystal always varies from said first temperature to said second temperature when the magnitude of said second signal varies from said first value to said second value; and

cooling means thermally coupled to said crystal for periodically decreasing the temperature thereof from said second temperature to said first temperature sequentially with respect to the increasing of the temperature of said crystal from said first temperature to said second temperature so as to enable the heating of said crystal to said second temperature and the cooling of said crystal to said first temperature at sonic frequencies.

2. The transducer of claim 1 for use in an environ- References Cited UNITED STATES PATENTS 2,962,695 11/1960 Harris 34011 X 3,126,347 3/1964 Swoboda 252-625 3,140,942 7/1964 Walter 75122 3,198,969 8/1965 Kolm et al 3104 3,238,396 3/1966 Schubring et a1. 310-4 OTHER REFERENCES Swoboda et al.: Evidence for an Antiferrornagnetic Ferrimagnetic Transition in Cr-Modified Mn Sb, Physical Reviews, May 15, 1960, pp. 509-511.

Cloud et 211.: Exchange Inversion in Mn Cr Sb, Journal of Applied Physics, Supplement, March 1961, pp. S56S.

RODNEY D. BENNETT, Primary Examiner.

RICHARD A. FARLEY, Examiner.

B. L. RIBANDO, Assistant Examiner.

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