GB2214031A - Ultrasonic transducer - Google Patents

Abstract

An ultrasonic transducer suitable for use up to a temperature of about 600 DEG C comprises a stainless steel casing (12) containing a lithium niobate piezoelectric element (18) and a backing material of a powder (38) which provides a partial pressure of oxygen and thereby prevents deterioration of the element (18) by oxygen loss or contamination. The powder might be of lithium niobate or magnesia. <IMAGE>

Description

Ultrasonic Transducer This invention relates to an ultrasonic transducer suitable for use in a high temoerature environment (up to about 6000C) both as a transmitter and a receiver of ultrasonic waves.

It has been suggested that lithium niobate might i useful as a piezoelectric material in an ultrasonic transducer for use at temperatures above about 3000C. Such an ultrasonic transducer might be suitable for use for example within a liquid sodium cooled fast reactor.

However it is also known that lithium niobate is chemically unstable at these temperatures, losing oxygen, its electrical conductivity simultaneously increasing se that it becomes less efficient as a transducer. For use within a nuclear reactor it is also desirable to minimize the effects of radiation on a transducer by avoiding use of plastics materials, which tend to be degraded by gamma irradiation, and, if the transducer will be exposed to a significant thermal neutron flux, by usinq lithium niobate with a very small proportion of the lithium-6 isotope, which has a high cross-section for an (n, alpha) reaction when irradiated by thermal neutrons.

According to the present invention there is provides an ultrasonic transducer comprising a lithium niobate piezoelectric element, and a backing material adjacent to one side of the element, the backing material consisting of a powder comprising a material which orovides a sufficient partial pressure of oxygen adjacent the element to prevent loss of oxygen by the element.

Preferably the trans(lucer incluries a stainles, steel casing for the piezoelectric element, and means for coilnressing the backing material into firm contact Jith the piezoelectric element. Desirably the power consists of particles smaller than about 90 micrometres in size, and the hacking material comprises powdered lithium niobate.

It may be possible to use magnesia instead, though this will only provide the oxygen physically adsorbed onto the surfaces of the particles.

The backing material has two effects. It has a similar acoustic impedance to that of the element, so that it absorbs ultrasonic signals from the element, and because of the large number of particle interfaces these signals are attenuated. Consequently the time for which the element rings after being energized is reduced, as is the pulse length emitted by the transducer. Equally the bandwidth for emission and reception of ultrasonic signals is increased.

Secondly it minimizes any chemical changes to the element. The powder acts as a physical barrier between the casing and the element, minimizing the diffusion of metallic elements from the casing to the element. And, by providing oxygen additional to that available from enclosed gas and the element, and so maintaining the oxygen partial pressure within the casing, it minimizes oxygen loss from the piezoelectric element. It will be appreciated that at high temperatures stainless steel tends to act as a setter for any gaseous oxygen, forming an oxide layer.

The invention will now be further described bv way of example only and with reference to the accomtranying drawings, in which: Figure 1 shows a sectional view of an ultrasonic transducer; and Fiqure 2 shows qranhically signals obtane' itl the transducer of riqiire 1 Referring to Figure 1, an ultrasonic transducer 10 comprises a cylindrical tubular casing 12 of stainless steel with flanges at each end to which are welded a circular stainless steel top plate 14 and a bottom plate 16.

A nominally 4 MHz lithium niobate Z-cut piezoelectric element 18 is fixed to a thin central diaphragm portion 20 of the hottom plate 16, which is of thickness half a wavelength at the nominal frequency, by vacuum brazing with a copper/silver eutectic braze (which may be titanium cored for use at higher temperatures) heated to just above its liquidus temperature. The element 18 is then coated on its back surface (upper surface as shown) with silver, by coating it with a silver-containing firing paste (such as A 3600 paste supplied by Johnson Matthey Chemicals Ltd., Royston, England), allowing it to dry, and then firing at 5400C for 20 minutes.Alternatively the element 18 may be fixed to the diaphragm portion 20 by diffusion bonding, by coating both the lower surface of the element 18 and the upper surface of the diaphragm portion 20 with silver as described above, then assembling and firing again at 5400C while pressing together for several hours.

The top plate 14 includes a tubular boss 22 which extends both inside and outside the casinq 12. R stainless steel sheathed, magnesia insulated, coaxial cable 24 extends through the boss 22 and is brazed to it. The cable 24 has a nichrome conductor 26 which makes electrical contact to the upper surface of the piezoelectric element 18 through a silver-coated nimonic 90 spring 28, the conductor 26 and the spring 28 being joined by a crimped stainless steel ring 30.

In the annular space between the boss 22 and the casing 12 are located two stainless steel pressure plates 32 spaced apart by four nimonic 90 disc springs 34. Three equally spaced screws 36 (only one is shown) extending through threaded holes in the top plate 14 enable the position of the upper pressure plate 32 to be adjusted.

The space below the lower pressure plate 32 within the casing 12 is entirely filled by powdered lithium niobate 38, graded by passing it through a 79 micrometre mesh. The powder 38 is compacted after assembly by pressure exerted on it by the lower pressure plate 32, by tightening the three screws 36.

Where the transducer 10 of Figure 1 is to be used immersed in a liquid, such as liquid sodium, a stainless steel cap (not shown) is desirably welded to the external end of the boss 22 and to the external periphery of the top plate 14, the cable 24 emerging through the cap. This ensures that the screws 36 are not exposed to the liquid.

Referring to Figure 2 there is shown graphically the reflected signal detected by the transducer 10 of Figure 1 when immersed in water, with a 20 mm diameter brass reflector 55 mm from the external surface of the diaphragm portion 20. The graph A shows the signal detected hy the transducer 10 as described above with lithium niobate backing powder 38, whereas the graph B shows the signal detected by a transducer differing from that of Figure 1 only in that no backing powder is provided, the space between the lower pressure plate 32 and the piezoelectric element 18 being empty. It will be observe3 that the effect of the backing powder 38 is to shorten the effective pulse length, by reducing the signal amplitude after an initial pulse of duration about 10.5 microseconds.

Claims (6)

Claims

1. An ultrasonic transducer comprising a lithium niobate piezoelectric element, and a backing material adjacent to one side of the element, the backing material consisting of a powder comprising a material which provides a sufficient partial pressure of oxygen adjacent the element to prevent loss of oxygen by the element.

Amendments to the claims have been filed as follows Claims 1. An ultrasonic transducer comprising a lithium niobate piezoelectric element, and a backing material adjacent to one side of the element, the backing material consisting of a powder comprisinq a material which provides a sufficient partial pressure of oxygen adjacent the element to prevent loss of oxygen by the element.

2. A transducer as claimed in Claim 1 including a casing for the piezoelectric element, and means for compressing the backing material into firm contact with the piezoelectric element.

3. A transducer as claimed in Claim 2 wherein the compressing means comprises resilient means for exerting a force on the backing material, and means for adjusting that force.

4. A transducer as claimed in any one of the preceding Claims wherein the powder consists of particles smaller than about 90 micrometres in size.

5. A transducer as claimed in any one of the preceding Claims wherein the backing material powder comprises lithium niobate.

6. An ultrasonic transducer substantially as hereinbefore described with reference to, and as showl in, Figure 1 of the accompanying drawings.