Technical Articles

The Use of Ceramic in Military Sonar Systems
February 2008

Military equipment and instrumentation requires the highest levels of precision and accuracy. Industry demands for the 'latest' and 'best' technologies, means designers and engineers are constantly striving to find ways to increase performance, reliability and range. 

High-performance ceramics have been critical elements in modern military instrumentation since World War II. Mechanical and electrical properties make them versatile materials that can be used in a range of applications. For example; electronics and optical ceramics are essential parts of military radar and sonar communication systems; ceramic components ensure accurate, consistent readings in aircraft instrumentation and engine monitoring.

This article looks at the use of ceramic components within instrumentation in sonar systems used in the sea.

Enabling accurate detection under the sea
One of the main uses of ceramic in sea-based military and defence applications is in sonar equipment. Sonar allows the detection of objects under the sea, to identify them and their position. It is used in a variety of vessels such as surface ships, submarines, torpedoes, mine hunting and decoy systems. 

First widely deployed in World War II, active sonar creates a pulse of sound and then listens for the reflection (echo). The pulse of sound is generally created electronically, using a Sonar Projector, which consists of a signal generator, power amplifier and electro-acoustic transducer. There are three types of transducer arrangement, those that transmit, those that receive and those that transmit and receive at the same time.

There are several materials from which sonar can be generated, for example magnetostrictive, piezoelectric ceramics and piezoelectric plastics. Designers striving to achieve greater accuracy are also now turning to piezoelectric composite materials. 

Piezoelectric ceramic vs magnostrictive technologies
An inherent property of the piezoelectric ceramic material means that when an electrical signal is passed to the ceramic it will develop strain. When this occurs at high frequency it generates the sound wave. Conversely, when a sound wave comes into contact with the ceramic it will come under stress and produce an electrical signal. The distance to an object can be measured by calculating the time from transmission of a pulse to the time of receiving the echo.

Ceramic transducers are more efficient than those that use magnostrictive technology, which is based on converting magnetic energy to mechanical energy. They have a good conversion rate of signals, with minimal energy loss and are more cost effective. Magnetostrictive technologies tend to be used for lower frequencies, as they can accept higher powers, however, they cannot be used to transmit and receive at the same time.

Industry demand to scan the water more accurately over greater distances, for example 100kms, means transducers must transmit at lower frequencies. Different configurations of transducers have been designed over the years to address this, including Tonpilz arrangements, where piezoelectric materials are sandwiched between a light, stiff radiating head mass and heavy tail mass, and pre-stressed tubes.

Pre-stressed tubes
Morgan Electro Ceramics manufactures pre-stressed tubes that give a high degree of accuracy for sub-sea detecting and sensing. They give a high acoustic output for a relatively lightweight structure. The tubes are pre-stressed during the manufacturing process with glass fibre wrapping which increases their reliability under high drive conditions. The process increases the maximum tensile stress, which, in turn, increases maximum drive voltage. The risk of ceramic failure through excess tension, which is a problem with standard piezoceramic tubes, is also minimized.

Pre-stressed tubes are available in a range of sizes and design. Small tubes, up to 100mm diameter can usually be manufactured from one piece of piezoceramic and larger tubes, up to 500mm diameter, are constructed with multiple tapered ceramic segments. Tubes are manufactured from piezoceramic materials, which have low dielectric loss and minimal self heating.

Piezoelectric composite materials
Latest developments for designers, to improve transducer performance, include the use of piezoceramic composite components constructed of pillars of conventional piezoelectric ceramic in a matrix of a polymer material. Manufacture of the composite element uses a technique called dicing and filling. Dicing uses a very high tolerance computer controlled saw giving small piezoelectric pieces, typically 1mm cross section or less depending on the operating frequency. Designers are also able to adjust the properties to deliver a much reduced acoustic impedance of the transducer element that produces a closer match to that of the surrounding water. This leads to a higher efficiency along with improved output levels and receive sensitivity.

Conclusion
Ceramic continues to play a vital role in many military applications. From its use in sonar, where it has been a key component for over sixty years, to the latest developments in optical guidance systems, the material is enabling the design of higher performance military instrumentation and equipment. Today's advanced ceramics, such as those supplied by Morgan Electro Ceramics, offer powerful physical, thermal and electrical properties which allow engineers to design efficient systems for harsh environments that ensure accuracy and reliability - whether being used at sea, on land or in the air.

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