Published on 23-Jun-2022

Positive Thermal Expansion – Simplified

Positive Thermal Expansion – Simplified

Do you know why you can’t bond traditional PZT to metal substrates for use at high temperatures? The answer is a thermal expansion mismatch! Read our latest blog to understand this phenomenon and learn what you can do to overcome this physical restriction. 

One thing many people don’t know is that when you pole a traditional piezoelectric material like PZT it gets thicker. Therefor when you heat that material up towards its Curie temperature, it gets thinner. This provides a negative thermal expansion coefficient.  

The negative thermal expansion can make it difficult to use the piezoelectric material in scenarios when there is a wide range of temperatures, for example, if you bond a piezo to a piece of steel, the steel gets larger and the piezo gets smaller. With this, you have got a big thermal expansion miss-match which can either lead to damage to the piezo or debonding or it can lead to inefficiencies of the system. 

Ionix HPZ is unique because it has a positive thermal expansion coefficient which makes it easier for our customers to use piezoelectric materials inside their sensors and actuators. 

Breaking it down 

  • Traditional PZT ceramics get smaller when heated, providing a negative thermal expansion 
  • If the PZT material is bonded to a piece of steel a thermal expansion miss-match would occur on heating or cooling leading to damage of the piezo or failure of the assembly 
  • Ionix HPZ provides a positive thermal expansion making it a unique characteristic compared to PZT 

Screaming Piezo 

To highlight this effect, we created a speaker – a disk of HPZ580 which has been bonded to a titanium sheet using Ionix proprietary bonding techniques, which can be applied to other metals including steels. This allows the sounder to operate at temperatures of up to 500°C. The titanium, bonding layer and piezo are all thermally matched which is critical otherwise the piezo would crack and de-bond.  

The video below shows our speaker working in an oven set to 300°C. The high temperatures are what has turned the titanium sheet blue, as there is a very thin layer of titanium oxide on the surface. During operation an applied signal voltage from the amplifier generates a strain in the piezo. This translates into vibrations in the titanium which then makes a sound that you can hear. This simple scenario demonstrates three Ionix technology features: 

  1. A proprietary method for bonding ceramics to metal substrates for use at high temperatures. 
  2. The thermal expansion matching of the piezoceramic with the metal substrate. 
  3. The high operating temperature of the Ionix HPZ ceramic. 

If you want more information on how a positive thermal expansion works or would like to discuss any of the other key benefits of our HPZ materials, get in contact today! 

Source: https://ceramics.ionixadvancedtechnologies.co.uk/2022/06/22/positive-thermal-expansion-simplified/



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