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Accueil du site > Divers > equipes de recherche > Matériaux Fonctionnels et de Structure (MFS) > Activités > Céramiques diélectriques >

19 mai 2008

Introduction

 

Under electric field, dielectrics materials can accumulate electrostatic energy that could be maintained without exterior energy input. Consequently, a dielectric material has to be an insulator. However, we often talk about non-insulating dielectric materials properties. (electrolytes, semi-conductors, metals...).

Presentation of the team research :

We are working on close collaboration with the industry, on two types of applications : multilayers condensators and hyper frequencies resonnators. We can find theses passif components in all electronic circuits. The essential economic stake for this kind of components is to decrease the manufactoring cost ( decrease of sintering temperatures, change of the electrode composition...) without altering the properties.
 

 Without electric field Electric field => dipoles formation (polarisation)

Charges move and set along the field. This is the origine of capacitence effect of dielectric materials.

 



The material response of the E applied electric field is the D shift of the dipoles. The lost energy is called ’’dielectric loss" written tan(δ).

According to the applications, different type of material could be used :

Type

ε

Caractéristiques / Propriétés
I <300 Paraelectric Very stable according the temperature and the frequency
II >300 Ferroelectric Not stable according the temperature and the frequency
III >1000 Mixte Insulator/Conduct Not stable according the temperature and the frequency



Modern machine uses multilayers capacitors which get altenatively insulating and conductive layers piled up to minimise the size and to increase global component performances :

We can distinguish very clearly on this diagram the external electrode (contact), the dielectric ceramic and the internal electrodes.

 

The BME-MLCC (Base Metal Electrodes - Multi Layers Ceramic Capacitors)

 

 


At this time, the used electrodes in the industry are made with Palladium/Argent. The aim is to substitute the Pd/Ag by the Copper cause of the price . (Price Pd >>>>Cu).

Two problems come into sight :

- The melting temperature of the copper is 1084°C. So we have to decrease the sintering temperature of the dielectric material below this temperature.

- It becomes essential to use a reductive atmosphere during the sintering to avoid copper oxydation.

We have got differents possibilities to decrease the material sintering temperature : 

- Stoechiometry modification.

- Doping.

- Adding of melting lithium compound (LiF, LiNO3...).

- Adding of glass phases (SiO2, B2O3...).

We are working on type I material class. They show a dielectric constant below 300 and very low dielectric loss. They are as well very stable according the temperature and the frequency. These perovskite like (ABO3) materials are very good for BME-MLCC applications. The table below shows the materials studied in our laboratory.

Material Sintering temperature ε
CaZrO3 1600°C 30
Ba(Zn1/3Ta2/3)O3  (BZT) 1500°C 30
Ba(Zn1/3Nb2/3)O3  (BZN) 1400°C 25
Ba(Mg1/3Ta2/3)O3  (BMT) 1600°C 25

 

 

The hyper-frequency resonators

 

The wanted properties for this kind of passif component are the following :

- Low dielectric loss.

- Resonance frequency has to be stable with the temperature.

In this case we are studying the same materials as before with the BME-MLCC :

- BMT, BZT, BZN


We focus on the decrease of the sintering temperature in air without any changes with the properties. We are using for that sintering addings like glasses, dopping or melting help compounds. You will see below an example of HF resonator made by TEMEX company (1.5 à 5GHz).

 

 

Contact : Sylvain MARINEL

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