New chemical formulation of Quartz material for Ultra Stable Resonators – NOCHIQRUS

The main purpose of this project is to improve the piezoelectric properties of quartz material by modifying the chemical formula of quartz material . Indeed, we have in mind to substitute a part of Si atoms with Ge atoms and/or another larger atom like Sn.
During a previous DGA/REI-financed project, a synthesis process of mixed SiO2-GeO2 nutrient and a new hydrothermal crystal growth process have been defined to grow crystals having Si1-xGexO2 general formula. The physical properties have been improved. The Ge-substitution allowed increasing the structural distorsion leading to a better thermal stability and to an increase of the electromechanical coupling factor. The elastic constants in the (x, y) plane have been reduced that confirms the increase of the electromechanical coupling factor. AT resonators have been performed from a 4%Ge-substituted quartz material. The quality factor (surtension) values remain good. Moreover, this quality factor value increases with temperature leading to high quality resonator at 80°C (working temperature of oscillators). This behaviour has to be explained for high-temperature resonator.
For this project, the study of the crystalline network will be continued either by increasing the Ge-content in the crystals or by introducing an other big atom (like Sn). The "Chemistry-Structure-Properties" will be developped particularly in the aim to better undertand the phenomenon which is at the origin of the quality factor increasing with temperature.
From an technical point of view, the interest of the 4%Ge-substituted quartz has been demonstrated in the previous contract with DGA. Now, it is important to fully master the crystal growth process in the aim to define an industrial process. The nutrient (20%Ge cristobalite type) synthesis routine with a thermal treatment has also to be master. In the same time, an hydrothermal process of nutrient synthesis will be evaluated at a laboratory scale. The study of the hydrothermal crystal growth will be developped at a pilote scale by using autoclaves of a volume of 1.5litre. The crystals issued from different growth runs will be used to improve the design of the resonators with SC-cut for oscillator applications.
The expected results are at several levels:
1- Fundamental study in the aim to increase the structural distorsion of the crystalline network. Two ways will be followed: Ge-content increasing and/or use of a co-doping with Germanium.
2- Growth of 4%Ge-substituted crystals having good crystal quality (good homogeneity of Ge-content and a low dislocation rate) with a reproducible process.
3- Define an industrial process of Si0.96Ge0.04O2 crystal growth.
4- Perform high performance resonators at 40MHz and 10MHz for Ultra Stable Oscillators
5- Understand the behaviour of the resonators at high temperatures.