Study of LGT crystal substituted to Quartz for TF applications – ECLATEMPS 2012

The first part of our job will consist in the comparative analyses between differrent suppliers

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In the field of numerous applications in the Frequency and Time domain, quartz crystal remains the leader. Nevertheless, « new » piezoelectric materials can exhibit properties compensating the inconveniences of quartz which are:
- a weak piezoelectric coupling factor,
- strong acoustical leaks at high temperatures and
- at least the evidence of a Curie transformation point which limits its use in temperature.

For instance, the interest of the GaPO4 crystal used as a temperature sensor in combustion engine is further proof. The Germanium doped quartz has a more distorted structure than pure quartz and presents a higher Curie temperature and a stronger coupling factor (see the conclusions of the REI n° 0534051). But, we have to admit that these “new” materials will not replace quartz in watches applications, or in the applications for which a very accurate oscillator is not needed, with a frequency lower than 100 MHz.

On the contrary, recent international publications and also new works done by FEMTO-ST showed that crystals belonging to the Langasite family (and particularly the LGT) allow Q.f products largely higher than the ones of quartz (up to 2.5 times). Furthermore, it was shown that stress and radiations sensitivities of these materials are better than those of quartz.
Furthermore, we also showed that the quality of these materials is very inhomogeneous, from one supplier to another and also within the “boule”. It seems difficult to obtain a “boule” with an homogeneous chemical composition along the pulling direction (generally the Z-axis). The measurements realized in the departement by EPM showed that the weight pourcentage of the Tantalate, including in the crystal, is 2 to 3 times different from the theoretical composition. The question is to define the influence of such inhomogeneity, coming from different crystals or in the same “boule”, on the quality of the resonator used for clocks (in Ultra Stable Oscillators).
To answer these questions, it is necessary to first establish a program characterizing different LGT samples coming from different suppliers and then to manufacture resonators in the qualified zones of these samples. To realize this program, we are associated to RAKON (in Argenteuil) whose expertise in the field of acoustic resonators is internationally recognized.
Our project proposal develops this program. If some resonators are only made to qualify the material, the other ones are defined to match the needs of the Frequency and Time domain.

So, beyond the study of the material quality, the goal of the present project is double:
- the fabrication of 10 MHz oscillators for telecom applications requesting very high stability (close carrier) and very low aging,
- the fabrication of 120 MHz resonators for defense applications with weak flicker noise.

At the end of this study, we should be able to write a business qualification procedure to qualify LGT “boules” or wafers and to indicate the expected results for Frequency and Time domain, including its applications at high temperatures. A third aim would be to acquire know-hows in the crystal growth process for such crystals in order to complete the piezoelectric materials “chain”.