Bernard Capelle

A quartz-like material: gallium phosphate (GaPO4); crystal growth and characterization

This investigation concerns GaPO4 crystal growth in sulphuric and phosphoric acid media through the slow heating and vertical reverse temperature gradient methods. Systematic study of growth parameters shows that the growth rate Vx is always much greater than the two other ones, Vz and Vy. The most interesting result is the ability to produce GaPO4 epitaxy on large berlinite seeds in sulphuric acid and to use them for GaPO4 crystal growth in phosphoric acid. The epitaxial fit and the crystalline quality have been checked by X-ray topography. The [OH] content, determined by infrared spectrometry, seems to prove a lower OH concentration than in case of berlinite when approximatively the same crystal growth conditions are used. First piezoelectric characterizations for resonators near the AT cut show GaPO4 to be a very promising piezoelectric material with a large coupling coefficient near 16%, a quartz-like thermal stability and a Q factor already nearly sufficient for the applications.

Topography and high resolution diffraction studies in tetragonal lysozyme

Abstract Two complementary approaches are used to enhance the usefulness of X-ray topographies obtained from protein crystals. First, the use of thin plate-like crystals in conjunction with a high intensity, collimated and small source size synchrotron beam produces a large beneficial effect on the level of detail and contrast of topographies for the quantification of local misalignment in the crystal lattice. Second, the recording of topography series along the rocking curve of a diffraction peak is proposed as a technique to combine the benefits of both rocking curves and topographies and produce very detailed data (“rocking maps”) on the spatial distribution of lattice misalignments and mosaic spread (“local rocking curves”). The most important crystal features controlling the observed contrast are growth sectors and inter-sector boundaries, clearly observed in the topographies. Systems of parallel fringes are observed in many of the topographies. Two alternative explanations for these fringes are discussed: (a) as moire interference fringes or (b) as Pendellosung fringes in a wedge shaped crystal volume. In both cases, growth sectors play a central role in the physics of fringe generation. Many observations suggest the presence of a relatively large component of dynamical diffraction in these crystals; the consequences of this new scenario are discussed.

Crystal growth of berlinite, AlPO4: Physical characterization and comparison with quartz

Abstract The feasibility of large crystals of aluminum phosphate has already been demonstrated, so as the interest of this material for obtaining BAW and SAW devices with attractive properties. Our purpose in this study was: (a) to synthetize crystals with low acoustical losses; (b) to specify the useful crystal orientations more accurately; (c) to design devices that take in account all the specificities and advantages of this material. With a view to obtain high perfection and high Q crystals, three hydrothermal crystal growth methods were investigated in H3PO4 solvent, compared and improved with the assistance of crystal characterization techniques and BAW device measurements. Experimental conditions used most frequently with the slow heating, the reverse vertical gradient and the horizontal gradient methods are reported together with specific features of each method as applied to grow berlinite. X-ray topography has shown that the best crystals have a dislocation density of 10 to 100 dislocations per cm2. X-ray topography examination above room temperature (25–150°C) has revealed temperature dependent quasi-reversible localized variations of strain in water-containing crystals. Transmission electron microscopy was used after high temperature precipitation of water to assess the H2O content of crystals as a function of some growth parameters and to determine the distribution of this impurity within crystals. Thickness shear resonators of several Y rotated cuts were measured to compare the growth methods. Some experiments with recently obtained crystals have demonstrated the feasibility of very high Q crystals and the possibility of obtaining superior thermal behaviour from this material. Experiments concerning the AT cut has demonstrated the possibility to obtain band-pass of filter of shift of oscillators twice that of quartz. Specificity of energy trapping in this material is then discussed. We conclude that waterless berlinite is of major interest for device applications.

Crystal growth and characterizations of quartz-like material: gallium phosphate (GaPO4)

This investigation concerns GaPO 4 crystal growth in phosphoric and sulphuric acid media by the slow heating and vertical reverse temperature gradient methods. Systematic study of growth parameters shows that the growth rate V x is generally much greater than the other two, V z and V y . The most interesting result is the ability to produce GaPO 4 epitaxy on large berlinite seeds in sulphuric acid and to use them for GaPO 4 crystal growth in phosphoric acid where the V x /V Z ratio can be close to 1. As for α-quartz and berlinite, V y growth rate being always very slow, the lengthening of these crystals in that direction is obtained by the use of several joined seeds accurately adjusted and spliced on a quartz plate. The epitaxial fit and the crystalline quality have been checked by X-ray topography. The OH content, determined by infrared and near infrared spectrometry, proves a lower OH concentration than in the case of berlinite when approximatively the same crystal growth conditions are used. Piezoelectric characterizations of resonators have been suitable to specify the AT-cut position, its very promising piezoelectric properties with a high thermal stability and a Q factor are already nearly sufficient for applications. The most interesting properties of these crystals, the piezoelectric properties, are compared with those of other known crystals, quartz and berlinite, and related to their crystal structure distortions. The observed evolution allows the prediction of some interesting unknown characteristics for other quartz-like materials.

Evaluation of high quality berlinite crystals grown in sulphuric acid medium

Abstract This investigation concerns berlinite crystal growth in sulphuric acid solvent through the reverse temperature gradient method and its consequences on crystalline quality and piezoelectric characteristics. Systematic study of growth parameters shows a high sensitivity of growth rates to crystal growth temperature, temperature gradient and acid concentration. The choice of good crystal growth conditions allows us to obtain crystals with a very good crystalline quality checked by X-ray topography. The -OH content, followed by infrared spectrometry, has been lowered to very small values and then electrical measurements of Y and Y-rotated resonators lead to high Q factors, first and second order temperature coefficients lower than those of quartz and a favourable coupling coefficients. For extensional bars, the existence of compensated cuts is also demonstrated.

Piezoelectric characterization and thermal stability of a high-performance α-quartz-type material, gallium arsenate

Piezoelectric measurements were performed on large single crystals (8 mm along the c direction) of an α-quartz-type piezoelectric material, gallium arsenate, GaAsO4, which allow us to extend the structure-property relationships in the α-quartz-type materials. These first measurements on Y-rotated-cut plates have shown that gallium arsenate is the highest-performance piezoelectric material of this group. As compared to the coupling coefficients of the other materials with the same structure (kSiO2=8%, kAlPO4=11%, and kGaPO4=16%), gallium arsenate exhibits the highest piezoelectric coupling coefficient of about 22%, as has been predicted by the structure-property relationships. Moreover, from these piezoelectric measurements, the C66′ elastic constant was determined and compared with elastic constants in quartz-type materials. The proposed value for the cut angle of the AT plane in GaAsO4 is −6.3°. In order to extend the previous thermal stability results, thermal gravimetric analysis (TGA) and x-ray diffra...

Crystallization within agarose gel in microgravity improves the quality of thaumatin crystals.

To prevent crystals from moving in orbit and sedimenting upon their return to earth, the model protein thaumatin was crystallized in agarose gel in the Advanced Protein Crystallization Facility during the eight-day Space Shuttle mission STS-95 (November 1998). The quality of tetragonal crystals grown in microgravity was compared with that of controls prepared in parallel in the laboratory. On the basis of their diffraction properties, microgravity crystals were more ordered than crystals grown in gel on earth (the latter being, on average, better than reference crystals obtained in solution on earth). It is concluded that protein crystallization within a gel in microgravity may yield crystals of superior quality by combining the advantages of both environments. A possible explanation for the positive effect of microgravity on protein crystallization in gels involving the better quality of the nucleus is discussed.

Influence of impurities on protein crystal perfection

Abstract A quasi-planar X-ray study has been done to assess the quality of crystals of pure thaumatin and of hen egg-white lysozyme as well as of lysozyme which was intentionally contaminated by structurally unrelated (ovalbumin and conalbumin) or related (turkey egg-white lysozyme) macromolecules. To investigate the behavior of different growth sectors, we have chosen crystals exhibiting well defined habit, namely crystals grown either in agarose gel or in silica gel. The main defect evidenced is a misorientation originating at the level of the nucleus: the parts grown in the +c and −c directions seem to be individuals twisted always clockwise with respect to the growth direction. The measured lattice plane tilt increases up to 3.5xa0min of arc when the contaminant content increases. In addition to this defect, we could measure for lysozyme relative lattice parameters differences (Δd/d) of the order of 10−4 between prismatic and pyramidal growth sectors. All these defects do not seem to have a major influence on the resolution limit but they have consequences on optical properties.

Properties of AT cut gallium phosphate resonators

Abstract Gallium phosphate is a quartz analogue that possesses more intense piezoelectric properties. We report here a study performed to determine more accurately the properties of resonators made using crystal orientations situated near the main cut presenting a zero temperature coefficient at room temperature. Recent crystals having a much improved quality were used to cut plano-convexe resonators with five orientations situated in a range of nearly two degrees around the previously determined angular position of this cut. The temperature coefficients of the resonance frequencies of the electrically excited shear modes and the electrical properties of the resonators were determined. The vibration modes were studied using synchrotron radiation X-ray topography. For all the studied cuts, parabolic thermal variations of the resonance frequencies were observed. The temperature of the extremum varies somewhat with the rank of the mode and also, but slowly, with the cut angle. On the whole a very good thermal stability is obtained for the resonators. Q factors higher than those previously measured were obtained together with, as predicted, very interesting values of the equivalent electrical scheme. The observed vibration modes are generally close to those computed using the Tiersten theory. They present several very interesting properties but display several particularities, such as often possessing a second shear component with a noticeable amplitude, which make them somewhat different from the mode of the corresponding quartz resonators.

New Materials and new Devices for Filtering in Radio-communication Systems

There is now a large market for the UHF filters made with piezoelectric resonators due to their large use in radio-communication systems. These filters are presently built with materials presenting only a moderate coupling coefficient and using very similar circuit topologies. In this contribution we demonstrate that this lead to a limitation of their performances and we discuss solutions to improve them. These solutions are found considering alternative filter topologies and/or other materials and/or new kinds of devices (resonators or monolithic filters)