Chemical Controlled Dissolution of LGS Samples: Comparison with Quartz and GaPO4
Though the use of LGS family crystals for filtering applications is now well known, we present here one of the best way to realize antimesa structures working at higher frequencies than a few tens of MHz. Generally, this kind of realizations requires dissolution depths higher than 100 mum. Consequently, the rough surface state does not remain constant: it becomes smoother or its roughness parameter can increase drastically. For that, we have compared the chemical dissolution of the crystal in four different etchants (HCl, HNO3, H3PO4 and NH4F.HF) for three crystal orientations: X-, Y-and Z-cuts (in LGS, Y-cut is similar to the AT-cut in quartz crystal). The parameters chosen to compare the efficiency of our selected solvents are the dissolution rate and the average roughness Ra. Furthermore, we have worked at 2 different temperatures: 60 and 80degC. The experiences show firstly that a X-cut plate is etched in HCl rapidly, compared to the Z-cut and so reveal the anisotropy of the material structure. Secondly, we observe that the dissolution rate of LGS in HCl solution is 8 to 10 times higher than in other solvents, at a given temperature. Unfortunately, it tends to reveal deep dissolution figures on the surface. From this set of experiments, we conclude that the best compromise is obtained with the ammonium bifluorure solvent which allows us to obtain a smoother surface state than initially.
Investigations on 10 MHz LGS and LGT crystal resonators
Materials in the LGT family are promising for designing bulk acoustic wave resonators with high quality factor. In our laboratory, we have manufactured a lot of planoconvex 10 MHz 5th overtone Y-cut resonators using LGS (langasite La3Ga5SiO14) and LGT (langatate La3Ga5.5Ta0.5O14) crystals. Our initial aim was to do noise measurements on these home-made resonators but problems occurred during manufacturing. It was the opportunity for further investigations. Indeed, we observed that the quality factor depends strongly on the energy trapping, the polishing method and the materials quality from one supplier to another. As for the quartz crystal, we have found that the material quality can be qualified by IR spectrometry whose resulting spectra exhibit absorption peaks more or less deep, linked to defects. These predominant criteria are not surprising but although they are nowadays quite well-defined in the case of quartz crystal resonators, they have to be defined again in the case of these LGS and LGT crystals. Then, a satisfying machining and polishing method has been first applied to elaborate high Q resonators. A comparison between different grades of LGS and LGT materials is established. In addition, LGT resonators are characterized by their motional parameters and frequency-temperature curves. Nevertheless, one of the main results is that the measured Q-f product is not the expected one. We present results of Q-factor versus radius of curvature and their comparison with the theoretical approach. It appears that an optimization should be performed. Right now the best resonator that we have made has got a Q-f product of 1.4 1013 on its 5th overtone (1.7 1013 on its 9th overtone). This result is slightly higher than the similar parameter obtained on a SC-cut quartz crystal resonator working at the same frequency.
Evaluation of CuAl2O4 as an Oxygen Carrier in Chemical-Looping Combustion
The chemical-looping combustion (CLC) process is a novel solution for efficient combustion with intrinsic separation of carbon dioxide. The process uses a metal oxide as an oxygen carrier to transfer oxygen from an air to a fuel reactor where the fuel, or gasification products of the fuel, reacts with the solid oxygen carrier. In this work, copper(II) aluminate (CuAl2O4) was assessed as a potential oxygen carrier using methane as fuel. The carrier particles were produced by freeze–granulation and calcined at 1050 °C for a duration of 6 h. The chemical-looping characteristics were evaluated in a laboratory-scale fluidized-bed reactor in the temperature range of 900–950 °C during 45 alternating redox cycles. The oxygen carrier exhibited reproducible and stable reactivity behavior in this temperature range. Neither agglomeration nor defluidization was noticed in any of the cycles carried out at 900–925 °C. However, after reactivity tests at 950 °C, soft agglomeration and particle fragmentation were observed. Systematic phase analysis of the Cu–Al–O system during the redox cycle was carried out as a function of duration of reduction and oxygen concentration during the oxidation period. It was found that the CuAl2O4 is reduced to copper(I) aluminate (CuAlO2; delafossite), Cu2O, and elemental Cu. The CuAlO2 phase is characterized by slow kinetics for oxidation into CuO and CuAl2O4. Despite this kinetic limitation, complete conversion of methane with reproducible reactivity of the oxygen carrier is achieved. Thus, CuAl2O4 could be a potential oxygen carrier for chemical-looping combustion.