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.
Etching Study of AT-Cut Cultured Quartz Using Etchants Containing Fluoride Salts, Hydrofluoric Acid, and Ammonium Bifluorlde
T h i s p a p e r r e p o r t s t h e r e s u l t s o f a chemical e t c h i n g s t u d y o f A T c u t c u l t u r e d q u a r t z when t h e e t c h a n t s h y d r o f l u o r i c a c i d and amnonium b i f l u o r i d e a r e m o d i f i e d w i t h f l u o r i d e s a l t a d d i t i v e s . These f l u o r i d e s a l t s s u p p l y p o s i t i v e i o n s o t h e r t h a n H+ and NH3+ a t a d s o r p t i o n s i t e s on t h e q u a r t z s u r f a c e . R e s u l t s show t h a t s p e c i f i c f l u o r i d e s a l t s a f f e c t sur face topography and e tch de fec ts . The a d d i t i o n o f ca l c ium, a lum inum, and l i t h ium f l uo r ide reduce e tch p i t s and improve surface topography when added t o h y d r o f l u o r i c e t c h a n t s a t 60°C. A d d i t i o n o f p o t a s s i u m f l u o r i d e r e d u c e s e t c h p i t and e tch channe l f o rma t ion wnen added t o h y d r o f l u o r i c a c i d o r ammonium b i f l u o r i d e e t c h a n t s a t 25°C o r 60°C.
Bodies Politic : Disease , Death and Doctors in Britain , 1650-1900
In spite of its intellectual, literary and comic brilliance, this book contains a dark and disturbing, but revealing, message. In some ways, my melancholy reading of Bodies Politic has inevitably been shaped by Roy's recent untimely death. Roy Porter was without doubt the finest social historian of medicine this country, or indeed the world, has produced. Expansive, humorous, intelligent, and incisive, his broad range of historical works has excited and educated several generations of students of history over the past thirty years. His death leaves an unfillable void within the global history of medicine. At the same time, however, this book itself, completed well before his death, is haunted by elements of despair and uncertainty that should make us carefully reconsider not only the purpose and direction of our professional inquiries but also our own mortality.