NetNotes
books, manuals are highly appreciated. Zhaoxia Zhou z.zhou@lboro.
ac.uk Fri May 20 Contrast from dislocations can only arise if the crystal is
oriented in such a way that the strain field scatters the Bloch waves set up by diffraction. Although dislocation contrast in aluminum was discovered by Peter Hirsch and co-workers back in 1956, Archie Howie and Michael Whelan explained the dynamical diffraction contrast of imperfect crystals with phenomenal results. Much of this is contained within what is considered to the ‘Te Bible’ of electron microscopy: Electron Microscopy of Tin Crystals by Hirsch, Howie, Whelan, Pashley & Nicholson. Several different editions of the book exist and it is still highly recommended as a text book. Mike Loretto’s book Defect Analysis in Electron Microscopy is a shorter, and in some ways more accessible book to consult, with helpful diffraction patterns and Kikuchi line maps in the Appendices that assist navigation to the required diffraction conditions. More recent books include Barry Carter & David Williams, Transmission Electron Microscopy, which is aided by a selection of some the most beautiful pictures of defects to have appeared in the past 55 years. Some of the best images seared into my mind date back from the 1970s! All of these books (and many more) will tell you that successful dislocation analysis requires setting up well known diffraction conditions for each crystal grain. Terefore, no single orientation can give you all the information you want. Identifying dislocation Burgers vectors requires multiple images of the same set of dislocations under diffraction conditions to see which ones disappear, i.e., dislocations are identified by a process of elimination. However, it is reasonably straightforward to find a set of reflections in which all dislocations will eventually appear. Terefore, the dislocation density can established only if all crystal grains have each diffracted strongly with several different reflections. Jon Barnard
jsb43@hermes.cam.ac.uk Mon May 23
TEM: LaB6 upgrade We have a JEOL 1200 EXII TEM and we were wondering if it is
good enough in which to use a LaB6. Te engineers seem to have mix opinions, as does the great Google. I would like comments from anyone who has or has had a 1200 fitted with a LaB6. If you have one, which one? Te microscope is behaving itself very well, no leaks everything is as good as it gets given its age. With one fitted do you notice any significant difference when working at the top end? John Mitchels
john.mitchels@gmail.com Tu Jun 16 LaB6 filaments have to work in a good vacuum. Tere are some
instruments like your TEM working with LaB6 in a gun pumped by DP or turbo, but most of the LaB6 TEM work with high vacuum in the gun provided by SIP. If your vacuum system is very good, you may use LaB6 but you must understand that you will work on an edge. Anyway, you may try without danger for your instrument and you will see if LaB6 stay safe during one year (good) or one month. Please note that you may have to use another Wehnelt cap for this kind of filament and you have to change distance between filament tip and Wehnelt cap. First heating of this filament must be done very slowly (about one hour) aſter almost one night of pumping. Working with LaB6 is very comfortable because the brightness is very high and it may increase significantly top end results. Nicolas Stephant nicolas.
stephant@univ-nantes.fr Tu Jun 16 I would not recommend to work with a LaB6 in a (T)EM pumped
by a DP or turbo only. For me, and from our 20 year experience, an IGP or ion getter pump (is SIP the same?) plus a nitrogen-trap is a must. Other people may comment on this or have other experience,
70
though. Te lifetime of a LaB6 in our TEM (heavily used by bio-people; though, no cryo) , pumped by ODP, plus IGP plus nitrogen trap (all are always on, during day time) is 3 years, up to 3.5 years, in the last 20 years of use, now. I was told that the lifetime in a turbo-pumped TEM (bio, too) is about 0.5 to 1 year only, at best. It is not my own experience. Everybody can decide what this means, and what to do. Reinhard Rachel
reinhard.rachel@biologie.uni-regensburg.de Tu Jun 16 IGP (ion getter pump) and SIP (sputter ion pump) are different
names for same device. Roger Ristau
raristau@ims.uconn.edu Tu Jun 16
We have the same model JEOL 1200 EXII as you do and it has
worked flawlessly for 25 years with LaB6 filaments. It has diffusion pumps not turbo. We get anywhere from 900 to 1500 hrs of filament life—and that is running at fairly high emission currents of ~20 µA. One caveat: it helps to keep a clean column (vacuum). We have a custom-built LN2 cold trap that lasts for up to 48 hrs. Peter Ingram
p.ingram@
cellbio.duke.edu Tu Jun 16 Use of LaB6 cathode in a vacuum system without an IGP and
differential pumping is possible but impractical. Even if all following rules will be enforced: have LN trap cold before heating filament; turn off filament heating before specimen exchange; never heat up filament before gun/column vacuum fully recovers aſter specimen (or film) exchange, LaB6 will not likely last for even 1000 hours. Much less at higher emission setting. Vacuum for LaB6 must be better then 10–6 Torr. Very difficult to maintain without IGP and differential pumping system. I consider LaB6 use justified when it lasts at least 2000 hours at low/medium emission. My average service time experience for LaB6 is above 3500 hours in a UHV system, with the record approx. 7000 hours and still going. Tis particular Denka cathode was installed on Philips CM-12 in 1997 and still works 14 years later. Vitaly Feingold
vitalylazar@att.net Fri Jun 17 Like Nicolas said here below, it’s all a matter of vacuum. Because
of the well pumped gun (usually turbo or ion pumped), LaB6 SEMs usually have a filament life of way above 1000 hours. If you take the same SEM and exchange the LaB6 filament for a Tungsten one, you’ll be able to get way above 1000 hours with the tungsten filament. So, in theory if your SEM gun has a very good vacuum, you should be getting a very high lifetime from each tungsten filament. If not, you could expect to get good result with the LaB6 filament, but keep the lifetime of your current tungsten one. Keep in mind that the LaB6 filament is exponentially more expensive than tungsten. It’d be best to consult with JEOL in any case before you decide to do this. Roy Golombick
roy@picotech.co.il Wed Jun 22 I beg to differ. I have not seen any significant increase in
tungsten filament life in an ion-pumped gun versus a normal gun in good condition in an SEM. Poor vacuum will definitely shorten a W filament’s life, but a better vacuum makes very little difference due to the fact that W filaments run hotter than LaB6 and evaporate faster. Te reason poor vacuum accelerates their demise is due to oxidation on top of evaporation. No W filament I’ve ever seen has lasted more than about 300 hours at operating temperature in an SEM, 10–7 T vacuum notwithstanding. My observations may not hold for a TEM because TEMs run at lower temperatures/emission currents and therefore evaporation rates should be significantly lower. Ken Converse
kenconverse@qualityimages.biz Wed Jun 22 I usually agree with your comments but this time I have to
throw in a curved ball. I oſten work with automated analysis SEMs which have a tungsten filament life of upwards from 2500 hours; admittedly we run at 11 micro amps emission with the filament well
www.microscopy-today.com • 2011 September
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