276
8.
MICROSCOPY
to intersect the lip of the aperture. Thus, the aperture quite probably
will become asymmetrically contaminated.
8.4. Aperture Cleaning and Manufacture
Whether or not an aperture can be cleaned and reused depends partly
upon what it is made of. Platinum apertures originally were popular
because of the inertness of this metal. The contaminating deposits which
form during electron bombardment, however, are almost as insoluble as
platinum itself, and there is no really satisfactory way of getting rid of
them. Most simply, platinum apertures can be heated in the oxidizing
portion of a Bunsen burner flame. Platinum will not vaporize at this
temperature, but some contaminants will, and relatively clean apertures
result. However, ashes and perhaps amalgams remain which accumulate
and sooner or later destroy the usefulness of the aperture. It has been
claimed that it is safer and better not to flame the aperture directly,
but to protect it by having it in a platinum crucible or over platinum
foil. An old aperture can be used in this way to protect a good one.
Platinum apertures can also be heated in vacuo in a tungsten boat to
a temperature just below vaporization. This can be judged by the incandescent color of the aperture. To learn when to stop heating the
aperture one should first deliberately vaporize an old aperture or two
while observing the color changes before this occurs.
A variety of chemical cleaning procedures have been proposed. Fused
potassium hydroxide works fairly well. Fuming sulfuric—chromic acid
mixtures are used. The RCA laboratories recommend the following somewhat elaborate procedure. A platinum or quartz crucible is required.
Pure potassium bisulfate is melted in this, and the aperture soaked for
at least 5 minutes. The aperture is then rinsed at least four times in boiling concentrated sulfuric acid. It is finally washed several times with
distilled water. The Siemens Company recommends a nearly identical
procedure.
Unfortunately not one of these methods will reliably remove all
deposits. Even when an aperture looks clean it may produce astigmatism
when it is reintroduced into the electron microscope. Thus, there has
been more than a little dissatisfaction with platinum, and other metals
have been used.
Molybdenum apertures recently have become available commercially.
8.
MICROSCOPY
to intersect the lip of the aperture. Thus, the aperture quite probably
will become asymmetrically contaminated.
8.4. Aperture Cleaning and Manufacture
Whether or not an aperture can be cleaned and reused depends partly
upon what it is made of. Platinum apertures originally were popular
because of the inertness of this metal. The contaminating deposits which
form during electron bombardment, however, are almost as insoluble as
platinum itself, and there is no really satisfactory way of getting rid of
them. Most simply, platinum apertures can be heated in the oxidizing
portion of a Bunsen burner flame. Platinum will not vaporize at this
temperature, but some contaminants will, and relatively clean apertures
result. However, ashes and perhaps amalgams remain which accumulate
and sooner or later destroy the usefulness of the aperture. It has been
claimed that it is safer and better not to flame the aperture directly,
but to protect it by having it in a platinum crucible or over platinum
foil. An old aperture can be used in this way to protect a good one.
Platinum apertures can also be heated in vacuo in a tungsten boat to
a temperature just below vaporization. This can be judged by the incandescent color of the aperture. To learn when to stop heating the
aperture one should first deliberately vaporize an old aperture or two
while observing the color changes before this occurs.
A variety of chemical cleaning procedures have been proposed. Fused
potassium hydroxide works fairly well. Fuming sulfuric—chromic acid
mixtures are used. The RCA laboratories recommend the following somewhat elaborate procedure. A platinum or quartz crucible is required.
Pure potassium bisulfate is melted in this, and the aperture soaked for
at least 5 minutes. The aperture is then rinsed at least four times in boiling concentrated sulfuric acid. It is finally washed several times with
distilled water. The Siemens Company recommends a nearly identical
procedure.
Unfortunately not one of these methods will reliably remove all
deposits. Even when an aperture looks clean it may produce astigmatism
when it is reintroduced into the electron microscope. Thus, there has
been more than a little dissatisfaction with platinum, and other metals
have been used.
Molybdenum apertures recently have become available commercially.