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452 Chapter 4.5: Construction Materials

There are several possible problems with glass systems. Glass is brittle and

therefore fragile. Although glass is strong under compression, it is weak under

tension. (A well-designed glass system will ensure that areas subjected to high

levels of mechanical stress will be in compression. For example, glass-to-metal

seals should contain layers of glasses with gradated thermal coefficients of ex-

pansion. The sequence of glasses selected should vary depending on whether the

part is heated or cooled during use.) Soft, soda lime glass and, to some degree,

borosilicate glass (Pyrex®) are vulnerable to thermal shock and soften at elevated

temperatures in excess of 500 to 600°C. The sodium in soft glass and the boron

in Pyrex® may chemically contaminate certain processes—especially processes

used for fabricating silicon microelectronic devices. (Quartz withstands thermal

shock more gracefully, softens at higher temperatures, and does not contaminate

silicon devices; manipulating quartz requires advanced glass-blowing skills be-

cause it has a strong tendency to devitrify if it is not annealed properly.) If a glass

or quartz system is not baked under vacuum for an extended amount of

time,

inter-

stitial and adsorbed water vapor outgassing limits ultimate pressure. Ultimately,

helium diffusion and permeation, especially for thin-walled systems at elevated

temperatures, limits background pressure.

4.5.2.2 Steel

Commercially available, nonmagnetic, 304, 316, or 317 stainless steel vacuum

system components are marketed by a number of companies. (Type 303 stainless

steel is not suitable for use in high vacuum because high-vapor-pressure materi-

als—

sulfur, phosphorus, and selenium—are included as alloying constituents to

improve machinability.)

Stainless steel systems offer a considerable number of advantages. Stainless

steel is strong and resistant to many chemicals. There are documented practices

and proprietary processes for cleaning and polishing stainless steel (see Chap-

ter

4.9).

Appropriate joining techniques allow a choice of tungsten inert gas (TIG)

welding, spot welding, high-temperature brazing, and silver soldering. When soft

copper or gold gaskets are used in valves and seals, steel systems can be baked to

desorb contaminants and reduce the ultimate pressure. Large-scale manufacture

of stock items and competition have reduced

costs.

Because of the wide variety of

stainless steel vacuum components that are conmiercially available, customized

chambers can frequently be assembled by simply bolting together stock items

selected from catalogues. Components not commercially available can be fabri-

cated by machinists and welders with some specialized training and care. A judi-

cious choice of stock components, careful cleaning and assembly, and regular

baking at elevated temperatures will result in a system capable of operating at ul-

timate pressures in the ultra-high and extra-ultra-high-vacuum range.

Several possible problems are associated with stainless steel. The raw stock is

4.5.2 Vacuum Chamber Materials 453

inherently expensive and

heavy.

Nonmagnetic stainless steels require more sophis-

ticated machining skills than other metals such as nickel and certain aluminum al-

loys.

(If stainless steel is not machined at just the right speed with sufficient cut-

ting fluid, it work-hardens.) As a result of phase transformations, nonmagnetic

stainless steels deform at temperatures in excess of 500 to 600°C. Stainless steel

is attacked by certain chemically aggressive gases and plasmas — fluorine and

chlorine, for example [6]. Heavy metal by-products eroded or sputtered from

stainless steel may introduce unacceptable contamination. (The silicon micro-

electronics fabrication community and the magnetically confined fusion commu-

nity are especially concerned with trace heavy metal contaminants introduced by

corroded or sputtered stainless steel fixtures.) Hydrogen diffuses within and per-

meates through stainless steel and limits ultimate pressure. Heating accelerates

diffusion and permeation rate. The diffusion rate also depends on composition.

Hydrogen permeates through chromium steel at

-j^

J^Q

the rate at which it trav-

els through pure iron. Adding carbon to iron to make mild steel increases the hy-

drogen diffusion rate. Carbon also generates a carbon monoxide background.

If hydrogen and carbon monoxide background pressures do not cause prob-

lems,

then "mild" (low-carbon, cold-rolled) steel is a fine vacuum material if the

surface is free of rust. Mild steels can be plated with copper, then nickel, or cop-

per and nickel followed by chromium to avoid rust problems (and increase sur-

face hardness). Mild steel represents a cost-effective alternative to stainless steel

for constructing large vacuum systems with conservative performance. Problems

arise if porous cast raw stock or raw stock with a high sulfur content is inadver-

tently procured. Of course, mild steel is magnetic and therefore inappropriate for

systems where strong magnetic fields are present.

4.5.2.3 Aluminum and Its

Alloys;

Anodic Coatings

Chapter

4.8,

"Aluminum-Based Vacuum Systems," provides detailed information

regarding the use of aluminum as a vacuum material. Aluminum's popularity as a

vacuum chamber material has increased in recent years. Aluminum vacuum sys-

tem components offer an attractive alternative to the helium diffusion problems

associated with glass and the hydrogen diffusion problems associated with steel.

(The rate at which hydrogen diffuses through aluminum is three to four orders of

magnitude lower than the diffusion rate through stainless steel.) As a general rule,

aluminum raw stock is less expensive than stainless steel. Aluminum can be an-

odized to increase its corrosion resistance to some chemically reactive working

gases.

Anodic aluminum oxide coatings have useful dielectric properties for elec-

trical insulation or shaping plasmas. Certain types of anodic coatings are ex-

tremely hard.

If aluminum components are machined in-house, and precise tolerances must

be maintained and low background pressures are desired, attention must be paid

454 Chapter 4.5: Construction Materials

to the selection of raw stock. Exposing this metal to a chemically corrosive envi-

ronment or specifying that piece-parts should be coated with a thick, homoge-

neous,

cosmetically attractive anodic coating imposes additional restrictions on

aluminum alloy selection. Cast aluminum alloys and cold-rolled plate stock may

be porous and unacceptable for use as vacuum materials. The 7000 series alu-

minum alloys contain a high-vapor-pressure alloying constituent—zinc; zinc also

interferes with homogeneous anodic film formation. The 2000 series aluminum

alloys contain copper as an alloying constituent. Copper may cause staining if this

alloy is anodized, or initiate localized corrosion in chemically aggressive ambi-

ents;

frequently these tendencies are not cause for concern, and the 2000 series al-

loys are good ultra-high-vacuum materials. Extra-ultra-high vacuum is obtained

most reliably in aluminum systems built from extruded 1000 series and 6000 se-

ries alloys. Machining these alloys in a dry, 95% argon, 5% air ambient signifi-

cantly decreases outgassing in state-of-the-art vacuum systems. Commercially

pure,

1000 series aluminum (and also the 5000 series alloys with magnesium) is

soft, exceedingly ductile, and difficult to machine to precise tolerances. The 6000

series alloys, especially 6061-T6, are relatively hard, machinable, and relatively

easy to weld or braze. Work-hardened 6061-T6 is hard enough to hold a knife

edge that forms a vacuum-tight seal with pure and/or annealed aluminum gaskets.

The 6000 series alloys, the 1000 series alloys, and the 5000 series alloys share the

advantage that they can readily be anodized.

The composition and microstructure of an anodic aluminum oxide coating can

vary widely, depending on the alloy selected and the anodization process parame-

ters [7]. Certain anodization processes, such as those performed around 0°C in

sulfuric or sulfuric plus oxalic acid solutions, form relatively dense, black, Martin®

or Sanford® "hard coats" that exceed chromium steel in surface hardness. As a

general rule, anodic coatings with extremely large pores form in phosphoric acid.

Coatings with pores around 10 to 30 nm in diameter form in room temperature sul-

furic and oxalic acid. Anodic coatings with extremely fine pores form in chromic

acid. In addition to the acid selected for the electrolytic bath, porosity also de-

pends on how the process is operated. As a general rule, increasing acid concen-

tration, electrolyte temperature, and cell voltage results in more porous coatings.

Drawing constant, direct current as opposed to rippled or pulsed current leads to

more porous coatings. Thicker coatings tend to be more porous. Some anodic

coatings are dyed for customers interested in cosmetic appearance and may not be

desirable for use in vacuum. Often coatings are sealed by immersion in boiling

water/salt solutions to improve corrosion resistance, and cosmetic appearance.

Each of these process variations can substantially alter the spectrum of residual

contaminants introduced into a vacuum system. Water and entrained electrolyte

by-products that desorb from these porous, anodized aluminum may affect pro-

cesses performed in these systems, especially at elevated temperatures.

Sometimes relatively thin, porous, drab brownish-yellow or green, phosphoric

acid plus chromic acid chemical conversion coatings, e.g., Alodyne®, are applied

4.S.3 Special-Purpose Materials

455

to aluminum

as a

low-cost alternative

anodization. Such coatings probably

de-

tract from vacuum performance.

Several problems may arise

aluminum alloy vacuum fixturing and chambers

are selected. Options

and

opportunities

for

avoiding

designing around some

of these problems

are

described

detail

Chapter 4.8.

The

crystal structure

of many aluminum alloys changes irreversibly

relatively

low

temperatures

(~400°C). Some alloys warp irreversibly

elevated temperatures. Aluminum

not

strong

steel

and

softens substantially

elevated temperatures (starting

above 250°C).

layer

dense, hard, relatively chemically inert, air-grown alu-

minum oxide that

is 3 to 30 nm

thick depending

thermal history

almost

al-

ways present.

the thin air oxide layer is breached

damaged, aluminum can

chemically reactive.

The air

oxide layer limits

the

number

techniques that

can

be used

bond aluminum. Aluminum solders that contain aluminum, zinc,

and

indium

as a

wetting agent

are

commercially available,

but

inappropriate

for

vac-

uum applications. Good-quality joints can be formed

inert gas welding.

4.5.3

SPECIAL-PURPOSE MATERIALS

Scientists

and

technologists frequently require electrical conduction

insula-

tion, thermal conduction

insulation,

and

lubrication

practical vacuum sys-

tems.

Sometimes people

are

concerned with costs—especially when they

are

building large, unique vacuum systems. This section provides guidelines

for se-

lecting materials

for

specific applications.

4.5.3.1 Electrical Conductors, Electrical Insulators,

Thermal Conductors, and Thermal Insulators

ELECTRICAL CONDUCTORS

Oxygen-free, high-conductivity (OFHC) copper is

excellent electrical conduc-

tor. Smooth, clean OFHC copper

is an

excellent vacuum material. Unfortunately

copper

chemically active and oxidizes readily when exposed to air, water, or car-

bon dioxide at elevated temperatures. Large areas

copper with somewhat rough

oxidized surfaces

(and

adsorbed water) will slow pumping substantially into

the

ultra-high

and

extra-ultra-high vacuum range.

ultra-high

and

extra-ultra-high

vacuum is required, less chemically reactive metals such as gold, chromium,

and/

or nickel

can be

plated

on the

copper surface.

electrodeposited chromium

gold coatings are deposited, the vacuum technologist must ensure that the electro-

plater will deposit

dense,

smooth coatings that are relatively free of organic bright-

456 Chapter 4.5: Construction Materials

ening and leveling additives with high vapor pressures. If the plating facility is

careless and does not adequately control the plating process, their chromium or

gold coatings may be rough, porous, and loaded with entrained hydrogen. Ultra-

high-purity gold coatings (deposited from a cyanide bath with no brighteners or

leveling agents) have a matte surface finish with a large surface area that may out-

gas excessive amounts of sorbed species. The best compromise may be to deposit

a thin, smooth, dense "bright" gold or chromium coatings from a bath that con-

tains some small concentration of organic additives, then vacuum bake the elec-

troplated fixtures to decompose and outgas entrained organics and improve vac-

uum performance. If the plated part will be used at elevated temperatures over

an extended amount of time, then a nickel undercoating should be deposited on

copper fixtures. This nickel coating serves as a diffusion barrier. It should also be

noted that both copper and gold sputter readily; copper and gold surfaces must be

protected from plasmas and particle beams.

Silver is an excellent electrical conductor. Unfortunately silver can be a chemi-

cally unstable material (see Section 4.5.3.4). Silver can be gold-plated to avoid

chemistry problems.

If only modest conductivity is required, then tantalum, rhenium, titanium, mo-

lybdenum, tungsten, nickel, or aluminum may suffice. Each of these metals has

both good and bad properties. A wise vacuum technologist will use different met-

als for different components to exploit good properties and avoid potential prob-

lems.

These metals have electrical resistivities that are higher than silver, gold,

and copper. Nickel, aluminum, molybdenum, titanium, rhenium, tantalum, and

tungsten sputter less readily than gold and copper. Molybdenum and tungsten are

refractory metals with relatively low thermal expansion coefficients and an affin-

ity for oxygen. They seal well with alumina and glass in electrical feedthroughs.

However, reactions between oxygen and tantalum, titanium, molybdenum, and

tungsten may be troublesome. These metals have a tendency to dissolve large

amounts of oxygen or form thick porous oxide layers in partial pressures of oxy-

gen at elevated temperatures. Hydrogen dissolves into titanium and tantalum, es-

pecially at elevated temperatures and causes serious embrittlement problems.

Tantalum and molybdenum are extremely ductile and somewhat unpleasant to

machine. Tungsten tends to be brittle and therefore somewhat difficult to manip-

ulate. Titanium's extreme hardness presents problems to unskilled machinists

with inferior

tools.

Rhenium wire is readily available but other forms of raw stock

are less common. Nickel and aluminum alloys, such as the 6000 series, are good,

cost-effective vacuum materials that are easily cleaned, shaped, and machined

and do not form thick porous oxide layers that outgas excessively or shed par-

ticles.

However, it should be noted that large pieces of nickel (and other magnetic

materials) rob substantial amounts of energy from RF plasmas (in magnetron

sputtering systems, for example) and heat-up. Thermal limitations associated with

aluminum are noted in the preceding section and in Chapter 4.8.

4.5.3 Special-Purpose Materials 457

ELECTRICAL INSULATORS,THERMAL CONDUCTORS,

AND THERMAL INSULATORS

As a general rule, the plastic insulation and enamels on commercially available

copper wire arc not suitable for use in vacuum and must be removed. (Additional

information regarding the use and behavior of organic compounds is included in

Sections 4.6.1 and 4.7.) If electrical insulation is required on a bare metal wire,

polytetrafluoroethylene (PTFE), the Teflon® formulation with the lowest vapor

pressure and the highest softening temperature is an appropriate insulator for use

down to 10"^ torn Alternatively, polyimide can be used [8]. Polyimide (Kapton®

and Vespel®) is an excellent vacuum material, with a vapor pressure comparable

to stainless steel at low temperatures. Resins containing polyamic acid (the Pyre

ML® and Pyralin® product lines from DuPont, for example) are available that can

be painted on vacuum fixtures and then dried and air cured to form a polyimide

coating. Not only can these resins be used to form electrically insulating coatings,

they can be also used as adhesives and binders. Unfortunately, polyimide is noto-

rious for generating water vapor when its temperature is allowed to rise above

150 to 200°C (depending on the curing temperature originally used to polymerize

the resin and subsequent thermal history). Fixtures operating at high temperatures

that require electrical insulation can be sheathed with glass, quartz, or ceramic

beads.

Many ceramics are good electrical insulators [9]. Many 100% dense, refractory

ceramics are excellent vacuum materials. In practice, ceramic stock is frequently

formed by sintering or hot-pressing powder stock and may be somewhat porous.

Porosity equates to large surface areas (with sorbed species) and virtual leaks. It

is a good practice to question potential ceramics suppliers about the density of

their product, especially open-celled materials, with tortuous passageways and

large expanses of internal surface area. Many suppliers of sintered or hot-pressed

ceramics and sputtering targets are willing to work with the vacuum technologist

in selecting a range of powder particle sizes and bonding parameters to optimize

the material's vacuum performance. It is desirable to use dense, close-celled ce-

ramic stock. If this is not practical, the second-best strategy may be to obtain

stock with an open-celled structure formed from relatively large-sized primary

particles, with a minimum amount of internal surface area and large openings that

pump down easily.

Frequently, vacuum users are concerned with building heaters and need a good

thermal conductor that electrically insulates the object being heated from resistive

graphite, refractory metal or nickel-chromium alloy heating elements [10]. Dia-

mond, with a thermal conductivity approximately equal to 40 times that of copper

at room temperature, is an excellent thermal conductor/electrical insulator fol-

lowed by beryllium oxide, with a thermal conductivity somewhat less than cop-

per, and aluminum oxide (preferably a-aluminum oxide — "corundum" or "sap-

458 Chapter 4.5: Construction Materials

phire").

Beryllium oxide particles are extremely toxic, and extreme care must be

exercised when working with this material. Boron nitride is used as an electrical

insulator on some commercially available heaters that are suitable for a wide va-

riety of vacuum applications. Unfortunately, boron nitride has a tendency to ad-

sorb water.

Cubic zirconia is an excellent thermal insulator. It withstands extremely high

temperatures and tolerates thermal shock well. Cubic zirconia spacers are par-

ticularly suitable for thermally isolating hot stages from supporting platforms to

minimize thermal mass.

4.5.3.2 Lubricants

Lubricants are used in a vacuum system for three reasons:

• In a ground glass joint or elastomer 0-ring sealed system, the joints and

0-rings can be lightly greased so they move, seat, and seal better. (Many vac-

uum technologists argue against greasing 0-rings.)

• Lubricants can be used between moving surfaces in the vacuum to reduce the

friction coefficient, wear rate, and particulate generation.

• Lubricants are used as anti-seizing agents. (Mating surfaces of identical met-

als,

e.g., stainless steel nuts and bolts baked at high temperatures in vacuum,

are especially likely to seize.)

Three types of lubricants may be suitable for vacuum applications:

• Wet, organic, or silicone-compound oils and greases

• Dry lubricants, e.g., PTFE (Teflon®) and metal dichalcogenide compounds,

e.g., molybdenum disulfide and tungsten diselenide

• Metal-on-metal combinations

The choice of a lubricant to use depends on a number of considerations relating to

the specific application, including

Operating temperature and vapor pressure

Sliding motion or rolling motion

Reactive gas, electron beam, or plasma ambient

Heavy or light loading

Extensive repetitive usage or moderate use

OILS AND GREASES

Wet lubricant oils and greases that are specially formulated for vacuum use [11]

are appropriate for providing lubrication on ground glass joints and elastomer

0-ring seals.

4.S.3 Special-Purpose Materials

459

Situations involving repetitive, rolling motion under heavy loads may demand

the use of a wet lubricant. Because oils and greases flow, they replenish areas ex-

periencing heavy wear, and are good lubricants for severe tribological situations.

Because oils and greases flow, they tend to creep over all surfaces in a vacuum

system. Taking care to design moving parts so a dry film lubricant or a metal-on-

metal couple will suffice is strongly advisable.

DRY LUBRICANTS

Dry lubricants are especially suitable for situations involving sliding motion

within the vacuum chamber with light loads and moderate usage. It may be ap-

propriate to machine selected moving parts from polytetrafluoroethylene. Propri-

etary preparations are available that contain a suspension of PTFE particles [12].

For light loads and moderate use, it might be sufficient to drive off the relatively

high-vapor-pressure vehicles and binders in these preparations and operate with

the particles that remain.

Alternatively, metal dichalcogenides compounds, in particular, molybdenum di-

sulfide and tungsten diselenide have a long and well-documented history of

use

vacuum lubricants. Compared to molybdenum disulfide, tungsten diselenide has

a lower vapor pressure, withstands higher temperatures, and is considerably more

expensive. Most proprietary preparations with molybdenum disulfide also con-

tain copious amounts of high-vapor-pressure organic dispersants and binders. The

temperatures required to drive off these organics in a feasible amount of time are

dangerously close to temperature where molybdenum disulfide degrades and de-

composes (150 to 200°C). Many of these preparations also contain large amounts

of graphite, which loses its lubricant properties in vacuum. Adherent coatings of

pure molybdenum disulfide with good lubricant properties can be deposited by

sputtering or by electrophoretic deposition [13].

If antiseize coatings on metal hardware are required, reagent-grade molybde-

num disulfide, tungsten disulfide, niobium diselenide, or tungsten diselenide pow-

der can be mixed with absolute ethanol or methanol and painted on screw threads,

etc.

After the alcohol evaporates, the fixtures can be assembled.

METAL-ON-METAL COUPLES

A judicious selection of different metals that are in moving contact with one an-

other may provide sufficiently low friction coefficients and wear rates so wet or

dry film lubricants are not needed. As a general rule, two metals that are in mov-

ing contact should be as different from one another as possible so adhesion and

seizing do not occur. One effective strategy is to deposit a soft, ductile metal coat-

ing such as tin, indium, silver, or gold on the part that is easiest to maintain or re-

place. A hard metal or ceramic is then used for the mating part. Martensitic hard-

460 Chapter 4.5: Construction Materials

facing, nitrided steels such as nitronic 60, carburized steels, titanium nitride coat-

ings,

hard-anodized aluminum, etc., can be selectively employed to fabricate

moving contacts that do not require lubricants as traditionally defined.

4.5.3.3

Unusual Vacuum

Materials

Frequently, materials not traditionally used for vacuum work are evaluated or

used—especially for large, unique systems where cost is important.

Certain organic materials are potentially useful vacuum materials. Polyvinyl

chloride (PVC) is reported to have a vapor pressure as low as stainless steel at

room temperature after a few hours pumping to remove water vapor. One com-

mercial supplier has developed a system for forming vacuum-tight seals for this

polymer [14]. (Trouble rapidly ensues if a misguided technologist attempts to use

commercially available PVC adhesives sold at hardware stores and plumbing sup-

ply houses to bond together PVC vacuum parts.) Polymethylmetacrylate (also

commonly called PMMA, "acrylic," "Lucite®," or "Plexiglass®") has been used

to construct a vacuum chamber for a large, pulsed power system at Sandia Na-

tional Laboratories in Albuquerque, New Mexico where cost, dielectric integrity,

structural strength, and weight are considerations. A high partial pressure of out-

gassed water was a major problem until this system was retrofitted with sufficiently

large cryopumps. Polystyrene divinylbenzene (Rexolite®) has a higher break-

down strength, useful structural strength, and a substantially lower water absorp-

tion/desorption rate than PMMA. It also costs substantially more.

Several years ago, there was intense interest in using concrete as a vacuum ma-

terial in the large tunnels and shafts used for nuclear weapons tests at the Nevada

Test Site and other explosive test chambers. Unfortunately, despite diligent atten-

tion to formulating vacuum-grade concrete, the best efforts could only be pumped

down to a pressure of 10 torr. Ultimately, steel liners were implemented to main-

tain a useful vacuum in the Nevada Test Site tunnels and shafts.

4.5.3.4 Materials to Avoid

In general, it is a good policy to consider all materials as unsuitable for use in vac-

uum until information to the contrary becomes apparent.

HIGH-VAPOR-PRESSURE MATERIALS

As a general rule, the vacuum technologist should be wary of organic materials,

especially in high-vacuum systems or at elevated temperatures. The vapor pres-

4.5.3 Special-Purpose Materials

461

sures of many practical polymers vary depending on their thermal history and the

degree of polymerization and cross-linking that has occurred. As discussed in

Section

4.6.1,

problems can arise as a result of high-vapor-pressure additives that

may vary from batch run to batch run—plasticizers, fillers, and binders — not the

polymer

itself.

Certain metals are unacceptable for high-vacuum work, e.g., lead, zinc, and

cadmium. This includes soft solders that contain lead; hard solders that contain

cadmium, cadmium-corrosion-resistant coatings on nuts, bolts, and screws, and

zinc,

which is an alloying constituent in aluminum solders, brass, some bronzes,

and the 7000 series aluminum alloys. Zinc is used as a corrosion-resistant coating

on galvanized steel.

Vapor pressure data suggest that brass and lead-tin solder are acceptable mate-

rials at foreline pressures. Brass is a good choice for foreline and roughing mani-

folds and fixturing. It is tempting to use lead-tin solder to join brass or copper

roughing and foreline manifolds and fixturing. Unfortunately, lead-tin solders

are not very strong. Continued use and vibration from a mechanical pump can

cause lead-tin solder joints to crack and leak. Cadmium-free silver solder and

indium-based solders are preferable to lead-tin "soft" solder.

Selenium, sulfur, and phosphorous have unacceptably high vapor pressures.

Selenium and sulfur are frequently used in metal machining fluids. The vacuum

technologist must take care that his or her machine shop avoids the use of such

products when machining vacuum fixtures. A copious stream of ethanol or iso-

propanol serves as a good cutting fluid for light machining. Vegetable shortenings

such as Crisco® expedite heavier machining operations and clean up readily in

soap and water. (Additional, proprietary selenium and sulfur-free machining fluids

are recommended in Chapter 4.8.) Phosphorous can inadvertendy be introduced

into a vacuum system in the form of electroless nickel coatings; hard, electro-

plated phosphorous nickel coatings; certain types of amorphous Metglas® metals;

phosphorous bronze and oxygen-free copper where phosphorous is used to scav-

enge oxygen. (This is not the same as oxygen-free high-conductivity copper).

CHEMICALLY UNSTABLE MATERIALS

Materials that chemically react at low temperatures should be regarded with skep-

ticism, especially for use in systems seeing elevated temperatures or plasmas. For

example, vapor pressure data suggest that silver metal is a fine vacuum material.

Unfortunately, silver readily oxidizes in lab air, and silver oxide reduces and lib-

erates oxygen in vacuum at temperatures slightly above 200°C. Silver is even

more likely to react with pollutants in air, e.g., sulfur, sulfur oxides, or hydrogen

sulfide, than with oxygen. These decomposition products are even more of a nui-

sance than oxygen.