Indium Seals

Indium seals are critical components used in a variety of industrial and technological

applications that require reliable performance at low temperatures and moderate

pressures. These seals leverage the unique properties of indium, a post-transition

metal discovered in 1863, which is highly ductile, malleable, and capable of forming

hermetic bonds without the need for heat.

The application of indium as a sealing material has a rich history that dates back

to the early 20th century. Indium was discovered in 1863 by German scientists

Ferdinand Reich and Hieronymous Theodor Richter using spectroscopic methods.

The element was named for the indigo blue line in its spectrum. Initially, indium’s

unique properties were not widely recognized, and it remained a scientific curiosity

rather than a material of industrial importance.

The first significant use of indium emerged in 1924 when it was found to stabilize

non-ferrous metals, marking its initial entry into industrial applications. However, it

wasn’t until World War II that indium found a large-scale application. During this period,

indium was used to coat bearings in high-performance aircraft engines, providing

protection against damage and corrosion. This application, while important at the

time, eventually became less significant as other materials and technologies were

developed.

Indium’s role in sealing applications started to gain prominence with the advent of

more sophisticated industrial and technological needs. The material’s unique properties

such as its softness, malleability, and ability to form hermetic seals without

requiring heat—made it particularly useful in applications involving low temperatures

and moderate pressures.

The ability of indium to form self-passivating oxide layers, which are easily removed

with an acid etch, further enhanced its suitability for sealing applications. This

property allowed indium to compensate for imperfections in mating surfaces, such

as ceramics, germanium, metals, or glass, without the need for reflow. As a

result, indium seals became less sensitive to mechanical shock, vibration, and low

temperatures compared to other types of seals.

The historical development of indium as a sealing material highlights its transition

from a relatively obscure element to a critical component in modern industrial applications.

This evolution underscores the ongoing innovation in material science and

the ever-expanding utility of indium in various technological domains.

Manufacturing and Fabrication of Indium Seals
Indium seals are highly valued in applications requiring low-temperature and
moderate-pressure environments due to their unique properties. The manufacturing
and fabrication processes are crucial to ensure the reliability and effectiveness of these
seals.

1. Indium Seal Mechanical Formation
Indium seals can be created mechanically, without the need for heat application. This
characteristic is particularly useful in scenarios where heating or the use of solder
flux, which might outgas, is not an option. By simply applying pressure, indium can
form an effective seal. To ensure the integrity of the seal, as many fasteners as
possible should be used to clamp the indium material.

2.Indium Sealing Purity
The indium material used for seals must be ultra-pure, with a minimum purity of
99.9%. This high purity level prevents the hardening of the material at sub-zero
temperatures and restricts impurities of elements with low vapor pressure [3]. In some
applications, even higher purity levels, such as 99.99% or 99.999%, may be required to ensure
a vacuum, hermetic, or cryogenic seal.

3. Indium Preforms and Wire
Indium can be fabricated into various shapes and sizes, including preforms and wires,
to fit specific applications. The thickness of the indium material is crucial; for instance,
flat seals can be as thin as 0.008 inches (0.2 mm) or as thick as 0.062 inches (1.6
mm), depending on the area of the mating surfaces and the required compressive
force. When using wire, it must be centered correctly to ensure a uniform seal
when compressed. In some cases, a small groove may be machined into the seal
area to guide the wire placement accurately.

4.Indium Seal Surface Preparation
Before the indium is placed on the surface to be sealed, proper placement of the
preform or wire is critical. The amount of pressure required to form the seal varies
depending on the application, and several trials may be needed to determine the
optimal pressure. Additionally, indium forms a self-passivating oxide layer that can
be removed with an acid etch, allowing the underlying metal to be compressed and
form a tight, hermetic bond.

5. Indium Application Techniques
Various techniques, including soldering and welding, are available for creating indium
seals. These methods are necessary to hermetically entrap cryogenic constituents
under vacuum-tight conditions [3]. Indium’s softness and compressibility make it ideal
for creating effective seals, even in harsh environments like outer space or cryogenic
temperatures, where it retains its malleability.