Microtubes are a basic component for a myriad of potential products. They are very small tubes (hundreds can fit in a human hair) that can be made from practically any material. Tubes larger than 1 micron diameter can be made with any cross-sectional shape desired. The significance of microtubes and microtube composites is that they provide the opportunity to miniaturize (even to the nanoscale) numerous products and devices that are currently in existence as well as allowing the fabrication of products that have to date been impossible to produce

Hoffman, Wesley P.

Upadhya, Kamleshwar

English

NASA Technical Reports Server

N94- 30452
THE UNIVERSAL APPLICATIONS OF MICROTUBES AND MICROTUBE COMPOSITES
Wesley P. Hoffman
Phillips Laboratory
OLAC PL/RKFE
Edwards AFB CA 93524-7680
Kandeshwar Upadhya
UDRI / Phillips Laboratory
Bldg. 8424
Edwards AFB CA 93524-7680
ABSTRACT
Microtubes, are a basic component for a myriad of potential products. They are very small tubes (hundreds can fit in
a human hair) that can be made from practically any material. Tubes larger than 1 micron diameter can be made with
any cross-sectional shape desired. The significance of microtubes and microtube composites is that they provide the
opportunity to miniaturize (even to the nanoscale) numerous products and devices that are currently in existence as
well as allowing the fabrication of products that have to date been impossible to produce.
_TRODUCTION
The world is becoming smaller each day. Not just in the field of transportation but more importantly in the
field of miniaturization. This is probably most evident in the electronics field with each new generation of integrated
circuits, i.e., from integrated circuits to... large scale integration to...very large scale integration to... ultra large scale
integration to...? These advances have had a dramatic effect on almost every aspect of modern civilization. However,
there are currently many other areas, such as: micromotors, sensors, detectors, microrefrigerators, and scanning
tunneling microscopes (STM) where miniaturization is also playing a crucial role.
In many miniaturization applications, there is a need for micro tubing for uses such as: connecting parts,
component cooling, sensors, and as probes, to name a few. If the proper micro tubing were available, numerous
components and systems could be miniaturized and others could be made more efficient. In addition, many
components and systems not currently in existence could become a reality.
Commercially, tubing is extruded, drawn or pultruded, which limits the types of materials that can be used
for ultra-small tubes as well as their ultimate internal diameters. As a result, ceramic tubes are currently available
only as small as lmm I.D., copper tubing is available as small as 0.05 mm I.D., polyimide polymer tubing is
available as small as 80 microns I.D. and quartz tubing is drawn down as small as 2 microns I.D. This means that
quartz is the only tubing available that is less than 10 microns in internal diameter. This quartz tubing is used
principally for chromatographic applications. Thus, until the availability of microtubes, only quartz tubing has been
available in micron dimensions and no tubing has existed with sub micron internal dimensions.
DESCRIPTION OF MICROTUBES
In contrast to tubing currently on the market, microtubes can be made from practically any material with
precisely controlled composition down to internal diameters less than 5 microns. (There is no upper internal diameter
limit.) In addition, for materials that can survive temperatures greater than 4000 C, tubes can theoretically be made
as small as 5 nanometers. To date, tubes have been made from metals (copper, nickel, aluminum, gold, platinum,
silver) , ceramics (silicon carbide, carbon, silicon nitride, sapphire), glasses (silica), polymers (teflon), alloys
(stainless steel) and layered combinations (carbon/ nickel, silver/sapphire) in sizes from 0.5 - 300 microns. ( By
comparison a human hair has a diameter of ~ 100 microns.) Some scanning electron microscope (SEM) micrographs
of these tubes can be seen in Figure 1.
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Figure 1. Examples ofmicrotubes. (a) 10 micron silicon carbide tubes. (b) 10 micron nickel tubes. (c)
26 micron silicon nitride tube. (d) 0.6 micron quartz tube.
Since the process does not involve pultrusion, extrusion or drawing but rather a fugitive tube forming
process, cross-sectional shapes as well as wall thickness can be very accurately controlled. A myriad of shapes have
already been made as seen in Figure 2. These micrographs should be sufficient to demonstrate that practically any
shape imagined can be fabricated. As seen in Figure 2, the wall thickness on the tubes can be held very uniform
around the tube. It is also possible to control the wall thickness along the length of the individual tubes as well as
among the tubes in a batch. To date, free-standing tubes with some mechanical strength have been made with wall
thicknesses as small as 0.01 microns. (Figure 3) There is no upper limit to wall thickness as will be discussed below.
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(a) (h)
(c) (d)
Figure 2. Above 1 micron inside diameter tubes can be made in any cross-sectional shape such as (a) 17
micron star, (b) 9 X 34 micron oval, (c) 59 micron smile, and a 45 micron uilobal shape.
The maximum length that these fibers can be made has yet to be determined because it depends on many
variables, such as, type of tube material, composition of sacrificial tube forming material, degree of porosity in the
wall, etc. It is possible that with a porous wall there is no limitation in length. For a non-porous wall the maximum
length would probably be measured in feet with there being a direct relationship between the tube I.D. and the
maximum possible length. However, for most applications conceived to date, the length need only be measured in
inches. If one does a quick calculation it is apparent that even "short" tubes have a tremendous aspect ratio. For
instance, a one inch long 10 micron I.D. Tube has an aspect ratio of 2500.
Currently, these tubes have been made by a batch process in the laboratory but the technique is equally
suited to a continuous process which would not only be more efficient but in some cases easier. Obviously, a
continuous process would reduce the cost which for most materials is already rather low because, unlike other
processes, expensive tooling is not needed. For many materials such as quartz, aluminum, copper, etc. the cost is
anticipated to be much less than $0.01/ inch. For precious metals such as gold or platinum the cost would be
significantly higher due to the cost of the raw materials.
110
(a) (b)
Figure 3. Tube can be structurally sound with very thin walls. (a) Unbroken tube. (b) Tube broken to
expose interior.
MODIFICATIONS FOR VARIOUS APPLICATIONS
Microtubes can be made free-standing, with wall thickness as small as 0.01 microns, or the walls can be
made so thick that the space between the tubes is filled producing a solid monolithic structure with micro channels
(Figure 4). These micro channels can be randomly oriented or they can have a predetermined orientation. Any
desired orientation or configuration of microtubes can be obtained by a fixturing process. Alternatively, composite
materials can be made using a material different than the tube wall as a "matrix" that fills in the space between the
tubes. The microtubes imbedded in these monolithic structures form oriented micro channels which like free-
standing tubes can contain solids, liquids and gases, as well as to act as conduits for all types of electromagnetic
energy. If the tubes are placed in a solid structure they can act as lightweight structural reinforcement similar to that
found in bone or wood. The cross-sectional shape of these reinforcement tubes can be tailored to maximize
mechanical or other properties.
The interior surface of these tubes can have practically any desired texture and degree of roughness which
would be advantageous in many applications. In addition, the interior Gr exterior surface of these tubes can be coated
with a layer or layers of another material ( Figure 5) to form, for instance, a multiple path conductor or a tube for
chemical reactions. When the tube is filled with another type of material, for example, it can become a sensor,
detector, or an element in an electron multiplier.
Depending on the application, the walls of microtubes can range from non-porous to extremely porous as
seen in Figure 6. In addition the tubes car. be made straight, curved or coiled as seen in Figure 7. For many
applications it is desirable to be able to interface microtubes with the macro world with a gradual decrease in the
internal diameter. This is possible by a tapering process in which the diameter is gradually decreased to micron
dimensions. Alternatively the tubes can be interfaced to the macro world through numerous types of manifolding
schemes.
APPLICATIONS AND USES
Microtubes appear to have almost universal application in areas as diverse as optics, electronics, medical
technology, and microelectromechanical devices. As a result, their specific function depends on the product in which
they are applied. Specific applications for microtubes are as diverse as chromatography, encapsulation, heat
exchange, injectors, micro-pipettes, dies, composite reinforcement, detectors, micropore filters, insulation,
111
Figure4.Solidcarbontube with microchannels. Figure 5. Sapphire tube with silver liner.
Figure 6.
Ca)
Examples of porous tube walls. Pores mainly in the sides (a).
Co)
Pores mainly longitudinal Co).
displays, sensors, optical wave guides, flow control, pinpoint lubrication, micro sponges, heat pipes, microprobes,
plumbing for micromotors and refrigerators, arrays for printers and xerography drums, etc. The technology works
equally well for high and low temperature materials and all applications that have been conceived to date appear
possible at this time.
112
Figure 7. Example of a curved nickel tube.
SUMMARY
The significance of microtubes and microtube composites is that they can be fabricated inexpensively out of
practically any material in a variety of cross-sectional shapes and in diameters orders of magnitude smaller than is
now possible. These tubes will provide the opportunity to miniaturize (even to the nanoscale) numerous products
and devices that are currently in existence as well as allowing the fabrication of products that have to date been
impossible to produce. For example, as electronic circuits become smaller and more compact, there is a greatly
increased need for micro-cooling. The solution to this problem currently does not exist. Microtubes can satisfy this
need in the form of micro-heat pipes or heat exchangers.
From the above discussion, it has become evident that microtube composites have become "solutions in
search of problems" and that future applications will be limited only by mankind's imagination.
Acknowledgment: The invaluable help of Hong Phan in fabricating most of the microtubes and of Jim King in taking
the SEM micrographs is greatly appreciated. The financial support of the Chemistry and Materials Science
Directorate of the Air Force Office of Scientific Research was responsible for the beginning phase of this work.
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The universal applications of microtubes and microtube composites

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