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A forming process for glass-coated microwire
A forming process for glasscoated m/crow/re A process for forming glass-coated highly resistive microwire with cores of copper, silver, or nichrome wire has been developed at Glass Developments Ltd. The wire has a maximum diameter of 50/~m The process for producing microwire was developed some five years ago and until recently has not been widely used. A small plant is installed at Glass Developments' works, but this only sees service when amounts of the wire are ordered.
The drawn metal is cooled by an oil bath. The wire passes through a small waterfall of oil. Without this the glass would stretch and the coating fall off or at best become too thin. The cold wire is wound round a reel at the base of the machine at a determined rate.
Technique The machinery is simple. It consists basically of a Radyne heater. Fig 1 shows the rudiments of the process. Wire, (copper, silver, nichrome, or even gold) is unwound from a reel and fed between the rollers at a determined speed. This is controlled by a transistor. It is most important to do this as the amount of molten wire produced at any one time is critical as shall be shown later. The wire is drawn into a glass tube. The base of the tube is surrounded by the heater coil which maintains a temperature of 1150°C. In the base of the tube the wire melts and forms a droplet of molten metal. The boro-silicate glass softens and melts in this area and it is possible to draw the glass-coated wire from the base of the tube. The amount of molten wire is critical, simply because too large an amount would cause the bottom of the tube to fall out and the molten metal to drop onto the floor or the feet of the operator. This also happens if the viscosity of the metal is too high. The correct viscosity is about 104p. The temperature gradient is from the inside to the outside. If it were in the opposite direction the metal would again be too viscous and the effect would be similar.
Bonding Bonding between glass and copper is good, but under stress and with other core materials the glass can be stripped off. Connections are not simple. The glass coat must be removed for soldering, either by crushing or etching.
Applications The most important application to date is the use of microwire as brain probes. The wires are stiff and act as free standing high strength electrodes. The core metal used for these probes is silver. Copper-core wire is used in microcircuits, gold in transistors and nichrome wire in microheaters.
Ack nowledgements Thanks are due to A. Arno of Glass Developments Limited for his help in the preparation of this article.
Coil I~lle
possed through these
ot a controlled rate
Gloss tube Heater coils
Oil both to cool wire ~
~.-.-~ I
~
Moltcn metal
:
Wire emerging from
Glass tube wall ( boro- sJlicot¢)
~
Metal wire
Wire melting .~,~i ~ ~
the gloss tube cooled In gloss
I .~\~ hot microwire, ...~.~-:~ without this wire stretches
\
,-----Half melted wire acts a s o flux This is important with
~
Teml~mture gradient Metal core Gloss coat
Micro#ireround ree~ w o u n d - - - - . ~
Forming process in outline
ox0
inclusions form
Fig 2
Detail of glass tube and wire
COMPOSITES March 1970
|67
The drawn metal is cooled by an oil bath. The wire passes through a small waterfall of oil. Without this the glass would stretch and the coating fall off or at best become too thin. The cold wire is wound round a reel at the base of the machine at a determined rate.
Technique The machinery is simple. It consists basically of a Radyne heater. Fig 1 shows the rudiments of the process. Wire, (copper, silver, nichrome, or even gold) is unwound from a reel and fed between the rollers at a determined speed. This is controlled by a transistor. It is most important to do this as the amount of molten wire produced at any one time is critical as shall be shown later. The wire is drawn into a glass tube. The base of the tube is surrounded by the heater coil which maintains a temperature of 1150°C. In the base of the tube the wire melts and forms a droplet of molten metal. The boro-silicate glass softens and melts in this area and it is possible to draw the glass-coated wire from the base of the tube. The amount of molten wire is critical, simply because too large an amount would cause the bottom of the tube to fall out and the molten metal to drop onto the floor or the feet of the operator. This also happens if the viscosity of the metal is too high. The correct viscosity is about 104p. The temperature gradient is from the inside to the outside. If it were in the opposite direction the metal would again be too viscous and the effect would be similar.
Bonding Bonding between glass and copper is good, but under stress and with other core materials the glass can be stripped off. Connections are not simple. The glass coat must be removed for soldering, either by crushing or etching.
Applications The most important application to date is the use of microwire as brain probes. The wires are stiff and act as free standing high strength electrodes. The core metal used for these probes is silver. Copper-core wire is used in microcircuits, gold in transistors and nichrome wire in microheaters.
Ack nowledgements Thanks are due to A. Arno of Glass Developments Limited for his help in the preparation of this article.
Coil I~lle
possed through these
ot a controlled rate
Gloss tube Heater coils
Oil both to cool wire ~
~.-.-~ I
~
Moltcn metal
:
Wire emerging from
Glass tube wall ( boro- sJlicot¢)
~
Metal wire
Wire melting .~,~i ~ ~
the gloss tube cooled In gloss
I .~\~ hot microwire, ...~.~-:~ without this wire stretches
\
,-----Half melted wire acts a s o flux This is important with
~
Teml~mture gradient Metal core Gloss coat
Micro#ireround ree~ w o u n d - - - - . ~
Forming process in outline
ox0
inclusions form
Fig 2
Detail of glass tube and wire
COMPOSITES March 1970
|67