January 1973 Popular Electronics
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
published October 1954 - April 1985. All copyrights are hereby acknowledged.
Liquid crystals have been with us for so
long now that it is hard to imagine a time when they were considered a
scientific laboratory entity. Before being controlled by electric fields for use
in alpha-numeric displays, the thermal properties of liquid crystals of the
cholesteric type found applications in temperature and power measurements. Since the colors scattered by cholesteric liquid crystals under incandescent
light are unique to a given temperature, measurement of temperature is possible
to an accuracy of better than 0.1 °C. Bendix (patent US3693084)
manufactured a liquid crystal microwave power density meter. Nematic liquid
crystals are the type found in displays (twisted nematic LCD's) and are
controlled by an electric field which causes light to be transmitted or blocked
at varying levels. In 1973 when this article appeared in Popular Electronics
magazine, Seiko had just introduced one of the first LCD wristwatches - the
Liquid Crystals for Electronics
Inexpensive readout devices that get brighter
with more ambient light
Imagine a 7-segment readout device that requires only 280 μW of power to operate
and becomes more visible as the ambient light increases. Also imagine a cathode
ray tube which appears normal in every respect, even to the image on the screen,
except that the image has been stored there for more than a year with no electrical
connection of any type and can be erased in milliseconds. Let your imagination run
wild and conjure up fantasies of a panel light that costs only a penny, a flat-screen
TV receiver, or a microwave fluoroscope. These devices either already exist or are
anticipated to arrive on the scene in the near future. They are all possible as
a result of liquid crystals.
As their name implies, liquid crystal substances exhibit properties of both a
solid and a liquid. Depending on their viscosities, liquid crystals can be poured
like water, easily assuming the shape of their containers. However, due to selective
reflection, white light striking a film of these crystals causes a different wavelength
to be reflected at different angles of incidence, resulting in the iridescent colors
typical of liquid crystals. Furthermore, since the crystalline structure which produces
selective reflection is inherently weak, any force that causes this structure to
shift or realign itself causes a different structure to be produced. This force
can be thermal, acoustic, electrical, magnetic, or even mechanical.
Fig. 1 - Liquid crystal microwave power density meter made by
Fig. 2 - Prototype liquid crystal digital readout for watches
and calculators. (Photo: RCA Labs.)
Here, we will consider only thermotropic liquid crystals. These are compounds
that exhibit a liquid/crystal phase, or "mesophase," at a temperature usually greater
than the ambient. Thermotropic crystals can further be divided into three categories,
two of which are of interest to us - cholesteric and nematic.
Cholesteric Liquid Crystals
Cholesteric liquid crystals, all derivatives of
cholesterol, are the best known thermotropic compound. Currently, they are the most
widely applied. The major property of these compounds is their ability to change
color under the influence of different stimuli, notably temperature.
When a layer of cholesteric crystals has been properly applied to a surface and
illuminated by an incandescent light, the crystals change from colorless to red,
yellow, green, then blue, and, finally, violet, as the temperature of the surface
to which the crystals are applied passes through the mesophase range. Raising the
temperature even more turns the crystals colorless. The process is reversible, with
the same spectrum of colors appearing in reverse order as the crystals are cooled
through the mesophase range.
Since the colors scattered by cholesteric liquid crystals under incandescent
light are unique to a given temperature, measurement of temperature is possible
to an accuracy of better than 0.1 °C.
Liquid crystals either mixed with a solvent or contained within 20-micron spheres
(encapsulated liquid crystals), which are suspended in a water slurry, are available
if you wish to apply them to surfaces like transistor heatsinks. To work with the
solvent-suspended or encapsulated liquid crystals, a black background is a must.
Some of the encapsulated types are sold in a blackened solution which leaves a black
surface when dry. To use unblackened crystals, the surface to which they are to
be applied must first be coated with a water-base black paint such as No. VL-447A
available from Van-Light (9770 Conklin Rd., Cincinnati, OH 45452) at $1.00 for 50
cc. Edmund Scientific (300 Edscorp Bldg., Barrington, NJ 08007) stocks both the
blackened and plain liquid crystals.
The liquid crystal solution, a clear or slightly cloudy yellow solution, is designed
to be air-brushed or aerosol-sprayed onto the painted surface, using a steady back-and-forth
motion. An even coating about 1-mil thick will provide optimum results. Too thick
a coating must be avoided. An excellent kit is offered by Liquid Crystal Industries
(460 Brown Ave., Turtle Creek, PA 15145); it contains 12 bottles of pre-blackened
liquid crystal solutions, a special aerosol applicator, and a Mylar hoop for indirect
testing. Available in low and high temperature versions, the kits are each priced
No color is visible until the spray coating dries and the surface being tested
is at a temperature within the range of the liquid crystal being used. Dim colors
mean the liquid crystal coating is too thin. If this is the case, put down another
Microelectronic calculators such as those above with liquid crystal
displays, are being manufactured in volume by such companies as North American Rockwell.
In the display shown at (A), each digit has seven segments, each with an electrical
lead, formed on glass plates with transparent tin oxide. Two glass plates are bonded
together with about 1/1000 of an inch between them. This space is filled with liquid
crystal material. Magnified drawing at (B) shows how material's molecules are normally
uniform; but when subjected to electrical field (C), molecules are upset so that
they scatter light rays and the area appears to glow. Different numbers are formed
(D) by subjecting selected segments to electrical field, which causes those segments
to glow brightly.
Instead of applying the liquid crystals directly to a surface, it is much easier
to work with them in encapsulated sheet form so that they do not become contaminated
and can be used over and over again. Memory liquid crystal sheets have been developed;
they turn black wherever the surface has exceeded the critical temperature. When
the temperature decreases, the black area remains black, but simple brushing of
the blackened area restores the original color. Experimental kits of all types of
sheet material are available from Edmund Scientific in a small kit, Part No. 60,756,
for $4.00 postpaid, or in a large kit, Part No. 71,143, for $10.00 postpaid.
The property of cholesteric liquid crystals to exhibit different colors with
changes in temperature has many direct applications in electronics. For example,
the alignment of infrared laser beams is a difficult task by traditional methods.
In comparison, either a sheet of liquid crystal materials or a sheet of metal with
a layer of liquid crystal on its rear surface will yield a good image of the beam
size and operating mode of the laser.
The visualization of intensities in a microwave beam has also been easily accomplished
by the use of cholesteric liquid crystals. A layer of liquid crystals is applied
to a thin sheet of Mylar placed in a microwave beam so that energy transferred to
the film heats up various segments in proportion to the amount of energy absorbed.
Since the crystals indicate specific temperatures, distinct color lines form a two-dimensional
plot of the microwave field intensity. An example of a device that uses this principle
is shown in Fig. 1. In this instrument. built by Bendix Laboratories, the temperature
range represented by the transition from red to blue is equivalent to a power spread
of 7 dB.
Nematic Liquid Crystals
Nematic liquid crystals are currently causing a big stir in the electronics industry.
Nematics are generally somewhat cloudy when viewed in bulk and tend to be a pale
yellow in coloring. In small quantities or thin films, the haziness disappears.
When placed under a microscope, the clear solution appears to have long wavy threads.
If the liquid is probed or otherwise disturbed, the threads greatly multiply, slowly
diminishing in number to the original quantity if no further disturbance takes place.
These threads represent minute changes in the index of refraction between adjacent
areas of the liquid. Under turbulent conditions, these area boundaries become many
in number and tend to scatter light as the liquid turns an almost opaque white.
To harness this turbulence, a cell can be fabricated to contain a solution of
nematic crystals between two electrodes. When a potential is applied to the electrodes,
a flow of ions is created which causes turbulence and turns the normally clear liquid
crystal solution to a whitish color. This process is known as "dynamic scattering."
The degree of whiteness and, correspondingly, the degree of reflected light, can
easily be controlled by the voltage applied to the electrodes. Due to electrolysis
depleting the liquid of ions, continued operation of the cell on dc will cause eventual
failure of the nematic material. However, if an ac driving voltage is used, ion
depletion will be greatly reduced.
Like cholesteric crystals, nematic crystals have a mesophase that must be observed
for proper operation. Until recently, nematic action was observed only in a narrow
temperature band around 230° F. Continued research, however, particularly by
RCA, is yielding nematic solutions usable over wider temperature ranges, including
One of the most obvious uses for a material that reacts as nematics do is the
currently popular 7-segment readout. An RCA prototype of a 7-segment readout is
shown in Fig. 2. Since there is so little force required to create turbulence in
a nematic, minimal power provides a good indication.
Optel (P.O. Box 2215, Princeton, NJ 08540) was first to market a 7-segment liquid
crystal readout device. Their No. 1003 display unit operates at 15-16 volts (ac
for greater than 10,000 hours). The numerals form in 15-20 ms and decay in 100-200
ms. More important, only 40 μW of power is required per segment. Although the rise
times are noticeable, they are acceptable in digital electronic clock, volt-meter,
and airport arrival/departure sign applications.
Fig. 3 - The diagram shows the physical arrangement of an experimental
transmission-type nematic liquid crystal cell.
Fig. 4 - Nematic storage-mode Reflicon tube and demonstrator.
RCA has introduced a four-number 7-segment display and plans a matching COS/MOS
IC for decoding and driving. Also projected are liquid crystal readout products
from Display Tek of Dallas and from Texas Instruments.
The Optel readout employs the reflective mode for imaging, making indication
in a dark room impossible without a light source but affording excellent image clarity
in areas of high-intensity ambient lighting. Displays employing the transmissive
mode have been built in prototype by several companies, but these require a power-consuming
lamp behind the panel.
The uses for nematic crystals are not limited to readouts. Since the transmission
of light can be controlled at will, it is possible to make automatically darkening
windows, light shutters for optical systems, and many other similar devices.
If the front and rear electrodes of a nematic cell are formed into a grid-like
configuration in which the front sheet of glass has vertically oriented electrodes
and the rear sheet has horizontal electrodes, only the nematic material at the crossover
point of the electrodes, when energized, reacts by turning opaque. With addressing
electronics, such a device is easily capable of displaying diagrams and images.
Making a Nematic Cell
For experimenters who wish to obtain first-hand knowledge of the nematic cell,
materials are available for making their own. These consist of a pair of glass sheets,
each coated on one side with a conductive material, a spacer, and the nematic liquid
crystal material. These items will permit fabrication of a transmissive cell. If
a reflective cell is desired, one of the sheets of glass is omitted and replaced
by a sheet of darkened metal (such as black-anodized aluminum).
Transparent conductive glass sheets can be obtained from several suppliers. Vari-Light,
for example, has 2 1/8" X 2" X 1/8" sheets for $2.40 (No. CG-80 tin-oxide coated)
and $1.50 (No. CG-75 gold coated) each. Larger sizes are available on special order.
Although the gold coating has lower unit area resistance, the tin-oxide coating
is slightly better since it transmits more light. When working with any type of
conductive glass, the side with the conductive film should never be handled or otherwise
contaminated. It should never be cleaned; to do so may damage the coating.
Once you have the conductive glass, a spacer of the same length and width must
be prepared from some nonreactive material such as 1-5-mil sheet Teflon. Cut a hole
of the desired size out of the center of the spacer material. Next, place a sheet
of the glass, conductive side up, on a level, flat surface and place over it the
spacer. Now, using a thoroughly cleaned medicine drop-per, deposit some of the nematic
solution on the sheet of glass within the confines of the cutout in the spacer.
Make sure that the total amount of nematic solution deposited is no greater than
the amount needed just to fill the hole and allow some room for expansion. Cover
the assembly with the remaining sheet of glass. To finish assembling the cell, use
a frame made from Plexiglas to hold it together. (Note: If you plan to cement together
the cell parts, do not use an epoxy compound; it may react with the nematic liquid
in the cell and ruin your efforts.) The procedure for assembling the cell is shown
in the drawing in Fig. 3.
To make a 7-segment readout assembly, it is necessary to remove only part of
the conductive coating on one of the glass sheets, leaving "islands" of conductor
to make up the segments and narrow bands to bring out to the bus bar along the edge
of the glass sheet. The Van-Light conductive sheets have bus bars which can be cut
into seven separate segments, each going to a separate segment of the display's
conductive coating. To form the segments, a Dremel "Moto-Tool" with an abrasive
rubber cone and a metal erasing shield can be used.
The nematic solution for your cell can be obtained from Van-Light as Part No.
VL-1047-N. It consists of a 5-gram bottle of liquid and sells for $12.80. The operating
temperature range of the solution is 10°-47°C, which includes normal room temperature.
Power requirements for your homemade nematic cell are minimal but will vary from
cell to cell due to the assembly techniques used by different experimenters. If
the nematic is from Liquid Crystal Industries ($15/gram, 5-gram minimum order),
there will be typically about an 8-volt threshold, with 22 volts optimum. Resistivity
is about 1010 ohms/sq cm, and the cell will yield a contrast ratio of
at least 20:1. Although the cell will certainly operate on dc, the noticeable rise
and decay times can be shortened by use of a 1000-Hz ac driving voltage. This experimental
cell can be used as a light shutter, but the rise and decay times are still too
lengthy to use it for modulating a laser beam with audio information.
By mixing a nematic crystal solution with a cholesteric solution (such as cholesteryl
chloride), in a weight ratio of 9:1, a "storage-mode" liquid crystal solution is
obtained. The solution is normally clear, but with 30 volts dc applied to it, it
turns a milky white. Removing the voltage, the mixture regains its normal transparency
only after several weeks. If desired, however, the material can be made transparent
at any time simply by applying a 50-volt, 4000-Hz signal to the electrodes. With
no power required to retain the image, the applications are virtually limitless.
One application has already appeared in the Model D-10 Reflicon®
tube produced by Optel and shown with its demonstrator/driver package in Fig. 4.
As can clearly be seen, the image shown on the screen of the disconnected tube is
stored without attached wiring. The particular tube shown held its image for more
than a year with little degradation.
Posted September 7, 2020