Liquid Crystal Temperature Sensors

The optical properties of the cholesteric phase provide a useful mechanism for a temperature sensor. Such devices may be found in pet stores (e.g., fish tank thermometers) or the hardware store (e.g., home temperature monitors) and are extremely cheap and relatively disposable. In a 

FIGURE 2.33 (a) Diagram showing the simplified structure of a reflective temperature sensor. Incident light with a handedness matching that of the material is selectively reflected from the liquid crystal structure, (b) The reflection spectrum from the sensor. 

FIGURE 2.34 A fish tankthermometer uses a reflective cholesteric film as the sensor. As the water temperature changes, the cholesteric helix varies in pitch, causing a change in the reflected wavelength of the material. 

---

A model for crystallization point of the urea melt sprayed into the granulator was developed based on acoustic spectra recorded from sensor position A during a trial period of 24 hours. A flow sheet of the liquid urea feed process can be seen in Figure 9.7. Sensor A is mounted onto an orifice plate inserted in the main supply pipeline of liquid urea (see Figures 9.6 and 9.7). The reference values used to calibrate the model are the crystallization temperature (called the jc point ), as determined by the pilot plant laboratory (heat table visual nucleation/crystallization detection). 

The first application described was as temperature sensors by using a chiral nematic liquid crystal, which displays different colors at different temperatures. It is also worth noting that many common fluids are in fact liquid crystals. Soap, for instance, is a liquid crystal, and forms a variety of liquid crystal phases depending on its concentration in water. 

In 1888 the Austrian botanist and chemist Friedrich Reinitzer, interested in the chemical function of cholesterol in plants, noticed that the cholesterol derivative cholesteryl benzoate had two distinct melting points. At 145.5°C (293.9°F) the solid compound melted to form a turbid fluid, and this fluid stayed turbid until 178.5°C (353.3°F), at which temperature the turbidity disappeared and the liquid became clear. On cooling the liquid, he found that this sequence was reversed. He concluded that he had discovered a new state of matter occupying a niche between the crystalline solid and liquid states the liquid crystalline state. More than a century after Reinitzer s discovery, liquid crystals are an important class of advanced materials, being used for applications ranging from clock and calculator displays to temperature sensors. 

Today liquid crystals are used as pressure and temperature sensors and as the display element in such devices as digital watches and laptop computers. They can be used for these applications because the weak intermoiecuiar forces that hold the molecules together in the liquid crystalline phase are easily affected by changes in temperature, pressure, and electric fields. 

Liquid crystals have found an important place in modem life. Just look around we see them in our clocks, computer displays, TV screens, telephones and calculators, car dashboards, photo-cameras, etc. Other applications include slide projection systems, spatial light modulators, temperature sensors and even liquid crystal lasers. In all these technical innovations, which appeared over the life of only a single generation, liquid crystals occupy a key position. This is because they consume a barely perceptible amount of energy when they change their state under external influences such as temperature, electric field, mechanical stress or whatever. In addition, there are very important biological aspects of liquid crystals. 

The first commercial application (1940 s) of liquid crystals was the preparation of a light polarizer. The second commercial application was their use as temperature sensors The third major application of liquid crystals dealt with commercial displays. Other current applications include polymeric and graphitic fibers and light attenuators. 

For measurements 2-5 mg of the sample were weighed into small aluminum pans. In case of lyotropic liquid crystals, pans with a wall thickness of 0.25 pm were used, instead of the conventionally used pans with a wall thickness of 0.1 pm. This ensured that the pans do not blow up in the unfavorable event of an increasing pressure due to solvent evaporation. The sealed pan as well as a second empty pan which served as reference, were then placed into two separate microfurnaces. The furnaces were heated with a constant heat rate while the temperatures of sample and reference were measured with two independent thermo-sensors. If a phase transition occurs in the sample, a temperature difference arises between sample and reference due to the transformation enthalpy. This temperature difference was compensated by an increased heating power in the colder sample chamber. The difference in heating power was recorded as measurement signal versus time. As time and temperature are related to each other via the heat rate, the resulting thermogram was depicted as heat flow versus tempaature. 

---