Presentation 1

Assignment On Liquid Crystal Thermography

Faculty of Engineering Jai Narain Vyas University Jodhpur

Submitted by- Submitted to-Raees Ahmed Quadri Prof(Dr.) S K SinghM.E. I sem. Mech.Engg (Thermal Engg.) Department

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Overview An Introduction of LCT & History A Brief about Liquid Crystals & its Properties What is Infrared Thermography & Thermogram? Comparison Of LCT and IR System Classification of Liquid Crystalss Manufacturing Processes of Liquid CrystalsI. Encapsulation Process or EncapsulateaII. Unencapsulation Process or Unencapsulatea Why Measure Temperature in Electronics Systems? Liquid Crystal Thermography— an In-depth Looka. Colour Descriptionb. Calibration of Liquid Crystalc. How to measure the surface temperature using LCT?d. Lighting & viewing e. General application

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An Introduction of LCT & History

Traditional techniques using temperature sensors can only measure the surface heat transfer at one location. If more detailed flow field information is required, a large number of sensors need to be placed and this may be limited due to constraint in space. Similarly, the heat flux measurement of an individual will be an average over its surface area. Therefore, there is a need to have a technique that can provide data over the whole region of the test surface and liquid crystal thermography

(LCT) is capable of serving this purpose . The LCT is extensively used in recent years by many investigators

like Leiner (1996), Schulz (1996), Ekkad and Pamula (2000), Ekkad and Han, Babinsky and Edwards, Wolferdorf, Wang, Gall and Holmes,Smith in 1997, 1996, 1993, 1996, 1986 and 1990 respectively for fluid mechanics related studies.

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An Introduction of LCT & History — LCT correlates the color response of a heat surface treated

with Thermo chromic Liquid Crystal (TLC) to temperature.What are Thermo chromic Liquid Crystals?Thermo chromic Liquid Crystals (TLC) are materials that

change their reflected color as a function of temperature when illuminated by white light. Hence, reflect visible light at a different wavelengths.

Thermo- TemperatureThermo chromic Chromic- Color

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An Introduction of LCT & History

Simplest Implementation— household temperature indicator process: A heated surface A liquid crystal with a known color-to-temperature response Example Fish-tank thermometers, Mood rings, Color sensitive coffee cup, etc.! Advance Implementation—Research quality thermograph A heated surface Calibration facility for temperature/color response Image acquisition and software for image analysis Example thermVIEW is a high resolution thermography system.

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Liquid Crystal Thermography

T(C) Red Green Blue Hue Saturation Intensity

39.9 107.881 115.662 93.13 81.92725 30.02203 105.5577

40.2 97.013 194.411 73.189 137.7078 101.4411 121.5377

40.3 82.043 195.849 82.042 138.7264 80.62878 119.9767

40.4 77.524 183.403 103.029 129.9105 92.05212 121.3187

40.5 76.838 167.748 126.139 118.8215 96.44293 123.575

40.6 77.547 152.984 149.791 108.3637 99.01782 126.774

40.7 78.146 143.139 161.472 101.3901 98.81294 127.585

40.8 79.067 132.841 175.107 94.09571 98.71083 129.005

40.9 80.288 119.712 188.993 84.796 97.10431 129.664

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Comparison of LCT and IR Systems

Sophistication level of measurement

Infrared (IR)

Liquid Crystal (LC)

IR vs. LC n= (IR/LC)

System– Cursory Full system Most desirable for spot, can be used for full system(not practical)

n= 3, can be used for temperature mapping. Useful for hotspot information, unless surface emissivity is given.

Board -- Evaluation Full board, must know emissivity, thus treat surface with an agent (black powder or paint)

Full board, must paint the board with black ink and liquid crystal

n= 1.5, can be used for temperature mapping. Useful for hotspot information, unless surface emissivity is given.

Component evaluation and Analysis

Full component, must know emissivity, thus treat surface with an agent (black powder or paint)

Full component, must paint the board with black ink and liquid crystal

n=1, both the system capable of this measurement. However LC can provide a more accurate number, but IR time to measurement is shorter.

Chip/Part Research quality Can be used for chip/part measurement. The constraints are on emissivity, spatial resolution (max 5 micron) and temperature averaging in the field of flow

Ideal for die and part measurements. Must treat the surface with paint & LC. Capable of measuring down to 1 microns.

n=0.1, this is sophisticated measurement, IR tends to be inaccurate & costly in this domain.LC provide advantage, while yielding more accurate results.

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A brief about liquid crystals & properties

Liquid crystals (LCs) are a state of matter that have properties between those of a conventional liquid and those of a solid crystal For instance, an LC may flow like a liquid, but its molecules may be oriented in a crystal-like way. There are many different types of LC phases, which can be distinguished by their different optical properties (such as birefringence). When viewed under a microscope using a polarized light source, different liquid crystal phases will appear to have distinct textures. The contrasting areas in the textures correspond to domains where the LC molecules are oriented in different directions. Within a domain, however, the molecules are well ordered. LC materials may not always be in an LC phase (just as water may turn into ice or steam).

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Liquid crystal is a unique substance, which exists between the solid and the isotropic liquid phase of some organic compounds.

It scatters incident light very selectively. Its optical characteristics involves the behavior of the molecular

structure. Each liquid crystal compound possesses a helical structure with a

characteristic pitch length. The helix pitch lengths are in the range of the wavelength of visible

light. The pitch length is altered by changing external stimulus(physical

change in structure), temperature(a main factor) and shear stress. Change in physical structure of an LC does not affect the fundamental

chemical structure, a liquid crystal coating can respond repeatedly to the same physical changes.

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A brief about liquid crystals & properties Liquid crystals are broadly classified in 4 categories…1. Thermotropic 2. Lyotropic 3. Metallotropic 4. BiologicalOur discussion is mainly on thermotropic liquid crystals, which can be further classified as: 2. Chiral nematic 3. Cholesteric There are two families of thermotropic liquid crystal materials, which have the helical

structure. These are: -I. Chiral nematic Phase: - The chiral nematic phase exhibits chirality(handedness). This phase is often called

the cholesteric phase because it was first observed for cholestrol derivatives. This phase exhibits a twisting of the molecules perpendicular to the director, with the

molecular axis parallel to the director. The chiral pitch, p, refers to the distance over which the LC molecules undergo a full

360° twist (but note that the structure of the chiral nematic phase repeats itself every half-pitch, since in this phase directors at 0° and ±180° are equivalent).

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In 1888, Austrian botanical physiologist Friedrich Reinitzer, working at the Charles University in Prague, examined the physico-chemical properties of various derivatives of cholesterol, which are now known as cholesteric liquid crystals.

He found that cholesteryl benzoate does not melt in the same manner as other compounds, but has two melting points. At 145.5 °C (293.9 °F) it melts into a cloudy liquid, and at178.5 °C (353.3 °F) it melts again and the cloudy liquid becomes clear. The phenomenon is reversible.

Reinitzer had discovered and described three important features of cholesteric liquid crystals,

1. the existence of two melting points

2. the reflection of circularly polarized light, and

3. the ability to rotate the polarized direction of light.

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Properties of liquid crystal is described by two phase temperatures comprising event temperature and clearing point temperature.

the lowest temperature where liquid crystal scatter visible light is called the event temperature.

At a temperature below the LC’s event temperature, LCs will be in the solid state and will appear transparent.

At a temperature above the clearing point temperature, it will enter the pure liquid state and will revert back to being transparent.

Liquid crystal phase transition-

LC event temperature

LC clearing temperature

Liquid stateSolid state

An arrow indicates

below event temperature

An Arrow indicates

above clearing

temperature

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Liquid Crystals…..

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Liquid Crystals….

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What is IR thermography & thermogram?

Infrared thermography, thermal imaging, and thermal video are examples of infrarred imaging science. Thermal imaging cameras detect radiation in the infrared range of the electromagnetic spectrum (roughly 9000–14,000 nanometers or 9–14 µm) and produce images of that radiation, called thermograms. Since infrared radiation is emitted by all objects above absolute zero according to the black body radiation law, thermography makes it possible to see one's environment with or without visible illumination. The amount of radiation emitted by an object increases with temperature; therefore, thermography allows one to see variations in temperature. When viewed through a thermal imaging camera, warm objects stand out well against cooler backgrounds; humans and other warm blooded animals become easily visible against the environment, day or night. As a result, thermography is particularly useful to military and other users of surveilence cameras.

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Infrared Thermogram of a Building & a Cat

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Infrared Thermogram & Thermal Imaging Camera..

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Clinical Thermogram

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Manufacturing process of LCs1. Encapsulation Process- the liquid crystal material is encapsulated in a 5-10

micron sphere suspended in a water based binder material-- provides excellent protection.

2. Unencapsulation Process- the material is in its native form-- susceptible to contamination, however, once applied, produces brilliant colors.

The thermal performance of LCs degrades rapidly due to chemical contamination and exposure to ultra-violet radiation. Therefore a manufacturing process known as micro-encapsulation or we can say encapsulation is used to protect the raw LC material and to make them more usable. The micro-encapsulation process offers chemical contaminant resistance and radiation protection.

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Why Measure Temperaturein Electronic Systems?

Temperature and reliability are synonymous—– Cooler devices imply better electrical operation and a longer

expectedlife.

Electrical Operation —– Semiconductor device operation experiences degradation at higher

temperatures. Reliability —– Hard failures (fuse like breakage) occur at high temperature– Material migration at the chip and component levels occurs at higher

temperature which can cause shorts and substrate cracking.– Activation energy (associated with the rate of failure) is exponentially

dependent on temperature.

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Liquid Crystal Thermography— an In-depth Look

a. Color Description- A color/temperature designator describes the response of a typical LC chemical make up. This helps in selecting the LC composition for a particular application. For example, “R35C5W” designates a formulation which signifies that the red start or event temperature of the LC is 35º C and the blue start temperature is 5º C above the red start temperature

This provides a crude estimate of the bandwidth of LCs. The LCs can be either narrow band or wide band formulations. The narrow band formulations have bandwidths below 1 or 2º C while

wide band formulations have band widths between 5º C and 30º C. Human eye decompose color into a combination of red, green and

blue (RGB) called the primary colors.

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Liquid Crystal Thermography— an In-depth Look

Many modern machine vision systems have implemented this natural tristimulus decomposition as an attempt to emulate the human vision system.

These system store the appropriate red, green and blue value needed to produce correspondingly matched color response at each point in an image.

This RGB color measurement model alone is difficult to use because it requires three values to interpret the temperature at each point. Therefore it is useful to have a quantity that will have a single value corresponding to a particular temperature.

It has been found that useful quantity for the above purpose is the hue defined in the standard HIS (hue, intensity & saturation) color system.

Hue value calculated for one color corresponding to particular temperature remains constant over large change in intensity or illumination. Most imaging software have a standard RGB to HIS transformation available, which is used for calibration of LCs.

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b. Calibration of Liquid crystal Calibration-

• Apply the TLC to a clean surface.• Subject the treated surface to known temperature levels.• Measure and record the color response of the TLC.Requirement-

• Uniform light source on the specimen• Isothermal calibration surface

Note:

This step is analogous to calibrating the voltage-temperature response of a thermocouple. In successive isothermal method, a temperature controlled test specimen and a color measurement system is used to generate LC color-to-temperature calibration data. Color image of the test specimen/surface is taken after bringing the test specimen and

LC to its event temperature. Then an average color value is computed and stored with the temperature of the test

specimen.

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b. Calibration of Liquid crystal

This process is repeated at subsequently higher temperature until the clearing point temperature is reached.

Uniform light source

Isothermal calibration surface

fig: Liquid crystal thermography set up

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b. Calibration of Liquid crystal Specimen preparation-

• To ensure good measurement, the goal is to have a smooth and contaminant free calibration and the test specimen surfaces.• Results are brilliant colors and accurate measurement. Preparation Process-

• Clean calibration and the test specimen surfaces (if possible)with alcohol and ensure that surfaces are dry.• Apply a “thin and uniform” coat of black paint to the test specimen and the calibration surface (place them side by side).• Dry the surfaces with a hot air gun at a mild temperature.• Spray or apply the desired TLC material to both surfaces simultaneously.

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d. Lighting and Viewing

Lighting and Light Source-

• A bright and stable white light source is required to obtain accurate and reliable reflected light intensity from a TLC coated surface.• The light source must be void of infrared (IR) and ultra-violet (UV) radiation.• Any IR energy present in the incident light will cause radiant heating of the test surface.• Extended exposure to UV radiation can cause rapid deterioration of the TLC

surface. this causes the surface to produce unreliable color-temperature response performance.

• Consistent light source settings and lighting-viewing arrangements between calibration and actual testing are essential to minimize color-temperature interpretation errors.

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High spatial resolution ThermoView system

Figure: ThermoView system

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System Features -

ThermoView is designed to be an accurate and easy to use temperaturemeasurement system for scientific and engineering applications.Some of the system’s features include:• Transient and steady state temperature measurement capabilities• Can be used for part (transistor) to board (PCB) level measurements• Spatial resolution to 1 Micron• Temperature accuracy to +/- 0.1ºC• A completely optical system based on visible light-- independent of surface

emissivity• Fast response liquid crystal for temperature measurement and data processing• Uses thermCAL (calibration system) for precision color-temperature calibration

of TLC materials

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●Includes a precision 2D traversing test table for accurate positioning of the test specimen• Fiber optic lighting for high intensity uniform illumination of the test specimen• Polarized optics to enhance image viewing and measurement accuracy• NTSC and PAL based video inputs• Micro and macroscopic optics• A state-of-the-art windows based user-friendly thermSOFT software for data acquisition and image processing.thermSOFT contains state of the art tools for image manipulation and data acquisition.

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d. Lighting and Viewing

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c. Temperature measurement using LCT

Why Use LCT as a Temperature Measurement Tool? Advantages-

• Flexibility of use in virtually any temperature measurement application frommicron sized electronic circuits to large scale gas turbine blades.• Ultra high (<1 micron) spatial resolution and non-destructive application for the device under test.• Ability to easily use common color video cameras and recorders as input devices to the system.• Customized and cost effective solution for many demanding applications.

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c. Temperature measurement using LCT

Two or three multiples less expensive than IR systems that offer poorer spatial resolution.

• Enables live tests by allowing to see temperature distribution through clear plastics (Lexan).

Deficiencies -• Not a quick tool for measurement since the test specimen needs to be treated by LC; unless one uses plastic films treated with LCs• Not a suitable tool for very large surfaces, i.e., system level tool.

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c. Temperature measurement using LCT How Does LCT work? The following steps are taken when measuring surface temperature with an

LCT systema. Select the optics suitable for the spatial resolution required.b. Select the appropriate liquid crystal and calibrate it.c. Coat the test specimen with black paint.d. Spray the test specimen with liquid crystal.e. Apply power to the test specimen and start the measurement.

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e. General Application of LCTLiquid crystals are currently used in many applications…1. Fluid mechanics and heat transfer studies2. Ergonomics studies3. Electronic component testing and medical (clinical) thermo logy. Fluid mechanics application includes flow and temperature field visualization, shear stress measurements and temperature measurements. Traditional flow visualization techniques used for high altitude visualization

(chemical and Oil flows) can be replaced by flow visualization using LCT. Liquid crystals have been used successfully at altitudes up to 15000 m. The state of the boundary layer present on an aircraft, dictates the heat transfer,

being low where the boundary layer is laminar and high when this becomes turbulent

The region of high heat transfer will be cool relative to those with low heat transfer region and this is reflected in the color display of the liquid crystals.

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e. General Application of LCT (continued…)

Medical thermo logy is a diagnosis technique which is prevalent from ancient greek civilization.

Physician would apply a thin mud slurry on to areas of their patients bodies to observe the pattern of drying and thereby determine the abnormal distribution of body heat. For example, if there is a damage in the nervous system i.e. in case of nerve injuries a temperature variations is observed in the area of the distribution of the injured nerve.

Similarly, temperature pattern over the skin does provides information regarding arthritis and breast cancer.

Liquid crystals can show the skin temperature field information and thus used as a tool for medical applications.

Liquid crystals can also be used to identify the thermal spots in high power density electronic components are used for electronic component performance studies.

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THANKS…..