Download 1 teachers` guidelines thin film waveguides and

NTSE - Nano Technology Science Education
Project No: 511787-LLP-1-2010-1-TR-KA3-KA3MP
TEACHERS’ GUIDELINES
THIN FILM WAVEGUIDES AND SOL-GEL PROCESSING
Contents
1. Foreword for Teachers
2. Introduction
3. Learning Process
4. Conclusion
5. Evaluation
The lesson plan is prepared to provide high school science teachers an introduction to sol-gel
from a different viewpoint, and products obtained from sol-gel which are used in modern
technology; waveguides. The plan is enhanced by both activities for classroom settings and
information provided on the web site (www.ntse-nanotech.eu ).
Target Group
Student groups aged 15-18
Duration
90 minutes
School Subject Areas
Physics – Chemistry
Lesson Goal
The “thin film waveguides and sol-gel processing” lecture explores the physical
phenomenon of reflection and waves. Students’ attention is drawn to the lesson by relevant
activities. It is aimed to provide an understanding of sol-gel, its products, including fiber
optics used in the internet communication.
Lesson Objectives
 To learn about properties of sol-gel and its products
 To describe the relation between internet and fiber optics
 To describe how sol-gel is used to fabricate thin films
Required Student Pre-knowledge
To know the following terms those exist in the reading text:
 Basics of nanotechnology
Material & Resources
 Reading text for the students
 Experiment videos
 Activity materials
1
Student Assessment
 Student worksheet
 Student quiz
Extension Ideas
1- Find out which country has the fastest
internet in the world*.
2- Assign students to write a paragraph about
“what determines the internet speed” by
searching the definition of bytes/second.
*According to Cisco, South Korea has the best
broadband service in the world, with an
average download throughput of 33.5
megabits per second –nearly three times the
speed of second-place Hong Kong– an
average upload throughput of 17 megabits
per second –more than twice that of Hong
Kong–,
and
has
100%
broadband
(1)
penetration.
FOREWORD FOR TEACHERS
Short Theoretical Background
An optical fiber is a flexible, transparent fiber made of high quality extruded glass (silica) or
plastic, slightly thicker than a human hair. It can function as a waveguide, or “light pipe”,
to transmit light between the two ends of the fiber.
Optical fibers are used in fiber-optic communications, permitting transmission over longer
distances and at higher bandwidths (data rates) than other forms. Fibers are used instead of
metal wires because signals travel along them with less loss and are also immune to
electromagnetic interference. Optical fibers include a transparent core surrounded by a
transparent cladding material with a lower index of refraction. The total internal reflection
keeps light in the core, and this causes the fiber to act as a waveguide.
Figure 1: A bundle of optical fibers. (2)
2
The index of refraction is a way of measuring the speed of light in a material. Light travels
fastest in a vacuum with the speed about 300,000 km/second. Index of refraction is calculated
by dividing the speed of light in a vacuum by the speed of light in some other medium.
When light traveling in an optically dense medium hits a boundary at a steep angle (larger
than the critical angle for the boundary), it is completely reflected. This is called total internal
reflection. This effect is used in optical fibers to keep light in the core. Light travels through
the fiber core, bouncing back and forth off the boundary between the core and cladding.
Because the light must strike the boundary with an angle greater than the critical angle, only
light that enters the fiber within a certain range of angles can travel down the fiber without
leaking out. This range of angles is called the acceptance cone of the fiber.(3)
Figure 2: A laser bouncing down an acrylic rod, illustrating the total internal reflaction of
light in a multi-mode optical fibre.(4)
The common name "sol-gel process" brings together a large group of methods for obtaining
materials from solutions, in which the gel formation is present at one of the process stages.
The most famous version of sol-gel process is based on the processes of controlled hydrolysis
of compounds, usually alkoxides M(OR)x (M = Si, Ti, Zr, V, Zn, Al, Sn, Ge, Mo, W, etc.) or
corresponding chlorides, in an aqueous or organic medium, usually alcohol.
At the first stage of sol-gel process the hydrolysis and polycondensation reactions lead to the
formation of a colloidal solution, i.e. sol, of hydroxide particles. Increasing bulk
concentration of the dispersed phase or other changes in external conditions (pH, solvent
substitution) leads to the intense formation of contacts between particles and the formation
of a monolithic gel, in which the solvent molecules are enclosed in a flexible, but fairly stable,
3D grid formed by particles of hydroxides. Concentration of sols followed by gelation is
carried out using dialysis, ultrafiltration, electrodialysis, evaporation at low temperatures, or
extraction.
A crucial role in the sol-gel process is played by the processes of solvent removal from the
gel (drying). Depending on the method, the synthesis can result in various products
(xerogels, ambigels, cryogels, aerogels). Common features of these products include the
preservation of the nanosizes of the structural elements and sufficiently high values of
specific surface area (hundreds of m2/g), although the bulk density can vary by hundreds of
3
times. Most products of sol-gel synthesis are used as precursors in obtaining oxide
nanopowders, thin films or ceramics.(5)
Sol-gel method also includes the polymer-gel process, in which the gel formation is achieved
by introducing a water-soluble polymer into the initial solution, followed by evaporation.
Freeze-drying or supercritical drying of polymer gels with subsequent heat treatment in an
inert atmosphere is used for the production of carbon cryogels and aerogels.(6)
Figure 3: Schema of sol-gel technologies and their products.(7)
INTRODUCTION
“To love beauty is to see light.”(8)
Victor Hugo
“There are two ways of spreading light: To be the candle or the mirror reflects it.”(8)
Edith Wharton
“Education is what remains after one has forgotten what one has learned in school.”(8)
Albert Einstein
Teacher starts a brainstorming session by asking the following questions (Together with
letting students to express their opinions/ideas, helping them to recall their previous
knowledge, and motivating them to realize/see the principles, concepts, and relations):
 How do we communicate via internet?
 What would you understand if I say: Entire electronic industry depends on thin films
today?
4
LEARNING PROCESS
Before the Lecture
Assignment:
 Providing students the text on “thin film waveguides and sol-gel processing” and
asking them to read it.
Before Class Preparations:
 Designing the room to watch the experiment videos.
 Preparing the materials needed in activities explained below.
In the Lecture
 Step-1: Asking the following questions about the text.
Question 1: What is intranet?
 Intranet is the generic term for a collection of private computer networks within an
organization. It uses network technologies as a tool to facilitate communication
between people or work groups to improve the data sharing capability and overall
knowledge base of an organization's employees.
Question 2: Where have you heard the name fiber-optic?
 In telecommunication practices.
Question 3: What are fiber optics?
 Fiber-optic lines are strands of optically pure glass as thin as human hair that carry
digital information over long distances.
Question 4: What is total internal reflection?
 Total internal reflection is a phenomenon that happens when a propagating wave
strikes a medium boundary at an angle larger than a particular critical angle with
respect to the normal to the surface. If the refractive index is lower on the other side
of the boundary and the incident angle is greater than the critical angle, the wave
cannot pass through and is entirely reflected.
Question 5: What is a sol?
 A sol is a colloidal suspension of very small solid particles in a continuous liquid
medium.
 Step -2: Starting the Activity-1 described in Students’ Guidelines.
 Step -3: Asking the following question about the activity.
5
Question 6: What did you observed in Activity 1? We all know that light travels in straight
lines – how did this happen?
 By the total internal reflection phenomenon. Light propagates by bouncing to the
boundaries of the optical fiber.
 Step -4: Starting the Activity-2 described in Students’ Guidelines.
 Step -5: Asking the following question about the activity.
Question 7: Why do you think we saw different colors from a clear nail polish?
 By the spread out of the polish into a thin film. The film reflects light differently
depending on its thickness.
 Step -6: Starting the Activity-3 described in Students’ Guidelines.
 Step -7: Starting the Activity-4 described in Students’ Guidelines.
CONCLUSION
Discussion of the following:
 What can be other materials that we can use to make fiber-optics
Any two transparent materials that have significant refractive difference with good
machineability and flexibility.
EVALUATION
A. Fill in the blanks with an appropriate expression. (5 minutes)
1- A …………….. is a stable dispersion of colloidal particles or polymers in a solvent.
2- …………….. is a chemical solution process used to make ceramic and glass materials in the
form of thin films, fibers , or powders.
3- A ……….. consists of a three dimensional continuous network, which encloses a liquid
phase.
4- An ………… is essentially a waveguide for light.
B. Select True (T) or False (F) for each of the following statements. (5 minutes)
( ) 1- There is a connection between the Internet and fiber optics.
( ) 2- Optical fibers have a core and cladding that surrounds the core.
( ) 3- Fiber Optics is a data-delivery system transmits light through glass fibers connects the
developed world delivers information 100.000 times faster than copper-wire technology.
( ) 4. The index of refraction of the cladding is higher than that of the core.
6
KEY
A. Fill in the blanks
1- sol
2- sol-gel
B. Write True (T) or False (F)
1- T
2- T
3- F
3- gel
4- optical fiber
4- F
REFERENCES
(1) http://www.guinnessworldrecords.com/world-records/10000/fastest-internet-connection(country)
http://www.dgtprojects.com/site/images/Guinness%20World%20Records.jpg
(Guinness World Record)
(2) http://en.wikipedia.org/wiki/File:Fibreoptic.jpg
(3) http://en.wikipedia.org/wiki/Optical_fiber
(4) http://en.wikipedia.org/wiki/File:Laser_in_fibre.jpg
(5) http://eng.thesaurus.rusnano.com/wiki/article847
(6) http://www.aerogel.org/?p=992
(7) http://smtp2.thewwwstore.com/chemat/images/Flowchat72.jpg
(8) http://www.brainyquote.com/quotes/keywords/light.html
7