However, it should be noted that nanotubes are not synthesized by rolling graphite sheet(s), tubes simply resemble rolled up graphite sheets. The following image illustrates the possibility of different forms of SWCNTs that can be related to rolling patterns of hexagonal networks of graphite sheets.
Figure 7.34
Different Forms of Single-wall Carbon Nanotube.
Figure 7.35
Single-wall Carbon Nanotubes.
Figure 7.36
This image is a nanometer carbon nanotube, filled with several cobalt nanoparticles.
It can be seen from the following tables that the carbon nanotube is lighter than aluminum but stronger than steel.
Material Elastic Modulus (GPa) Strain (%) Yield Strength (Gpa) Density
Single-wall carbon nanotube
Multi-wall carbon nanotubes
Steel
Aluminum
Titanium
Comparison of Stability, Electrical and Thermal Properties of CNTs with Other Material Used Currently Properties Nanotubes Current Materials
Size (diameter) MWCNT:
SWCNT:
Electron beam lithography can create lines wide, a few nm thick
Temperature stability Stable up to in vacuum, in air Metal wires in microchips melt at
Thermal conductivity Predicted to be as high as at room temperature Nearly pure diamond transmits heat at
Field emission Can activate phosphors at if electrodes are spaced micron apart Molybdenum tips require fields of and have very limited lifetimes
Current conductivity Estimated at Copper wires burn out at about
As a result of these extraordinary properties, CNTs promise ``a tiny revolution." Their unique and extreme properties allow them to be used in a variety of engineering disciplines:
Magnetic nanotube
Molecular quantum wires
Thermal protection
Hydrogen storage
Single electron transistors
Field effect transistors
Supercapacitors
Field emission flat panel displays
Solar storage
Electron microscope tips
Reinforcement of polymer
Electromagnetic shielding
Reinforcement of armor materials
Dialysis filters
Nanogear
Data storage
Nanotube actuator
Collision-protection materials
Nano tweezers
Controlled drug delivery/release
Nano balance
Conducting composites
Nano electronics
Batteries
Nano lithography
Artificial muscles
Nanotube reinforced composites
Synthesis of Carbon Nanotubes
Carbon nanotubes can be produced by several techniques, such as chemical vapor deposition, arc discharge, laser ablation, high-pressure carbon monoxide (HiPCO). Most of these processes take place in a vacuum or with process or carrier gases. In the CVD method, carbon-containing gas (such as acetylene, ethylene, ethanol, methane, etc.) and carrier gas (ammonia, nitrogen, or hydrogen) are heated at in the presence of metal catalyst particles (such as nickel, cobalt, iron, or a combination) in a reaction chamber. Nanotubes grow at the sites of the metal catalyst; the carbon-containing gas is broken apart at the surface of the catalyst particle, and the carbon is transported to the edges of the particle where it forms the nanotubes. Graphite is used as a precursor in the arc discharge and laser ablation methods.
Color and Nanotechnology
Chemical compounds are the origin of color in most objects (natural, synthetic, and food). For example, the chemical formula of methyl orange (dye) is dimethylaminoazobenzene-sulfonic acid sodium salt. This color changes to bright yellow as the (acidity indicator) changes.
Figure 7.37
Methyl orange
Figure 7.38
dimethylaminoazobenzenesulfonic Acid Sodium Salt
Some metals in bulk form also possess color. For example, gold is a yellowish orange color when its dimensions are more than . The color changes to green when the particle size is and to red/ruby at . Similarly, silver is yellow at , but blue at . These forms of tiny crystals of gold and silver are termed nanocrystals.
The stained glass windows in churches are good examples of gold and silver nanoparticles or nanocrystals. Medieval artisans unknowingly became nanotechnologists when they made red stained glass by mixing gold chloride into molten glass. That created tiny gold spheres, which absorbed and reflected sunlight in a way that produced a rich ruby color. While some of these stained glasses were made more than 1000 years ago, their color has maintained its brightness and saturation.
Figure 7.39
Stained glass window
Quantum Dots (QDs)
Quantum dots are basically nanocrystals and possess properties of a semiconductor with unique optical properties. Sizes of QDs range between ( atoms) and color changes with the size of QDs. Sometimes they are also referred to as artificial atoms.
Figure 7.40
Quantum Dots
QDs are normally semiconducting materials and the band gap (energy gaps between conduction and valence bands) can be tuned by size of the QDs. Besides colloidal gold and silver, other possible QDs are composed of periodic groups of II-VI, III-V, or IV-VI. Cadmium Selenide CdSe is a good example of solid QD.
Because of their unique electrical and optoelectronic properties, QDs can be used in several application areas such as in solar cells, displays, light emitting devices (LEDs) and life sciences. QDs will replace present organic dyes used as biosensors and biomedical imaging.
Further reading: http://www.evidenttech.com/quantum-dots-explained.html
Risks Factors?
Because materials at the nanoscale behave differently than they do in their bulk form, there is a concern that some nanoparticles could be toxic. Nanoparticles are so small that they could easily enter living cells and cross the blood-brain barrier, a membrane that protects the brain from harmful chemicals in the bloodstream. More powerful weapons, both lethal and non-lethal, may be created using nanotechnology. Because of their light weight, a small quantity of useful or harmful nanomaterials could easily be smuggled into the wrong hands.
References / Further Reading
Nanoproducts
The following Web sites are good sources of nanoproducts:
http://www.nanoshop.com/
http://www.nnin.org/nnin_nanoproducts.html
http://www.nanotechproject.org/inventories/consumer/browse/products/
Virginia Physics Standards of Learning
This chapter fulfills sections PH.4, PH.10, and PH.14 of the Virginia Physics Curriculum.
Chapter 8: Biophysics (Medical Imaging)
David Slykhuis. "Biophysics", 21st Century Physics Flexbook.
Ultrasound
Benefits of Ultrasonography
Non–invasive: the probe does not breach the tissue and reliable imaging can be recorded without surgery.
Inexpensive and routinely available in North America.
Provides a clearer picture of soft tissues that do not image well using ionizing radiation.
Provides flow rates for blood using Doppler technique.
Provides immediate images which can be used to guide other procedures or surgeries.
Can be repeated reasonably often.
Can be used on possibly pregnant women and on fetuses.
Ultrasound imaging provides a view of the human body that is not accessible by other means. While more energetic electromagnetic beams like rays penetrate the body including bones, ultrasound imaging or sonography uses sound waves to image soft tissues. Ultrasound imaging/sonography has been used to image fetuses in pregnant patients. This is especially important because the fetus is sensitive to many kinds of energetic probes which might otherwise be used. rays, for example, are known to cause changes in fetal DNA.
The ultras
ound image can be produced in real time so that the image can be used to guide surgical procedures. The equipment is routinely available in North America for a nominal cost. Advanced ultrasound machines can use the Doppler Effect to determine speed of blood flow in arteries and veins. By analyzing the blood velocity, physicians can locate aneurysms and blood clots.
Risks and Shortcomings of Ultrasonography
No known risks, but major medical organizations such as the World Health Organization have discouraged the popular practice of imaging fetuses to determine sex or to take “home movies.”
Very limited ability to image structures with bone or air. See the section on impedance.
Very long period (>30 minutes) of ultrasound associated with damage in small rodents.
Ultrasound has been generally recognized as a safe procedure when used for medically significant imaging. The recent popularity of making home movies of the developing fetus has been discouraged by major medical organizations. There is some danger of thermal heating of the tissue by long exposures at high power. A recent study found that prolonged exposures of over thirty minutes to developing rat fetuses produced some genetic damage. This is an area of ongoing research.
More Risks
One of the risks of any diagnostic device is that the energy beam is thermalized by the body; that is, the incoming energy heats the tissue. Because homeostasis (maintaining the same temperature) is the hallmark of mammals, changing the temperature of target tissue is a problem.
As a worst case scenario, assume that the probe delivers of energy to a cylinder of tissue deep and in diameter for 15 minutes. Also assume that there is no blood flow to the affected area so that the heat stays in the cylinder and that the technician does not move the probe for the entire 15 minutes.
The energy delivered to the tissue is .
The change in temperature of the tissue can be compued by assuming that the energy of the ultrasound is converted into heat:
where is the change in the internal energy associated with a temperature rise , is the mass of the tissue, is the heat capacity, and is the temperature. Substituting for the mass , where is the density and is the volume, yields the second equation. Solving for the change in temperature gives the third equation. The final temperature change is about , which is appreciable. Clearly, there is the possibility of changing the temperature of the target tissue under extreme circumstances.
How Can an Ultrasound "See"?
The Rayleigh criterion gives the resolution of waves, whether the wave is light, sound, or any other kind. It states that the wavelength must be at least as small as the object in order to "see" it. When the wavelength is larger than the object, then diffraction occurs. This “smears” out the beam so that an image cannot be formed. The same process applies to computer screens. If the image to be displayed is smaller than the pixels on the screen, then the image cannot be represented. The best results for the computer screen are when the images are much, much bigger than the size of the pixels that make up the computer display.
For medical imaging, the smaller the object to be imaged means the smaller the wavelength (and the higher the frequency) of the imaging beam.
Figure 8.1
Rayleigh criterion for different wavelengths
Rayleigh criterion:
The limit to imaging an object (if everything else is perfect) is diffraction.
This means that the wavelength of the incoming detector beam can be no smaller than the size of the object.
When the minimum of one peak just overlaps the maximum of the next peak, the two peaks are resolved.
If the peaks are closer, then they cannot be told apart.
Choosing the Best Frequency
The frequency of the ultrasound beam depends on two factors: the speed of sound in the tissue and the wavelength of the imaging beam. Note that the smaller the wavelength, the higher the frequency. And, of course, the wavelength is fixed by the size of the object to be examined.
The frequency of the required ultrasound depends on the wavelength and the speed of sound in the material:
where is the speed of sound in the substance and is the wavelength of the sound wave.
The speed of sound in a solid or a fluid depends on the density of the material, , and the stiffness of the material.
For fluids, this is the bulk modulus, , while for solids this is usually Young’s modulus, .
The speed of sound in human tissue varies by more than a factor of three. The speed of sound depends on the density of the tissue. While human bone is fairly dense, subcutaneous fat is much less dense than water. The overall density of a human is just about that of water. How do you know? Because the average human just barely floats in water. The more muscle and bone that a person has, the lower that person floats.
The other factor for determining the speed of sound is the stiffness of the material. Again bone is fairly stiff while subcutaneous fat is not. The stiffness is measured by the bulk modulus for fluids or Young’s modulus for solids.
Material Velocity
air
human soft tissue
human brain & amniotic fluid
liver
kidney
blood
muscle
skull-bone
fat
(Taken from http://www.yale.edu/ynhti/curriculum/units/1983/7/83.07.05.x.html)
Which references Christensen, E. E., Curry,T. S., Dowdey, J. E.: Introduction to the Physics of Diagnostic Radiology. Philadelphia: Lea & Febeger, 2nd Edition; 1978: Chapter 25.
Most soft tissues where the ultrasound is most effective have a speed of sound that is about , which is about five times faster than the speed of sound in air. Everyone is familiar with watching a distant event like a lightning strike where the light arrives almost immediately, but the sound of the thunderclap arrives some time later (about 5 seconds for every mile away).
Now Try This!
You can demonstrate the change in the speed of sound with the change in density by using dry coffee creamer (or hot chocolate mix) in a coffee cup. Put a couple of spoonfuls of the fatty creamer in the bottom of the cup and carefully add hot water so that the cream stays on the bottom. Strike the cup with your spoon so that the cup "rings." Now stir the creamer into the hot water while continuing to strike the side of the cup. As the density of the fluid changes, the notes will change.
Just a Bag of Water
The first thing to notice is that the speed of sound in choosing the best frequency is very close to that of water (why?) and is about five times faster than the speed of sound in air.
So in order to image an object that is cm in size, the frequency of the ultrasound probe that travels through muscle should be . In general, to image a smaller object, the frequency must be increased. Why?
The speed of sound in human tissue is about that of water, . If the size of the target is then
or about . As the size of the target decreases, the frequency increases. To image a target that is , the imaging beam must have a frequency of . The normal range of diagnostic ultrasound is .
Now try some problems.
What is the frequency required to image an object that is in diameter?
What is the frequency required to image an object that is in diameter?
A typical medical ultrasound is . What is the smallest object that can be imaged with this wave in human tissue?
Echolocation
The process of imaging is the same as the echo-locating sonar of a submarine or a bat. The observer sends out a brief pulse of ultrasound and waits for an echo. The pulse travels out, reflects off the target and returns. The ultrasound machine uses pulses because the same device acts as both transmitter and receiver. If it continually sent out sounds, then the receiver would not hear the much softer echo over the louder transmission. The duty cycle of the ultrasound imager is the amount of time spent transmitting compared to the total time of transmitting and listening.
Figure 8.2
&
nbsp; Reflected Wave
The pulse travels out and returns to the transducer where it is converted to electrical signal.
But the same device is both sender and receiver.
Duty cycle: emit pulse, wait, and listen.
Same procedure as SONAR.
Wait Time
In order to be as efficient as possible the machine should send out the next pulse just after the target pulse arrives.
To calculate the wait time:
Notice that there is a very short return time for the echo. Some bats do this naturally and even change the duty cycle as they close in on their prey. Essentially, as soon as the bat hears an echo, it sends out a new chirp for additional information.
The duty cycle is the amount of time that the probe is producing a pulse compared to the time that it is listening.
The size of the object that can be imaged with the transducer is a function of wavelength, therefore, the user should move to a transducer with the highest frequency and smallest wavelength.
But, sound waves that travel are subject to attenuation, i.e., gradual loss of intensity.
CK-12 21st Century Physics: A Compilation of Contemporary and Emerging Technologies Page 18
