Download crystalline solids report instructions introduction

Crystalline Solids
Revised 2/8/16
CRYSTALLINE SOLIDS
Adapted from Experiments by A.B. Ellis et al, ICE
REPORT INSTRUCTIONS
All in-lab work for this experiment must be recorded or attached to the ELN. Create a Pre/InLab
page in this week’s Experiment folder. Typical lab report sections are NOT required. Because
of limitations with the number of models, you will probably need to perform the parts of this
experiment out of the order indicated. Many pictures and screenshots should be attached in the
observations section. Students will work in pairs. Each pair will turn in a pictorial group report
that should consist mostly of annotated pictures. (Make a picture book showing what was
learned in lab.)
INTRODUCTION
Material science, which encompasses the traditional scientific disciplines of chemistry, physics,
and engineering, is the study of the synthesis, composition, and properties of solids. In this
experiment the composition and properties of one class of material – crystalline solids containing
cube-like building blocks called cubic unit cells. Read “Crystal Structures with Cubic Unit
Cells” before proceeding further.
Both physical and computational models will be used to
further understand the structure of solids in this experiment. Also, the properties of nitinol and a
superconductor will be investigated.
SAFETY PRECAUTIONS
Safety goggles and lab aprons must be worn in lab at all times. Liquid nitrogen is extremely cold
(–321°F!). Contact with skin may result in severe frostbite. If any liquid nitrogen spills on
clothing, remove the clothing immediately, as the trapped liquid will cause severe frostbite to the
skin beneath the clothing. Do not touch any metal dipped into liquid nitrogen until it has
warmed to room temperature. Do not place liquid nitrogen in a closed container; it can rapidly
expand and explosively shatter a container that is not properly vented. Use plastic tweezers to
handle superconductors and rare earth magnets; the solids may be toxic. The solid state models
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contain small spheres and rods - if any are dropped on the floor, pick them up to prevent slips or
falls. Wash your hands before leaving lab. Report any spills, accidents, or injuries to your TA.
PROCEDURES
Before starting the experiment, the TA will ask you to do a quick demonstration or talk-through
one of the following:
1) How to pour liquid nitrogen.
2) Assemble the superconductor setup for this experiment (without the liquid nitrogen).
Make sure you watch the videos on the course website and read the documents to prepare. These
demonstrations will be done every week. Everyone will have presented at least one topic by the
end of the quarter. The demonstrations should be short (>1 min) and will be graded.
Part A: Models and Odyssey
Before coming to lab look through the following Odyssey modules: 99 – The Structure of
Elemental Solids, 102 – Crystal Cubic Lattices, 103 – Stacking Orders, 104 – Interstitial Holes,
105 – Ionic Solid Structures. You will need to take screenshots from these modules to match up
with the pictures you take of the marble and stick models in lab. These screen shots can be taken
during or after lab, but you must look at the Odyssey modules while looking at the marble and
stick models in lab so that your pictures will be comparable.
In lab, you will find the material to build one of four generic (all colorless) models with cubic
unit cells: simple (primitive) cubic, body centered cubic, cubic close packed, and face
centered cubic. Your TA will assign you to one of the models and provide you with the
instructions for building it. Make sure you compare your model with like models built by other
groups. You are responsible for photographing all 4 generic models, not just the one your group
built. And, five (colorless and colored) models of specific substances with cubic unit cells are
also available: CsCl, NaTl, NaCl, and CaTiO3 (perovskite & fcc). These models will only be
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identified by the letters A through E. It is up to you to correctly determine the identity of the
solid represented by the model.
Take pictures of models demonstrating the various properties of solids with cubic unit cells.
Annotate the pictures with lines, labels, and descriptions that demonstrate your knowledge of the
concepts listed below. Compliment these pictures with screenshots from the Odyssey modules
showing the same orientations and/or concepts. Annotate the screenshots in the same way. You
will be graded by how thoroughly and concisely you illustrate your knowledge of these concepts.
(Note: Concisely means you do in with the least amount of pictures, writing, etc. needed.)
•
lattice points
•
types and locations of holes
•
type of packing & packing order
•
coordination numbers
•
type of unit cell
•
empirical formula
•
differences between generic model
•
density of solid
•
number of atoms per unit cell
•
which models are different
•
appearance a single unit cell
representations of the same solid?
If a specific substance cannot be found in Odyssey, use a combination of related solids in
Odyssey (and/or figures found on the web) to compliment the pictures of the marble and stick
models. You must have a picture from a model in lab and a screenshot from Odyssey for every
model and you must annotate (put labels directly on the pictures and screenshots).
Part B: Actual Solids & Models
Nitinol (NiTi)
A nickel-titanium alloy called "nitinol" is a memory metal that can be twisted or bent without
causing crystal defects and returns to its original shape when heated. At high temperatures,
nitinol atoms arrange in the orderly crystalline austenite form that resists distortion and “pings”
when dropped. At low temperatures, nitinol arrange in the more disordered martensite form that
can easily be twisted and bent and will “thud” when dropped. The difference in the unit cells of
the two forms explains these observations: austenite has a regular body centered cubic structure
while martensite has a related, but distorted structure. During the process of cooling, the high
temperature austenite exists until the transition temperature (the point of solid-solid transition
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from austenite to martensite) is reached.
If the alloy is bent while in the lower temperature
martensite phase, gentle heating of the metal above the transition temperature (into the austenite
phase) restores the original shape. If a new permanent shape is desired, the metal can be
annealed (programmed) at very high temperatures (in a flame) to retain the "memory" of a new
shape.
Slightly altering the 1:1 ratio of Ni to Ti (i.e., Ni0.99Ti1.01) changes the transition
temperature. Therefore, at room temperature one sample of nitinol can be in the martensite form,
while another can be in the austenite form.
Create your own procedure in lab that varies the temperature of nitinol rods to use their acoustic
properties to find the transition temperature for the alloy. Also, using the flame of a candle,
anneal a new shape into a thinner nitinol wire and devise a procedure that shows that nitinol has
the memory of that shape. Inspect the models for the two forms of nitinol labeled 1 & 2. Which
is austenite and which is martensite? Follow the same procedures in Part A to demonstrate your
understanding of nitinol’s crystal structure. Explain your results with the rod and wire with
respect to the crystal structures.
The “1-2-3” Superconductor (YBa2Cu3O7)
The "1-2-3" refers to the yttrium, barium, copper atom ratio. In this experiment the interaction
between the cooled “1-2-3” superconductor and a samarium magnet will be observed.
Understanding this requires the knowledge of band theory. Band theory is an electronic model
for solids that explains conductivity by assuming a higher energy level exists above valence
electrons called a "conduction band". (Figure 1).
Figure 1. Band Theory for solids.
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The electrons in this band are not attached to (localized on) individual atoms, but are free to
move (delocalized) throughout the entire solid. Electrons can always cross the small band gap of
conductors (metals) with the input of ordinary thermal energy (room temp) and create a current.
Increasing the metal’s temperature excites atoms causing electron scattering in the conduction
band and increases resistance. Conversely, decreasing temperature in ordinary metals decreases
resistance. The resistance, however, never does go to zero because electrons are scattered by
defects in crystalline structure. However, at very low temperatures, some metals (or metal
oxides) undergo a solid-solid transition and the resistance drops to zero, allowing electrical
current to flow without hindrance. This remarkable phenomenon of superconductivity allows
current to flow indefinitely because of a unique pairing of electrons – Cooper’s pairs. The
combined momentum of Cooper pairs prevents electron scattering. Resistance drops to zero
below some critical phase transition temperature (Tc). Above Tc, the Cooper pairs dissociate,
superconductivity ceases, and the solid becomes normal conducting material.
The Tc for
YBa2Cu3O7 is 95 K.
When the magnet is brought near the YBa2Cu3O7 superconductor cooled in N2(l) (77 K), the
magnet’s magnetic field lines penetrate the superconductor inducing a current. This current
creates an opposing magnetic field in the superconductor repelling the magnet and leading to its
levitation. (Figure 2). The height at which the magnet is levitated reflects the tendency to
minimize the total energy of the system: The internal energy of the superconductor increases as
the magnet moves closer to the surface and the gravitational potential energy increases as the
magnet moves further away from the surface.
magnet
perpendicular current
induced in
superconductor
super
conductor
magnetic field induced
in superconductor
(perpendicular to current
& antiparallel to magnet's field
Figure 2. Induced current and magnetic field of the superconductor.
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Superconductor
1. Check out a magnet from your TA. (Loss of magnet will result in a 5 point deduction.)
Using plastic tweezers, place the larger superconductor pellet on a stack of pennies in the
center of a cutoff Styrofoam cup. The pellet should be level with the top edge of the cup.
(Scrape off loose material from the pellet with a spatula. If the pellet is broken, use the
largest piece, flat side up.)
Use plastic tweezers to place the smaller magnet on the
superconductor.
2. Carefully pour N2(l) into the cup, covering the pennies and the bottom of the pellet. Touch
the magnet gently with tweezers - it should spin. Take a video and picture. Allow the N2(l) to
evaporate so the pellet and magnet warm back to room temperature. (To avoid frostbite, do
not touch pellet or magnet until warmed.)
3. The unit cell of YBa2Cu3O7 is created by stacking 3 pervoskite unit cells. (Figure 3). Inspect
the two models for the superconductor labeled 3 & 4. Which model is face centered cubic?
Which is perovskite?
Follow the same procedures in Part A to demonstrate your
understanding of the superconductor’s crystal structure.
Compare these models to the
CaTiO3 models.
Cu2+/Cu3+
Ba2+
O2Y3+
Figure 3. The unit cell of YBa2Cu3O7.
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