MEEN Nanotechnology Issues in Manufacturing

Transcription

1 MEEN Nanotechnology Issues in Manufacturing Nanotechnology Concepts 28 October 2004 Dr. Creasy 1 In this lecture we review the types of nanoparticles, platelets, and fibers of interest. 1

2 Earth, Soccer, and C60 Earth d = 12.7 Mm Soccer ball d d = 0.22 m C60 d = 0.7 nm 10 7 /10-1 = /10-9 = TSCD We can check the scale of Buckminster Fullerene particles with this comparison: A regulation size soccer ball is 220 mm in diameter. The combination of hexagonal and pentagonal structures that form the soccer ball are identical to those that form a fullerene at the atomic scale. The diameter of the scoccer ball compared to the diameter of the earth is approximately the same as the diameter of C60 compared to the scoccer ball. 2

3 Early Report: Nanoparticle Hazards? (SMU, Dallas TX) Nanoparticle (fullerenes) effects on lipid peroxidation in the brains of fish Normalized Rate Clean Water Environment 0.5 PPM TSCD Althought fullerenes have not yet found an application. They are used to study the effect of loose nanoparticles on living creatures. One recent university study found that fullerenes caused a rapid increase in the rate of tissue degradation in fish when present at 0.5 parts per million. Studies like this one must be conducted in advance of large scale production of nanoconstituents. 3

4 What is the Young s Modulus of a Carbon Nanotube? 5 5 TPa? (5000 GPa) 1 1 TPa? (1000 GPa) 720 GPa at 9-12% 9 Strain E E = σ/ε = FL 0 /A L Pitch fiber 830 GPa; 0.5% strain IM7 fiber 303 GPa; 2% strain TSCD The previous discussion concerned nanoparticles, that is, components that have nanoscale features in all three dimensions. If we keep a nanometer size in two dimensions and let the third approach more familiar minron sizes, we would have something like the carbon nanotube. Researchers are looking at possible electrical, chemical, and structural applications of carbon nanotubes. In the case of structural applications, we have no clear determination of the modulus of a carbon nanotube! An early paper presented a calculation of the Young s modulus at 5 terapascal. However, a subsequent analysis showed that, since one must define a cross sectional area. the first calculation had used a tube thickness that did not account for the cloud of electrons around the nucleus of each carbon atom. That analysis dropped the Young; s modulus to 1 terapascal, which is still a tremendously stiff material. However, for those of us hoping to use nanotubes in composite materials, Pipes noted that the tube does not fill up with resin as shown the lower left image. Since the fiber is empty, we must use the entire cross section instead of the wall thickness as the area. This drops the tube stiffness to 720 GPa. Large, but in the same range as conventional carbon fibers. However, the strain to failure of the tubes are much larger so they might still be worth using. 4

5 Platelets: Nanoscale in 1 Dimension and Microscale in a Plane Montmorillonite Clays Intercalation. 1 1 nm thick. 1 1 x 1 µm plate. mposit/nano/struct2_1.htm TSCD If we now allow two of the dimensions of our particles to reach micron size, we have a platelet. Clay particles have found some success in increasing the stiffness of thermoplastics. There is much work to do in effectively separating and distributing the platelets through the polymer. These clays are much like the solid here in College Station. They swell and shrink by great amounts as a fluid in introduced and removed. Each platelet is a ceramic structure that is about 1 nm thick. These are ionically bonded with a free space, which is called a gallery, between platelets. The platelets are separated by introducing oligimers (short chain polymers) that are compatible with the clay at one end of the chain and compatible with the polymer along the chain and at the other chain end. 5

6 Exfoliation Maximizes the Effect TSCD This slide shows exfoliated platelets. The short chains are swollen, perhaps by treatment with solvents and the separated platelets can be blended with the matrix polymer. The sketch of the oligimer at the lower right shows an end group with a net positive charge. This can bond ionically with a negatively charged site on the clay. 6

7 Why Use Clay Platelets? Adding 5 w/0 clay to nylon-6 6 provides unique performance enhancement. 40% higher tensile strength 68% higher tensile modulus Heat distortion temperature increased from 65 (nylon-6) to 152 C TSCD And clay is cheap because it is dug from the ground. If you want it, you can buy two rail car loads of clay tomorrow. It could be a long time before two rail car loads of nanotubes are ever produced. 7

8 Switch to PDF File Problems in dispersion of nanoparticles TSCD The PDF file shows the problems in dispersing nanoscale components in a polymer melt. 8

9 Polymer Actuators Peter Sommer-Larsen Light, flexible, noiseless actuators with stroke, force and efficiency similar to - or better than - that of human muscles; such is the promise of polymer actuators TSCD 9

10 Why Polymers? Compared with Silicon Devices... Many cycles might be possible Flexible Tailored compliance = 1/stiffness Less friction (beware of sticky polymers) TSCD TSCD Also, polymers could have a softer touch than metal or ceramic devices. 10

11 A Thermally Actuated Polymer Micro Robotic Gripper for Manipulation of Biological Cells Ho-Yin Chan1 and Wen J. Li TSCD This polymer microactuator can grasp single cells without damaging them. 11

12 Chan & Li Use µfabµ Methods to Make Acutator Combine metal and polymer on a silicon surface. Bimaterial Strip: Use difference in thermal expansion to bend the strip. The polymer is parylene,, which is vapor deposited dimer TSCD 12

13 Polymer Flexure Adds Function without Joints TSCD 13

14 Capture of Zebrafish Embryo TSCD 14

15 Polymer Provides a Compliant Capture Surface TSCD This system is thermally actuated. It is like a bimetallic strip. The activation temperature must be compatible with the cells studied. As the size of the device grows smaller, we must use other means to activate them. Electric motors and solenoids lose their effectiveness. For example, a solenoid must have a good number of turns of conductor in order to generate a driving force. As the component shrinks it becomes more difficult to make a coiled conductor. 15

16 Why Dielectric Actuation? Many elastomers have good dielectric strength and could provide actuation. The issue is the compliant electrode layer TSCD However, small scales make electrostatic devices possible. Opposite charges on the top and bottom surfaces of a sheet of material have a large effect if the thickness of the sheet is small and the dielectric constant of the material is large enough to support the charge density neeeded for actuation. Also, recall that silicon based mems devices shown earlier in the semester are driven by oscillating static charges on comb drives. 16

17 Acrylic Elastomer TSCD This acrylic elastomer extends to 4 times its initial width when it is compressed by an electrostatic charge. 17

18 Maxwell Pressure for a Compliant Capacitor For large pressure we need large dielectric constant and large electric field strength in an elastomer TSCD The pressure generated is equal to the product of the materials dielectric constant, the permittivity of free space, and the square of the electric field. E, the electric field, is equal to the applied voltage V divided by the thickness of the polymer sheet, z. Note that z is reduced by the actuation, so E rises, but it must never exceed the breakdown limit of the polymer. 18

19 A Silicone Rubber Actuator Pelrine et al. Silicone Elastic Energy Density (J/cm 3 ) 0.22 Maxwell Pressure (MPa) 1.36 Strain (%) 32 Young's Modulus (MPa) 1 Electric Field Strength (V/µm) 235 Dielectric Constant (1 khz) TSCD The electric field strength shows that devices must be a few microns think if we want to avoid using thousands of volts for the system. The following finite element analysis demonstrates this scale effect. 19

20 A 10 mm Cube Apply ±1 V TSCD TSCD Apply a 2 v differential charge at the top and bottom surfaces of a 10mm cube of silicone will not provide any performance. 20

21 Increase Voltage to ±1.175MV TSCD The dielectric limit for a 10 mm cube would support an applied voltage of ±1,175,000 volts. 21

22 Nodal Forces Range from N in the corners to N in the center TSCD FEA shows that the forces generated at each node ranges from ot newtons. 22

23 3D Symmetry Boundary Conditions One plane each of X and Y symmetry. The bottom Z surface cannot move in Z TSCD We can look at the deformation of the cube when subjected to this great electric field. Symmetry conditions were set to keep the model stable in virtual space. 23

24 Performance Compression Stress: 0.67 MPa Movie--> 10mm cube.avi TSCD When the mesh was fully refined, the compression stress reach 670 kpa. The movie shows the deformation. 24

25 Performance 1 mm thick Movie Movie 1mm sheet.avi TSCD If we change the cube into a 10 mm by 10 mm by 1 mm thick sheet, we reduce the total voltage to ± 118 kv and get better performance. The thickness is reduced by 69% and the sheet extends by 45%. 25

26 Scaling Cell size Thickness Load Condition Displacement ZZ, XY Stress (MPa) 2.5 mm 10 mm E6 V mm (60%) mm (42%) 1 mm 10 mm E6 V mm (67%) 5.54 mm (55%) mm 10 mm E6 V mm (68%) 5.85 mm (58%) mm 1 mm ±117,500 V mm (69%) mm (45%) 25 µm 100 µm ±11,750 V µm (69%) mm (45%) 2.5 µm 10 µm ±1,175 V 0.25 µm 1 µm ±117.5 V TSCD This table shows the results of the analysis. The first column is the mesh size in the finite element model. The second column is the thickness of the elastomer. The third column shows the maximum potential that we can apply to the elastomer at each thickness. The fourth and fifth columns show the displacement in Z, X, and Y, and the generated stress. The last two rows are left as a homework assignment. 26

27 Gripper: Short Model ±1175 V Fixed bottom surface. 10 µm thick. Movie: Movie: Gripper1.avi TSCD We can start to design and analyze active devices that use this mechanism. If we want to make a gripper that uses this activation method, we can combine the electrostatic model with the elastic model and find the resulting behavior. 27

28 Gripping Action TSCD The movie shows the gripping action dynamically. However, further analysis is needed to find the effective gripping strength at the tips of the fingers. 28

29 Bending Cylinder 1 1 mm diameter 10 µm thick active layers TSCD We can progress to more complex elements. Consider a complex extrusion of a fiber that contains multiple electrostatic actuators. These are coextruded in meso to micro scale dies and then drawn down to smaller sizes if appropriate. This cylinder contains four regions of actuation within an elastomer. 29

30 Bending Cylinder 100 µm diameter 10 µm thick active layers TSCD TSCD These active regions may be powered in unison, in combination, or individually. 30

31 500 µm Long Bending Element Places enough material away from clamped BC to allow free motion. All elements deactivated so that a free element could be tested TSCD A short length of this active filament cannot overcome a gripped end condition and perform a function. However, at a length of 500 microns the cylinder can bend. Longer lengths would bend to a greater extemt. 31

32 Trial Activation TSCD This model shows the effect of firing a single actuator. 32

33 Single Actuator Active Displacement is mm. Movie: Movie: CylinderSingleActiveElement.avi TSCD The movie shows the displacement. 33

34 Two Actuators Improve Bending Vector sum of deflections increases magnitude of bending by 42% Displacement is mm Movie: Movie: CylinderTwoActiveElements.avi TSCD Two neighboring actuators generate more bending from the vector sum of the displacements. 34

35 Four Actuators Extend the Element Element gets 8% longer. Displacement is mm Cross section is more rectangular. Movie: Movie: CylinderFourActive.avi TSCD When all four actuators are charged the cylinder extends rather than bends. 35