Revision as of 09:15, 25 January 2014

Diamondoid molecular elements (DMEs) are structural elements or machine elements at the lower physical size limit. They are produced via mechanosynthesis and are often are highly symetrical. Since metals are unsuitable (they lack directed bonds and tend to diffuse) diamondoid materials must be used.

• DM machine elements (DMMEs) (examples) like bearigs and gears have completely passivated surfaces.
• DM structural elements (DMSEs) (example) are minimally sized structural building blocks that are only partially passivated. They expose multiple radicals on some of their surfaces that act as AP welding interfaces to complementary surfaces. The step of connecting surface interfaces is done in assembly level II and is irreversible.

Name suggestion: since DMEs are somewhat of a cross between crystals and molecules why not call them "crystolecules"

• DME ... Diamodoid Molecular Element (stiff - small - minimal)
• DMME ... D.M. Machine Element
• DMSE ... D.M. Structural Element

Diamondoid molecular machine elements

Images of some examples.

Types

Bearings

DMME bearings exhibit superlubrication. In the case of diamondoid rotative bearings this looks like described here: E.Drexler's blog: Symmetric molecular bearings can exhibit low energy barriers that are insensitive to details of the potential energy function.

The occurring friction is orders of magnitude lower than the one occurring when liquid lubricants are used in macro ore microscopic (non AP) bearings E.Drexler's blog: Phonon drag in sleeve bearings can be orders of magnitude smaller than viscous drag in liquids.

DMME bearings can be built such that the force between bearing and axle is anti-compressive further lowering dynamic drag but also lowering stiffness possibly down to zero. E.Drexler's blog: Bearings can be stable despite attractive interactions between their surfaces

If badly chosen the combined symmetry of bearing and axle can create a bistable tristable or an other low symmetry configuration. This should usually be avoided. Some symmetry considerations can be found here: Zyvex; Ralph C. Merkle: A Proof About Molecular Bearings.

A tutorial on bearing design can be found here: A Low-Friction Molecular Bearing Assembly Tutorial, v1

Friction elements

Gears

Single rows of protruding atoms can be used as gear teeth. Considerations about stiffness as in superlubrication for DMME bearings are relevant [more details needed]. A simple pair of inter-meshing straight bevel-gears have higher bumpiness than well designed DMME bearings. This can be reduced by making the gears helical. Example designs are needed. Using more than one row for a gear tooth will lead to more "bumpiness" but also potentially higher transmittable torque. Further investigation needed

Fasteners

Details can be found on the locking mechanisms page.
Enclosed radicals could be used to make very compact reversible connectors (name suggestion: covaconns - for covalent connectors)

• expanding ridge joint

Others

[Todo: gears, pumps, telescoptc rods .... DME issues lack of ball curvature & DMSEs?]

Sets

Minimal set of compatible DMMEs

In electric circuits there is one topological and three kinds of basic passive elements.
Adding an active switching element one can create a great class of circuits.
0) fork node; 1) capacitors; 2) inductors; 3) resistors

Those passive elements have a direct correspondences in rotative or reciprocating mechanics namely:
0) planetary or differential gearbox [*]; 1) springs; 2) inertial masses; 3) friction elements
[*] and analogons for reciprocating mechanics

But there are limits to the electric-mechanic analogy. Some mechanic elements often differ significantly from their electric counterparts in their qualitative behavior. Two examples of elements quite different in behaviour are:

• transistors & locking pins
• transformers & gearboxes

With creating a set of standard sizes of those elements and a modular building block system to put them together creating rather complex systems can be done in a much shorter time.
Like in electronics one can first create a schematics and subsequently the board.

To do: Create a minimal set of minimal sized DMMEs for rotative nanomechanics. Modular housing structures standard bearings and standard axle redirectioning are also needed.

To investigate: how can reciprocating mechanics be implemented considereng the passivation bending issue

Diamondoid molecular structural elements

An example of a diamondoid molecular structural element (DMSE). The bright red spots are open bonds.

sets

• standardized building block systems
• housing structures
• standard corner pieces connecting the various crystallographic planes
• in edge passivation with hydrogen can be problematic
• issue of non androgynous sinterfaces
• brackets for sub bond length positioning [[1]]

General properties of DMEs

DMEs with carbon, silicon carbide or silicon as core material can be can have internal structure like

• diamond / lonsdaleite
• or other possibly strained sp³ configurations.

Due to the lack of defects the ultimate tensile strength of larger DMEs lies above diamond of thermodynamic origin.

Forces from compressive and tensile stresses

It can be helpful or at least satisfying to get something of an intuitive understanding for the consistence or "feel" of DME components.

As the size of a rod of any material shrinks linearly (in all three dimensions) the area of the cross section shrinks quadratically. Consequently when keeping tension/compression stress constant the forces fall quadratically and one arrives at very low forces. [Sacling law: longitudinal force ~ length^2] This can be seen nicely in the low seeming inter-atomic spring constants. E.g. the equilibrium position spring constant of an bond in diamond (sp3 carbon-carbon-bond) is about 440nN/nm or 0.44daN/cm (1daN~1kg).

In order to get a feel for these forces one can transform atomic spring constants unchanged to the macrocosm. This can be done by letting the number of parallel and serial bonds grow equally so that the changement of stiffness through serial and parallel connection of bonds compensate. Here for convenience 10,000,000,000 bonds are assumed to be chained serially. We must apply this scaling to the number of parallel bonds too but here it divides up in each dimension of the cross-section sqrt(10^10) = 100,000. With the diamond bond (C-C sp3) length of 1.532Amstrong and area per bond of 6.701Amstrong^2 = (2.59Amstrong)^2 one gets a diamond string (with square cross-section) of 1.532m length and 25.9um thickness side to side (half a hair) that retains the atomic spring constant of 440N/m or 0.44daN/cm (1daN~1kg) If you bind up a half liter bottle of water with that (somewhat dangerous knife like) string it will bend around 1cm.

Putting one end of the sting in a vacuum filled square piston that seals tightly shows how little effect everyday pressures have at the micro and nanocosmos. Taking 1bar = 10^5N/m^2 ambient pressure the string experiences a force of only 67.1µN and elongates 0.152µm an invisible amount.

Though as seen bonds are rather compliable DMEs are still hard diamond since hardness is closely related to tensile and compressive stress which is scale invariant. The small force representation of high pressures might be a bit counterintuitive and hard to grasp.

By making the compliance at the nanolevel experiencable the model with the weight on the one bond equivalent diamond string should make one (maybe obvious) practical thing clear. That it is very effective to focus forces.

In mechanosynthesis conical tips can easily focus forces down to a more compliable size level. Not much of a size difference is needed. Nanoscale manipulators in the machine phase can hold back on their supporting structures they're mounted to. It is easy to create DMEs with high internal strains such as strained shell cylindrical structures, press fittings, structures under high tensile stress and more. Great amounts of elastic energy can be stored (permanently or temporarily).

Example of safely usable pressures from Nanosystems section 2.3.2.: Assuming ~1% strain the required stress is ~1% of diamonds young modulus. 10nN/nm^2 = 10GPa = 1000daN/mm^2 (1daN~1kg) this is 20% of the tensile strength of macro-scale diamond with natural flaws. Flawless mechanosynthetically assembled diamond will be capable of handling more stress.

[Todo: add info about shearing stress]

Surfaces

When viewing the thickness of a surface as the distance from the point of maximally attractive VdW force to the point of equally repulsive VdW force (experienced by some probing tip) the thickness of the surface relative to the thickness of the diamondoid part is enormous. This makes DMEs somewhat soft in compressibility but not all that much as can be guessed by the compressibility of single crystalline graphite which is a stack of graphene sheets.

[Todo: add further relevant scaling laws & example calculation]

VdW sticking

[Todo: add calculation of how much surface is needed to securely overcome the characteristic thermal energy -- to locking mechanisms?? -- techlevel I related too ...]

Acceleration tolerance

[Todo: add calculation of a block on a neck model]

External links

• Design of Nanomachines using NanoEngineer-1