Matter Realisations

Table of Contents

Brainstorming Document

This document is not an engineering document. It is a form of brainstorming. If you read this document and decide to try some ideas based upon it, that's fine. If you directly try my ideas, this document is prior art. To me, the information is an obvious application of the art of materials science and engineering, and inelligible for patent. That doesn't mean you can't make money on it. I would appreciate getting some recognition however.

I may have time available to work on this idea, either specific concepts or the general approach, should you be interested.

Introduction

In-floor heating is something that many home owners appreciate; it's nice to wake up and put your feet onto a warm floor on a winter morning. In-floor heating is also quite comfortable, as we have a large fraction of the viewable surface in a room at an elevated temperature, and hence receive radiant heat from it. We are considering the hydronic (water based) systems here, not electrical.

Current Practice

The water pipes through which the warm water circulates, either go above the floor structure or underneath it. If the pipes are above, they are commonly embedded in a lightweight concrete, although there are composite wood systems in use (and possibly other things). If the pipes are underneath the floor structure, insulation needs to be used to minimise how much of the heat can possibly move downwards. With under floor structure placement, the heat must conduct through the floor structure.

Concrete is a pretty good conductor of heat, typically about 5 times better than the better woods. Concrete conducts heat well in all directions, consequently you should insulate where the floor contacts outer walls, or add insulation at the outer walls where the floor contacts the walls.

A higher temperature is required for wood floors to drive a comparable amount of heat to the top surface of a wood floor of the same thickness. Wood and polymers are typically more chemically active at the temperatures concerned, consequently degradation of the wood or the heating system due to reactions assisted by the wood can be of concern. Any degradation is likely to be of long term concern, not short term.

Improving Heat Transfer

The plywood and oriented strand board (OSB) currently used in most floor systems is already a composite. Altering the composite system for heat transfer is possible.

Resin Thermal Improvement

In electronics, one often sees the use of thermal greases or thermal adhesives to help improve heat transfer. One common way to make a grease or adhesive thermally conductive, is to add something to it that is good at conducting heat. If you add enough, you can increase thermal conductivity significantly. Hopefully other properties haven't changed enough to make this new material unusable. At low concentrations, additives typically make "linear" contributions, but there can often be some critical loading at which this extra phase develops a continuous path through the material, and properties change drastically.

It might be possible to do something similar with wood, add a thermal component to the adhesives. This thermal additive would likely become part of the adhesive part of the system. Unless something was done to the interior plys to improve heat transfer across the ply, the resulting thermal plywood would not see much improvement across the thickness, but could see considerable improvement in the plane. Which is the opposite effect as desired. The resin phase of OSB also tends to run in the plane of the sheet, and would be expected to change in a similar way.

It might be possible to put slices in the plys (or strand chips) that largely serve to separate fibres for a short distance, to allow the resin to penetrate and bridge the ply or strand. This would probably increase conductivity in all directions, resulting in similar behaviour to concrete. Either type of modification on plywood or OSB would probably also require a large lead time, since it might require recertification of this thermal plywood (OSB, ...) for service.

Third Phase Thermal Bridges

Another idea would be to place needles/fibers of something else across the thickness of the wood. Processes borrowed from textiles might work to do this. At or near room temperature, diamond is the best conductor of heat. Some forms of graphite nearly approach diamond in thermal conductivity, and are likely to be more economical. :-)

Tabulated values for the thermal conductivity of "graphite" exhibit high variance. The chemistry and microstructure both play a strong part in the thermal conductivity of a graphite. Some of these numbers would scarcely give anyone reason to try anything like this, and some of the numbers have graphite being just a little poorer than diamond.

Pure, monocrystalline diamond is about 20,000 times better at conducting heat than the better woods. If we wanted to double the bulk heat transfer through a wood sheet, we would want to replace 1/20,000 of the cross sectional area of the wood with diamond. From an acoustic consideration, picking fibres that are at most 1/32 inch in diameter, we could double the heat transfer across a 4x8 sheet of plywood by employing about 0.23 square inches of diamond. If each diamond fiber had a diameter of 1/32 inches, we would need about 300 fibers in a 4x8 sheet of plywood to double its thermal conductivity.

If instead of diamond, we found a graphite that was only 2000 times better than wood (or 10% of that of diamond), we would need about 10 times that many fibers. I don't think 3000 fibres per sheet of plywood is a technological stretch.

It might be a bit much to run a needle through a hard sheet of 3/4 inch plywood, but it might be possible at high temperature when the plys were soft and the adhesive not finished curing. This doesn't matter though. The huge difference in thermal properties at least makes the proposal interesting. Finding a way to position that small number of small fibers is probably more of a development problem.

The upside is that we end up with wood subfloor sheathing that doesn't require as hot a circulating water temperature as is now seen. This should result in fewer problems related to aging, oxidation and degradation. Since the heat transfer engineering is directional, we don't need to add extra insulation along the house perimeter to insulate the edges of the wood.

Possible Downsides

Are there downsides to doing this? Graphite is not only a heat conductor, it is also an electrical conductor. That these small conductors are placed far apart makes it unlikely that this would cause a problem.

This improved heat transfer is not what you are looking for in the case of fire. But with the fibers being so small, it's possible that something could accidentally or purposely shroud these fibers from the fire, and hence make this thermal plywood behave more like ordinary plywood. If the fibers were to preferentially oxidise, they might let smoke/pollutants through. The small cross sectional area suggests this is unlikely, but probably needs testing. Is graphite preferentially attacked by fungus, insects or rodents?

Acoustic Considerations

In the sound business, contractors will apparently try to track down all perforations in the envelope larger than 1/32 inch. This size is considerably smaller than the wavelength of a 20 kHz tone in air. This size presumably comes from some kind of testing, I haven't seen any scientific justification for it. The graphite fibers aren't going to act like hollow pipes, but more like nails, which are about as bad.

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