Talk:Thermal conductivity

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Contents 1 Definitions 2 Example values for common elements OK? 3 lamda? 4 U-value and K-Value 5 Cleanup in aisle four 6 Source for values 7 aerogel TC? 8 Querying aerogel's record low thermal conductivity 9 Further on querying aerogel's record low thermal conductivity 10 Disambiguation 11 An inconsistency with the diamond article 12 Constants for wood 13 Beryllium oxide 14 Merge Thermal resistance to Thermal conductivity 15 Diamond and electrical semiconductors. 16 Cleanup-list Tag 17 Power lines? 18 List of thermal conductivities

[edit] Definitions

Please put less technical definitions within the first paragraph of all entries. I'm thinking 4th grade level would be appropriate, followed by more exact at 8th, 12th and then post-doc. In addition, a good example using house insulation and electrical schematic analogies would be appropriate.

In the section "First definition," the article states that "To physicists [1], thermal conductance is ..." and the footnote [1] points to the well-known and authoritative NIST Special Publication 811 which actually does not define or use the term "thermal conductance" anywhere. This link is not really good evidence or corroboration of the assertion that there is a standard definition of thermal conductance. (It is, however, a good place to find authoritative definitions of thermal conductivity, -insulance, -resistance, and -resistivity.) Perhaps it should be moved?

Other semi-authoritative references like the CRC Handbook of Chemistry and Physics, 2002-203 edition don't define thermal conductance either. (Although they do have conductivity.)

Does anyone have a more authoritative definition of thermal conductance as having the dimensions W / (m^2 K) (which can be more generally stated in terms of SI base units as: kg / (K s^3) ) Preferably from an authoritative standards body, and not some individual's webpage or questionable textbook.

I have a feeling that "thermal conductance" is a not-well-standardized term, if it's proper at all.

I also find the phrase "to physicists" very questionable, as if physicists don't or shouldn't follow the SI but rather have their own standards. (Last time I saw this in Wikipedia, it was a rather confused physicist arguing that "in physics," a pound was a measure of force, not mass.) This phrase should probably be removed.

By the way, I'm interested in the right terminology because I'm building authoritative references in my programming language/calculating tool Frink.

No, the NIST doesn't define thermal conductance, but as I recall it does define electrical resistance and conductance as reciprocals of each other. I felt I was on pretty safe ground assuming thermal conductance was the reciprocal of thermal resistance. Thats also how it works in "building trade" units.

I've since added a few more web references that look fairly reliable: [1] and for the future[2]. The first says that the terms I've used - including thermal conductance - are in ISO 31-4:1992, Quantities and units - Part 4: Heat. Another new reference ([3], thermophysics FAQ 5) is much better for "builders" units.

I've replaced "To physicists" with "For general scientific use". I would have put "in SI units" or something similar, but I think maybe only some of these units are defined as part of the SI, while others are derived. After all, "bulider's" thermal resistance is loosely "in SI" if it's expressed as (K m^2)/W. And for SI we should really refer to the BIPM rather than the NIST.

Andy G 18:25, 10 Sep 2003 (UTC)

[edit] Example values for common elements OK?

Updated table of examples with values and reference to CRC. Please do not change without citing source AND discussion on "Talk" page. The following values were omitted from table due to lack of reference data. Values below from previous table:

Fiberglass 0.04

Expanded polystyrene ("beadboard") 0.03

--Az7997 14:07, 2005, Sep 20

I've added Titanium, copied from http://www.efunda.com/materials/elements/TC_Table.cfm?Element_ID=Ti; many other elements are available from that source, just change the URL accordingly. Happily, that data agrees with wikipedia for other elements. (I added titanium because it is unusually low for a metal.) mdf 02:37, 14 January 2006 (UTC)

Is there a good reason that the list of examples has items out of order? If not, could they be sorted?

Also consider thermal conductivities of the elements (data page) from our chemical elements data references as a central resource. Femto 11:51, 14 January 2006 (UTC)

I've added two entries for different types of thermal grease using the data on that page. With reference to future edits/culls, I argue that these entries are of specific interest because they are designed to optimise this characteristic (and yet, it's so poor in comparison to the metals it's used with), and because when working with the other materials listed, thermal grease will often be a factor in the design. --ToobMug 01:51, 3 March 2006 (UTC)

Reformated table, cleaned up a lot of garbage, and clarified text regarding the table. I removed a lot of material from the list that was either: 1) not referenced, 2) not very insiteful (e.g. multitude of metals previously listed), 3) many items didn't even have values, but were listed in the table!

I propose that additions to this list meet the following criteria: 1) values must be in units of W/(m K) 2) source must be sited, in the same section/manner as existing ones 3) value and source must be significant compared to those already listed. #3 is the the big one. From the existing table, people will have a good idea of which link to follow if looking for k values for different types of materials. If you want to re-add your thermal grease data, please do so for ONE grease, and link to where this information can be obtained. The table is most definitely not supposed to be an extensive list of k values. That's what the CRC and other existing references are for, not wikipedia!

--AZ7997 07:35, 25 March 2006 (UTC)

As I said, using the data on that page. This time I've added a citation just for you. --ToobMug 02:00, 23 January 2007 (UTC)

The typical thermal conductivities table is a train-wreck, now. Given its title, it's farcically overloaded with qualifiers and, for some reason, even electrical conductivity. Reading values out of the table is burdensome, and in some cases even just identifying the material is a task. The most fascinating example would have to be granite, which lists its components but links to oxides of the elements actually named, suggesting that what it says is different from what was meant. And what good is the overblown title of the Electrical Conductivity column doing anybody?

Typical values are, by definition, not definitive and so all the specifics of the measurements aren't helpful. All they do is degrade readability of the table. And Wikipedia should be an adequate source of information for most materials listed. Does that really need to be listed as a reference?

Also, If anybody thinks that the table should be reduced again, then I'd like to point out that removing copper and aluminium (again) would be a big mistake. They're notable for their common use for their thermal properties and they set important reference points by which to understand the relative merits of other materials.--ToobMug 02:00, 23 January 2007 (UTC)

The list is not very visually pleasing. How about moving the main list to it's own article and then providing 5-10 common substances in this list? 10:(Aluminium, copper, ice, wood, concrete, snow, air, water, soil, granite) 5:(copper, wood, air, concrete, snow) --Tunheim 21:50, 8 February 2007 (UTC)

Alright, but I'm copying the cleanup tag forward, because even on its own page, the list maintains its train-wreck status. --ToobMug 13:40, 10 February 2007 (UTC)

[edit] lamda?

The books I've read on this subject use k or lowercase kappa instead.--Tenfour 23:00, 3 August 2005 (UTC)

Agreed, k or kappa is often the variable used. --Anonymous

I added "k" to the article. -- Kjkolb 22:51, 17 December 2005 (UTC)

[edit] U-value and K-Value

I've found some sources that claim these are equivalent and some say that K is used when it is a single type of material or a single layer and U is when multiple layers are combined. -- Kjkolb 22:51, 17 December 2005 (UTC)

U-value is often used with non-homogenous/anisotropic materials such as insulators while k-value often refers to homogenous/isotropic materials --Wavestorm

I did a small edit to your comment (replaced the initial space with a colon) since it was screwing up my screen. I hope id didn't loose me too much karma ;) --Tunheim 22:02, 8 February 2007 (UTC)

[edit] Cleanup in aisle four

The Measurement section desperately needs cleanup.

[edit] Source for values

I came across this information for the thermal conductivity of various material and the author cites the information came from a book by Hugh D. Young. The site is http://hyperphysics.phy-astr.gsu.edu/hbase/tables/thrcn.html

 LDCorey 05:50, 21 February 2006 (UTC)

[edit] aerogel TC?

This article says 0.003W/Km, aerogel article says 0.017W/Km. --njh 01:25, 29 March 2006 (UTC)

Aerogel article contained no citation for 0.017W/Km value. I have changed that article and added referece. Hopefully future changes will include citations and we can evaluate values based on the quality of the source.

[edit] Querying aerogel's record low thermal conductivity

As a follow-up to the preceding comment, could an expert please verify/comment on the extremely low claimed thermal conductivity: 0.003 W/m.K - best insulator in Guiness Book of World Records - for SEAgel. (The SEAgel Wikipedia page doesn't repeat this claim.) The explanation given for it being a good conductive insulator is: "Silica aerogel is a good conductive insulator because silica is a poor conductor of heat" Yet a Google search returns an unremarkable thermal conductivity for silica, as might be expected: http://www.azom.com/details.asp?ArticleID=1114 reports 1.3 (1.4) W/m.K for quartz (fused silica). Since the aerogel is mostly air, a reasonable expectation would be a heat conduction value similar to air, as I understand is the case with the best (i.e. closed cell) insulators based on air. This article states the conductivity of air to be 0.0262 W/m.K and the Engineering Toolbox, http://www.engineeringtoolbox.com/thermal-conductivity-d_429.html, gives it as 0.024 as well as giving a generic entry "Insulation materials 0.035 - 0.16". If I could hazard a guess, would it be possible that someone somewhere has inadvertantly stuck in an extra zero in the the SEAgel conductivity and this error has been propagated? (Compare with the iron content of spinach, which I understand used to be a factor of 10 too high in many nutrition tables.) Or could there instead be some valid physical reason why SEAgel does an order of magnitude better than its dominant constituent, air?

Expert comments would be much appreciated ...

Thanks, 202.74.220.117 02:26, 30 March 2006 (UTC)Bruce P.S. I have also put this comment in http://en.wikipedia.org/wiki/Aerogel , where the low value (0.003) also appears.

The change was by Az7997

(cur) (last) 08:20, 30 March 2006 Az7997 (→Silica aerogel - changed thermal conductivity value to CRC value and ADDED REFERENCE, WHAT A CONCEPT!!!) My understanding is the aerogel is comparable with closed cell (perhaps a factor of 2 better). --njh 02:33, 30 March 2006 (UTC)

I wrote an explanation about why aerogel is a good insulator (it is not because it is made of silica), but this NASA web page explains it better. NASA did not invent aerogel, but they developed it further. 0.017 W/mK is a frequently quoted number. It is often rounded to 0.02 W/mK. I think that sometimes it is mistakenly written as 0.002 and 0.003. One company that makes aerogel claims that it has gotten it down to 0.011-0.013. An aerogel fact sheet by NASA says that it ranges from 0.016 to 0.03. Part of the variance depends on what properties are wanted for the aerogel. For example, if you want it to be more rugged, you might have to increase thermal conductivity. Some NASA aerogels are significantly higher in thermal conductivity because of their application. I recommend using the values from NASA, either 0.016 or as a range 0.016 to 0.03. -- Kjkolb 04:00, 30 March 2006 (UTC)

[edit] Further on querying aerogel's record low thermal conductivity

Thanks to njh and Kjkolb for very helpful replies to my query. As further information, Google returned a reference http://eetd.lbl.gov/ECS/aerogels/satcond.htm that quotes a typical total conductivity of 0.017 W/m.K for silica aerogels (i.e. the original value in the table that agrees well with the 0.016 recommendation of Kjkolb, and that was replaced by the value 0.003 W/m.K that I query). This is actually a well written and informative article on the thermal conductivity characteristics of aerogels and is part of an excellent more general write-up on aerogels, http://eetd.lbl.gov/ECS/aerogels/satoc.htm , that I would recommend as a reference for http://en.wikipedia.org/wiki/Aerogel . At the risk of complicating this discussion, The thermal article also hints at another way that a thermal conductivity of 0.003 W/m.K might conceivably have been reported: they themselves report a similar minimum value for a silica aerogel with added carbon of ~0.0042 W/m.K . Crucially, however, this is by *evacuating* the aerogel - which is clearly not legitimate for a thermal conductivity table. (To show how absurd this would be, the record-holder for insulating materials in the Guinness Book of Records would then have to be for the trace gases in an ultra-high vacuum and that thermal conductivity would be dominated by radiative transfer and would depend on the temperature and emissivity of the vacuum walls rather than the thickness of the vacuum - in short, the coefficient of a total thermal conductivity with units of W/m.K ceases to make much sense for vacuum environments.) To give a second recommendation, I agree with Kjkolb that the entry in the table could be 0.016 or 0.017 W/m.K for a "typical silica aerogel". Or instead it could be left out of the table since the table doesn't yet even include such common and important substances as water: the value for water is 0.58 W/m.K, given in http://www.engineeringtoolbox.com/thermal-conductivity-d_429.html . This is a good comprehensive reference and my third recommendation is to add this to the general references for this topic, http://en.wikipedia.org/wiki/Thermal_conductivity . Regards, 202.74.218.111 11:32, 30 March 2006 (UTC)Bruce P.S. I will also post this on the comments page for http://en.wikipedia.org/wiki/Aerogel .

[edit] Disambiguation

The name "K-Value" is also used to describe molecular size of polymers [4]. --Jak123 18:18, 18 April 2006 (UTC)

[edit] An inconsistency with the diamond article

This article states that diamonds doped with boron have a higher thermal conductivity then pure diamonds. The diamond article states the opposite. Which is correct?

- Pure diamond will have a higher thermal conductivity as the intrroduction of impurities into the system will lead to mass defect scattering and will decrease the maximum thermal conductivity achievable. shepplestone

[edit] Constants for wood

This source suggests that the thermal conductivity can be 4-5 times higher in wet wood. The source also claims (from its references) that the conductivity can be 1.7-2.25 times higher along the grains compared to radially.

[edit] Beryllium oxide

It would be helpful to insert a row for Beryllium oxide in the table of thermal and electrical conductivities. DFH 20:55, 23 November 2006 (UTC)

[edit] Merge Thermal resistance to Thermal conductivity

I hope this is an obvious merge, and I will add a redirect to Thermal resistance, unless there is any resistance to this.+mwtoews 21:49, 24 November 2006 (UTC)

Right-O. Done. +mwtoews 00:15, 26 November 2006 (UTC)

[edit] Diamond and electrical semiconductors.

Most diamonds (with some exceptions) are electrical insulators (see diamond article). This article says that diamond is an electrical semiconductor. —The preceding unsigned comment was added by 129.241.94.20 (talk) 09:59, 15 December 2006 (UTC).

[edit] Cleanup-list Tag

I've written my rant next to other relevant discussion above. I'm adding this note at the bottom of the talk page because I expect it's where people will look to see why the tag was added. --ToobMug 02:15, 23 January 2007 (UTC)

[edit] Power lines?

In the list, iron is marked as the material used in power lines. AFAIK, no power lines (this side of ~1930?) are of iron or even stainless steel, but rather aluminium. Correct?

81.233.228.251 18:52, 6 February 2007 (UTC)

[edit] List of thermal conductivities

I brached off the section list of thermal conductivities into its own article. What do you think? --Tunheim 12:14, 9 February 2007 (UTC)