Vizualizare sursă pentru „Difuzivitate termică”

{{Termodinamică|expanded=proprietăți}}
În [[transmiterea căldurii]] '''difuzivitatea termică''' este o [[proprietăți intensive și extensive |proprietate intensivă]] definită prin [[raport]]ul dintre [[conductivitate termică |conductivitatea termică]] și [[înmulțire (matematică) |produsul]] dintre [[densitate]] și [[capacitate termică masică |capacitatea termică masică]] la [[presiune]] constantă.<ref>{{en icon}} {{cite book|editor-last = Lide |editor-first = David R. |year = 2009 |title = CRC Handbook of Chemistry and Physics |edition = 90th |place = Boca Raton, Florida |publisher = [[CRC Press]] |isbn = 978-1-4200-9084-0 |page=2-65}}</ref> Este o măsură a vitezei transmiterii [[căldură |căldurii]] în interiorul unui [[material]]. [[Unitate de măsură |Unitatea sa de măsură]] este m<sup>2</sup>/s. Difuzivitatea termică este de obicei notată cu {{mvar|a}}<ref name=H>''Hütte'', vol. I, București: [[Editura Tehnică]], 1951, p.&nbsp;592</ref><ref name=LG><!-- Prof.dr.ing. -->Lucian-Gheorghe Gavrilă, [https://cadredidactice.ub.ro/gavrilalucian/files/2011/05/fdtou-curs-09.pdf ''Transfer termic conductiv în regim staționar''] (curs, 2011), [[Universitatea „Vasile Alecsandri” din Bacău]], accesat 2024-07-03</ref>, dar pot fi întâlnite și notațiile {{mvar|α}}, {{mvar|h}}, {{mvar|κ}},<ref>{{en icon}} {{cite book|last1=Hetnarski |first1=Richard B. |last2=Eslami |first2=M. Reza |title=Thermal Stresses - Advanced Theory and Applications |year=2009 |publisher=Springer Netherlands|location=Dordrecht|isbn=978-1-4020-9247-3|pages=170|edition=Online-Ausg.|doi=10.1007/978-3-030-10436-8}}</ref> {{mvar|K}},<ref name = AJP>{{en icon}} {{citation|last1=Unsworth|first1=J.|last2=Duarte|first2=F. J. |title=Heat diffusion in a solid sphere and Fourier Theory|journal=Am. J. Phys.|pages=891–893|doi=10.1119/1.11601|volume=47|bibcode = 1979AmJPh..47..981U|issue=11| year=1979}}</ref> sau {{mvar|D}}. 

Formula este:<ref>{{en icon}} {{cite book|last=Lightfoot|first=R. Byron Bird, Warren E. Stewart, Edwin N.|title=Transport Phenomena|publisher=John Wiley and Sons, Inc.|year=1960|isbn=978-0-471-07392-5|at=Eq. 8.1-7|url-access=registration|url=https://archive.org/details/transportphenome00bird}}</ref>
:<math>a = \frac{\lambda}{\rho c_p}</math>
unde
: {{mvar|λ}} este [[conductivitate termică |conductivitatea termică]], în W/m•K.
: {{mvar|c<sub>p</sub>}} este [[capacitate termică masică |capacitatea termică masică]], în J/kg•K,
: {{mvar|ρ}} este  [[densitate]]a, înkg/m<sup>3</sup>.
Împreună, {{mvar|ρc<sub>p</sub>}} poate fi considerată capacitatea termică volumetrică, în J/m<sup>3</sup>•K.

Așa cum se vede din ecuația căldurii,<ref>{{en icon}} {{citation|last1=Carslaw|first1=H. S.|last2=Jaeger|first2=J. C.|year=1959|title=Conduction of Heat in Solids|edition=2nd|publisher=Oxford University Press|isbn=978-0-19-853368-9}}</ref>
:<math>\frac{\partial T}{\partial t} = a \nabla^2 T, </math>
o modalitate de a vedea difuzivitatea termică este ca raportul dintre derivata în funcție de timp a [[temperatură |temperaturii]] și curbura acestei derivate, cuantificând viteza cu care concavitatea temperaturii este „netezită”.<ref>{{en icon}} {{cite book |last=Dante |first=Roberto C. |title=Handbook of Friction Materials and Their Applications |year=2016 |publisher=Elsevier |doi=10.1016/B978-0-08-100619-1.00009-2 |pages=123-134}}</ref><ref>{{en icon}} {{cite book|last=Venkanna|first=B.K.|title=Fundamentals of Heat and Mass Transfer|url=https://books.google.com/books?id=IIIVHRirRgEC&pg=PA38|access-date=1 December 2011|year=2010|publisher=PHI Learning|location=New Delhi|isbn=978-81-203-4031-2|page=38}}</ref> Într-o substanță cu difuzivitate termică ridicată căldura se transmite repede prin ea, deoarece substanța conduce căldura repede în raport cu capacitatea sa de căldură volumetrică.

Difuzivitatea termică este adesea măsurată prin metoda pulsului laser.<ref>{{en icon}} {{Cite web |url=http://www.netzsch.com/en/home/ |title=NETZSCH-Gerätebau, Germany |access-date=2012-03-12 |archive-url=https://web.archive.org/web/20120311084633/http://www.netzsch.com/en/home/ |archive-date=2012-03-11 |url-status=dead }}</ref><ref name="Parker">{{en icon}} {{cite journal
 |author1=W.J. Parker |author2=R.J. Jenkins |author3=C.P. Butler |author4=G.L. Abbott |title=Method of Determining Thermal Diffusivity, Heat Capacity and Thermal Conductivity
 |journal=Journal of Applied Physics
 |volume=32  |issue=9|page=1679
 |year=1961
 |doi= 10.1063/1.1728417
|bibcode = 1961JAP....32.1679P }}</ref> Aceasta constă în încălzirea unei benzi sau a unei probe cilindrice cu un impuls scurt de energie la un capăt și urmărirea schimbării temperaturii (reducerea amplitudinii și defazarea pulsului) la mică distanță.<ref>{{en icon}} {{cite journal
 |author1=J. Blumm |author2=J. Opfermann |title= Improvement of the mathematical modeling of flash measurements |journal=High Temperatures – High Pressures |volume=34  |issue=5 |page=515 |year=2002 |doi= 10.1068/htjr061 }}</ref><ref>{{en icon}} {{cite conference |last=Thermitus |first=M.-A. |editor= Gaal, Daniela S. |editor2=Gaal, Peter S. |title=New Beam Size Correction for Thermal Diffusivity Measurement with the Flash Method|conference=30th International Thermal Conductivity Conference/18th International Thermal Expansion Symposium|conference-url=https://web.archive.org/web/20100128105338/http://www.thermalconductivity.org/|booktitle=Thermal Conductivity 30/Thermal Expansion 18|url=https://books.google.com/books?id=F9row3bxLuYC&pg=PA217|access-date=1 December 2011|date=October 2010|publisher=DEStech Publications|location=Lancaster, PA|isbn=978-1-60595-015-0|page=217}}</ref>

== Difuzivitatea termică a unor materiale și substanțe ==
{| class="wikitable sortable"
|+Difuzivitatea termică a unor materiale și substanțe<ref>{{en icon}} {{cite book| title=Introduction to Heat Transfer|edition= 3rd|publisher=McGraw-Hill| year=1958| last1=Brown |last2= Marco}}  and {{cite book| last1=Eckert |last2=Drake| title=Heat and Mass Transfer| publisher=McGraw-Hill| year=1959| isbn=978-0-89116-553-8}} citată în {{en icon}} {{cite book| first=J.P. |last=Holman| title=Heat Transfer| edition=9th| publisher=McGraw-Hill|year= 2002| isbn=978-0-07-029639-8}}</ref>
|-
! Material !! Difuzivitate<br/>termică<br/>(mm<sup>2</sup>/s) !! Surse
|-
| Grafit pirolitic, paralel cu straturile ||1220|| 
|-
| [[Diamant]] ||1060–1160|| 
|-
| [[Carbon]] la 25&nbsp;°C
|216,5
|<ref name="Casalegno2010" />
|-
|Heliu (300 K, 1 atm)
|190
|<ref name="baierlein">{{en icon}} {{cite book|title=CDC Handbook of Chemistry and Physics|publisher=Chemical Rubber Publishing Company|year=1992|editor-last=Lide|editor-first=David R.|edition=71st|location=Boston}} citat în {{en icon}} {{cite book|last=Baierlein|first=Ralph|url=https://archive.org/details/thermalphysics00ralp|title=Thermal Physics|publisher=Cambridge University Press|year=1999|isbn=978-0-521-59082-2|location=Cambridge, UK|page=[https://archive.org/details/thermalphysics00ralp/page/372 372]|access-date=1 December 2011|url-access=registration}}</ref>
|-
| [[Argint]] pur (99,9 %) ||165,63||
|-
| [[Hidrogen]] (300 K, 1 atm)
|160
|<ref name="baierlein" />
|-
| [[Aur]]  ||127||<ref name="eleccool">{{en icon}} {{cite journal|url=http://www.electronics-cooling.com/2007/08/thermal-diffusivity/ |title=Materials Data| author=  Jim Wilson | date= August 2007 }}</ref>
|-
| [[Cupru]] la 25&nbsp;°C  || 111||<ref  name="Casalegno2010">{{en icon}} {{cite journal|title= Measurement of thermal properties of a ceramic/metal joint by laser flash method |journal= Journal of Nuclear Materials |volume=407 |issue=2|page=83 |author1=V. Casalegno |author2=P. Vavassori |author3=M. Valle |author4=M. Ferraris |author5=M. Salvo |author6=G. Pintsuk | year= 2010 |doi=10.1016/j.jnucmat.2010.09.032|bibcode = 2010JNuM..407...83C }}</ref>
|-
| [[Aluminiu]] || 97|| <ref name="eleccool"/> 
|-
| [[Siliciu]]
|88
|<ref name="eleccool" />
|-
| Al-10Si-Mn-Mg (Silafont 36) la 20&nbsp;°C   || 74,2|| <ref>{{en icon}} {{cite journal |author1=P. Hofer |author2=E. Kaschnitz |title=Thermal diffusivity of the aluminium alloy Al-10Si-Mn-Mg (Silafont 36) in the solid and liquid states |journal=High Temperatures – High Pressures |volume=40 |issue=3–4 |page=311 |year=2011 |url=http://www.oldcitypublishing.com/HTHP/HTHPcontents/HTHP40.3-4contents.html |access-date=2024-07-21 |archive-date=2016-03-04 |archive-url=https://web.archive.org/web/20160304113208/http://www.oldcitypublishing.com/HTHP/HTHPcontents/HTHP40.3-4contents.html |dead-url=yes }}</ref>
|-
| Aliaj de aluminiu 6061-T6 || 64 ||<ref name="eleccool"/>
|-
| [[Molibden]] (99,95%) la 25&nbsp;°C || 54,3||<ref>{{en icon}} {{cite conference|conference=17th Plansee Seminar |title= Measurement of the Thermophysical Properties of Pure Molybdenum |author1=A. Lindemann |author2=J. Blumm | year= 2009 |volume=3 }}</ref>
|-
| Al-5Mg-2Si-Mn (Magsimal-59) la 20&nbsp;°C  || 44,0|| <ref>{{en icon}} {{cite journal |author1=E. Kaschnitz |author2=M. Küblböck |title=Thermal diffusivity of the aluminium alloy Al-5Mg-2Si-Mn (Magsimal-59) in the solid and liquid states |journal=High Temperatures – High Pressures |volume=37 |issue=3 |page=221 |year=2008 |url=http://www.oldcitypublishing.com/HTHP/HTHPcontents/HTHP37.3contents.html |access-date=2024-07-21 |archive-date=2016-03-04 |archive-url=https://web.archive.org/web/20160304131424/http://www.oldcitypublishing.com/HTHP/HTHPcontents/HTHP37.3contents.html |dead-url=yes }}</ref> 
|-
| [[Staniu]]
|40
|<ref name="eleccool" />
|-
| [[Abur]] (1 atm, 400 K)
|23,38
|
|-
| [[Fier]] || 23||<ref name="eleccool" />
|-
| [[Argon]] (300 K, 1 atm)
|22
|<ref name="baierlein" />
|-
| [[Azot]] (300 K, 1 atm)
|22
|<ref name="baierlein" />
|-
| [[Aer]] (300 K)
|19
|<ref name="eleccool" />
|-
| [[Oțel]] AISI 1010 (0,1&nbsp;% carbon)  || 18,8 || <ref>{{en icon}} {{cite book|last=Lienhard|first=John H. Lienhard, John H. |title=A Heat Transfer Textbook|year=2019|publisher=Dover Pub|page= 715|edition=5th}}</ref>
|-
| Oxid de aluminiu (policristalin)
|12,0
|
|-
| [[Oțel]] cu 1&nbsp;% carbon || 11,72 || 
|-
| Si<sub>3</sub>N<sub>4</sub> cu [[nanotub de carbon |nanotuburi de carbon]] la 26&nbsp;°C
|9,142
|<ref name="Koszor2009">{{en icon}} {{cite journal|author1=O. Koszor|author2=A. Lindemann|author3=F. Davin|author4=C. Balázsi|year=2009|title=Observation of thermophysical and tribological properties of CNT reinforced Si<sub>3</sub> N<sub>4</sub>|journal=Key Engineering Materials|volume=409|page=354|doi=10.4028/www.scientific.net/KEM.409.354|s2cid=136957396}}</ref>
|-
|Si<sub>3</sub>N<sub>4</sub> fără nanotuburi de carbon, la 26&nbsp;°C
|8,605
|<ref name="Koszor2009" />
|-
| Steel, stainless 304A at 27&nbsp;°C   || 4.2|| <ref name="eleccool"/>
|-
|Pyrolytic graphite, normal to layers
|3.6
|
|-
| [[Oțel]] inox 310 la 25&nbsp;°C   || 3,352||<ref>{{en icon}} {{cite journal |author1=J. Blumm |author2=A. Lindemann |author3=B. Niedrig |author4=R. Campbell |title=Measurement of Selected Thermophysical Properties of the NPL Certified Reference Material Stainless Steel 310 |journal=International Journal of Thermophysics |volume=28 |page=674 |year=2007 |doi= 10.1007/s10765-007-0177-z |issue=2 |bibcode = 2007IJT....28..674B |s2cid=120628607 }}</ref>
|-
| Inconel 600 la 25&nbsp;°C  || 3,428|| <ref>{{en icon}} {{cite journal |author1=J. Blumm |author2=A. Lindemann |author3=B. Niedrig |title= Measurement of the thermophysical properties of an NPL thermal conductivity standard Inconel 600|journal= High Temperatures – High Pressures |volume=35/36 |issue=6 |page=621 | year= 2003–2007 |url=http://www.perceptionweb.com/abstract.cgi?id=htjr145 |doi=10.1068/htjr145}}</ref> 
|-
| [[Cuarț]] || 1,4|| <ref name="eleccool"/>
|-
| [[Gresie]]
|1,15
|
|-
| [[Gheață]] la 0&nbsp;°C
|1,02
|
|-
| Dioxid de siliciu (policristalin)  || 0,83||<ref name="eleccool"/> 
|-
| Cărămidă obișnuită
|0,52
|
|-
| [[Sticlă]] pt. ferestre
|0,34
|
|-
| Cărămidă, adobe
|0,27
|
|-
| [[Policarbonat]] la 25&nbsp;°C  || 0,144||  <ref  name="HTHP3536pp627">{{en icon}} {{cite journal |author1=J. Blumm |author2=A. Lindemann |title= Characterization of the thermophysical properties of molten polymers and liquids using the flash technique |journal=High Temperatures – High Pressures |volume= 35/36 |issue=6 |page= 627 | year= 2003–2007 |doi=10.1068/htjr144 |url=http://www.eyoungindustry.com/uploadfile/file/20151027/20151027211034_96662.pdf}}</ref> 
|-
| [[Apă]] la 25&nbsp;°C
|0,143
|<ref name="HTHP3536pp627" />
|-
| [[Politetrafluoroetilenă]] (PTFE) la 25&nbsp;°C
|0,124
|<ref>{{en icon}} {{cite journal|author1=J. Blumm|author2=A. Lindemann|author3=M. Meyer|author4=C. Strasser|year=2011|title=Characterization of PTFE Using Advanced Thermal Analysis Technique|journal=International Journal of Thermophysics|volume=40|issue=3–4|page=311|bibcode=2010IJT....31.1919B|doi=10.1007/s10765-008-0512-z|s2cid=122020437}}</ref>
|-
| [[Polipropilenă]] (PP) la 25&nbsp;°C  || 0,096|| <ref  name="HTHP3536pp627"/> 
|-
| [[Nailon]]
|0,09
|
|-
| [[Cauciuc]]
|0,089–0,13
|<ref name="AJP" />
|-
| [[Lemn]] de pin
|0,082
|
|-
| [[Parafină]] la 25&nbsp;°C  || 0,081||<ref  name="HTHP3536pp627"/> 
|-
| [[Policlorură de vinil]] (PVC)   || 0,08|| <ref name="eleccool"/>
|-
| [[Ulei]] de motor (lichid, 100&nbsp;°C) || 0,0738||
|-
| [[Alcool]] || 0,07||<ref name="eleccool"/> 
|}

== Note ==
<references />

== Vezi și ==
* [[Lista proprietăților termodinamice]]
* [[Termodifuziune]]
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