DSC Baseline Improvements Obtained by a New Heat Flow Measurement Technique

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1 DS Baeline Impovement Obtained by a New Heat Flow Meauement Technique obet L. Danley, Pete A. aulfield TA Intument, 109 Luken Dive, New atle DE ABSTAT Nealy all diffeential canning caloimety (DS) eult ae affected by the intument baeline heat flow. Weak tanition, fo example gla tanition may not be unambiguouly detected if the intument baeline i not taight. Meauement of heat capacity i influenced by all of the defect in the intument baeline. A new technique fo meauing heat flow in a DS employing the heat flux pinciple i decibed that yield geatly impoved intument baeline. A fou-tem heat flow meauement equation i ued which include light intument imbalance that contibute to baeline defect. A DS heat flow eno that ue the fou-tem heat flow equation i decibed. The eno ha themally independent ample and efeence poition and two diffeential tempeatue meauement. INTODUTION Among the moe impotant pefomance chaacteitic of a DS i the intument baeline, i.e. the eidual heat flow ignal of the DS when opeated empty. The DS i a twin intument, compiing a ample and a efeence caloimete within a common themal encloue, whee the two caloimete ae aumed to be identical. The output of the DS i the diffeence between the heat flow meaued by each of the caloimete. A numbe of advantage ae gained by the ue of twin caloimete including cancellation of heat leakage and tempeatue ditubance common to both caloimete (1). It follow that if the two caloimete ae identical and ymmetically poitioned within the DS encloue, the diffeential heat flow ignal of the empty DS hould be zeo. Howeve, all DS, whethe heat flux o powe compenation give nonzeo heat flow meauement when opeated empty, demontating that the intument i not ymmetical. Heat flux DS typically have upeio baeline when compaed to powe compenation DS, owing to the much malle tempeatue diffeence between the encloue and the caloimete in heat flux DS. Intument aymmety and baeline cuvatue contibute to eo in heat capacity meauement in DS (2) and MDS (3). In vitually all DS, the meaued ignal i the diffeence between the tempeatue of the ample and efeence poition of the eno. Within the intument the T ignal i conveted to a heat flow ate uing a tempeatue dependent popotionality facto: q& E( T ) T (1) 1 TA268

2 the popotionality facto E(T) depend upon the geomety and mateial of contuction of the diffeential tempeatue eno. In commecial intument, the heat flow popotionality facto i geneally detemined by the manufactue and i aumed to be common to all intument of a given type. To detemine the elationhip between the meaued tempeatue() and heat flow, a mathematical model of the meauement appaatu i ued. Analyi of the heat flow meauement i baed on the lumped heat capacity method whee a themal ytem i epeented by themal eitance and heat capacitie (4). Thi type of analyi ha fequently been ued to model DS (5-7). Figue 1 how an equivalent cicuit that may be ued to epeent the heat flow meauement in a DS. Each caloimete compie a themal eitance and a heat capacity; ubcipt and indicate the ample o efeence caloimete. T and T ae the meaued tempeatue of the ample and efeence caloimete and T 0 i the tempeatue of the DS encloue. The meaued heat flow ae q, the heat flow to the ample and it pan and q, the heat flow to the efeence and it pan. Thi model doe not include the ample, efeence o thei pan. q q T T T 0 Figue 1 DS Heat Flow Meauement Model Pefoming a heat balance give the ample and efeence heat flow in tem of tempeatue T, T, T 0 and eno themal paamete,,, : T T dt q& 0 (2) T T dt q& 0 (3) The diffeence i taken between the ample and efeence heat flow and the two diffeential tempeatue ae ubtituted: T T T T0 T0 T Afte eaangement, the eult i a fou-tem heat flow equation giving the diffeence between the ample and efeence heat flow: 2 TA268

3 T q& 1 + T0 1 + ( ) dt d T The fit tem i identical to equation (1), the conventional DS heat flow. The econd and thid tem eflect imbalance between themal eitance and heat capacitie of the ample and efeence caloimete. The fouth tem i a heat flow aociated with diffeence in heating ate between the ample and efeence caloimete, it i geneally zeo except when a tanition i occuing in the ample, o duing MDS expeiment (8). A pinciple diffeence between thi heat flow equation and thoe peviouly ued in DS i that the caloimete have not been aumed to be identical. To ue thi equation, the eno heat capacitie and themal eitance mut be known, the DS mut include the two diffeential tempeatue meauement and the ample and efeence caloimete mut be independent. A new heat flux DS and eno have been deigned to meet thee meauement equiement (9) and a calibation pocedue allow the eno themal paamete to be detemined. alibation involve the detemination of the value fo,, and. The calibation pocedue include two identical contant heating ate expeiment; the fit i pefomed with the DS empty and the econd with apphie dik without pan on the ample and efeence poition. Fo the empty DS expeiment, the ample and efeence heat flow ae et equal to zeo, the heat balance equation (2) and (3) ae olved, thu giving the caloimete time contant: T dt 0 0 T T dt d T The time contant ae a function of tempeatue. Fo the apphie dik expeiment, the ample and efeence heat flow fom equation (2) and (3) ae et equal to the heat flow to the apphie dik, which equal the poduct of apphie ma, heat capacity and heating ate. The ample and efeence apphie dik heating ate ae aumed to be equal to the heating ate of the ample and efeence caloimete. Thi i a eaonable aumption, a apphie ha no fit ode tanition in thi tempeatue egion. Thee equation ae olved to give the caloimete heat capacitie: mcapph T0 1 dt mcapph T0 + T 1 dt d T Whee, the caloimete time contant wee detemined by the fit expeiment. Themal eitance ae found uing the time contant and heat capacitie: 3 TA268

4 Thu, themal paamete of the caloimete a a function of tempeatue ae detemined. The twin caloimete eno aembly i hown in Figue 2, it include poviion fo meauing tempeatue T, T 0 and diffeential tempeatue T and T 0. It wa deigned uch that the ample and efeence caloimete ae themally independent, in othe wod, heat flow in one caloimete doe not affect the tempeatue of the othe. The main body of the eno i contantan and conit of a thick flat bae and a pai of thin wall cloed end cylinde integal with the bae. The thin wall ection ceate the themal eitance and the flat end uface hold the ample and efeence. A thin chomel dik i welded to the Figue 2 - Tzeo DS Seno Aembly undeide of each platfom and function a an aea themocouple junction to educe enitivity to vaiation of contact between eno and pan (10). A chomel wie i welded to each chomel dik. A contantan and a chomel wie ae welded to the cente of the bae tuctue. The bae uface i bazed to the ilve DS encloue, which make the bae of the eno aembly iothemal. Diffeential tempeatue T i meaued between the chomel wie attached to the chomel dik and T 0 i meaued between the chomel wie attached to the ample chomel dik and the eno bae. T i meaued between the ample chomel wie and the bae contantan wie. T 0 i meaued between the contantan and chomel wie attached to the eno bae. T 0 i ued to contol the tempeatue of the DS. EXPEIMENTAL Intument baeline can wee pefomed uing a TA Intument Q1000 DS that incopoate the heat flow eno aembly and heat flow meauement method 4 TA268

5 decibed above. Baeline wee un at 20 /min. uing the efigeated cooling ytem (S). alibation of the eno paamete wa pefomed uing the S at a heating ate of 20 /min. The DS cell and S wee puged with nitogen. ESULTS AND DISUSSION A compaion of the empty DS baeline of a Q1000 with S and a 2920 DS with S i hown in Figue 3, thi compaion i paticulaly apt, becaue the 2920 i notable fo excellent baeline pefomance. The Q1000 baeline i upeio in evey way, the tat-up offet i much malle, the baeline i damatically taighte and lope i geatly educed. A expected of a twin caloimete, the empty intument heat flow ignal i vey nealy zeo eveywhee duing the expeiment. Notice that the heat flow cale i 1.0 mw and that the Q1000 data extend fom 80 to Heat Flow (mw) Heat Flow Q1000 Heat Flow Tempeatue ( ) Figue 3 - Intument Baeline ompaion Figue 4 how a plot of intument baeline poduced by the Q1000 DS uing the S. Eight conecutive can wee made between 90 and 400 to demontate baeline pefomance. The total ange of heat flow vaiation i le than mw. All baeline ae vey taight, have vey little lope and vey mall tat-up offet. Between any two conecutive baeline the vaiation i no geate than mw. Thee eult how that excellent baeline pefomance may be obtained ove a vey boad ange of tempeatue with excellent epeatability. Among the benefit of the baeline impovement achieved by the Tzeo DS ae the ability to meaue vey weak tanition (11). 5 TA268

6 0.2 Heat Flow (mw) Tempeatue ( ) Figue 4 - Q1000 Intument Baeline Uing S ONLUSIONS Beginning fom fit pinciple, a new DS heat flow meauement technique ha been developed. Unlike taditional DS, the intument ha not been aumed to be pefectly ymmetic and the meauement include the effect of eno aymmety. A new DS eno ha been developed that include two diffeential tempeatue meauement o that the meauement model can be ued. A calibation technique ha been developed that give a moe thoough chaacteization of the DS eno and include the effect of aymmety. One of the impotant benefit of the Tzeo DS technology i a damatic impovement in intument baeline pefomance. EFEENES 1. W. Hemminge and G. Höhne, aloimety Fundamental and Pactice, 1984, V. B. F. Mathot, in: aloimety and Themal Analyi of Polyme, V.B.F. Mathot (Ed.), 1994, A. Bolle, et.al., J. Themal Analyi, 1997, v J. P. Holman, Heat Tanfe, 4 th Edition, 1976, A. P. Gay, in: Analytical aloimety,. S. Pote, J. F. Johnon (Ed.), 1968, v A. Baxte, in: Themal Analyi,. F. Schwenke, P. D. Gan (Ed.), 1969 v G. W. H. Höhne, W. Hemminge H.-J. Flammeheim, Diffeential Scanning aloimety, 1996, L. Danley, P. A. aulfield, Poc. 29 th N. Ame. Them. Anal. Soc., L. Waguepack,. L. Blaine, Poc. 29 th N. Ame. Them. Anal. Soc., U.S. Patent 4,095,453, Diffeential Themal Analyi ell, P. A. aulfield, L.. Thoma, Poc. 29 th N. Ame. Them. Anal. Soc., TA268

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