Page 329 - Instrumentation Reference Book 3E
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Thermal analvsis  313

             depending  on the  required  speed  and  accuracy   temperatures,  the  liquidus and the  solidus tem-
             of the analysis. Figure  15.14 was obtained using   peratures.  At  both temperatures,  the  change  of
             a  30-minute  plating  time.  Good  precision  and   state of the metal  releases latent heat. The tem-
             accuracy  can  be  obtained  in  concentration   peratures  at  which  the  liquidus  and  solidus
             ranges  as low as 0.1 to  1Opg per  liter  and this.   occur can be measured by  the use of equipment
             combined  with  the  fact  that  small  samples  can   made by Kent Industrial  Measurements  Ltd. To
             be  used,  means  that  the  technique  is  most   make the measurement,  a  sample of  liquid iron
             attractive  for  trace-metal  characterization  in   is  poured  into  a  special  cup  made  from  resin-
             the analysis of  air, water,  food, soil, and biolo-   bonded  sand into which a small type K thermo-
             gical samples.                           couple  is  mounted,  Figure  15.15. As  the  iron
                                                      cools  and  passes  through  its  two  changes  of
                                                      state  its  temperature  is  monitored  by  the ther-
                                                      mocouple.  The  graph  showing  the  cooling
             15.4 Thermal analysis                    against  time,  Figure  15.16,  has  two  plateaus,
                                                      one  at  the  liquidus  and  one  at  the  solidus.  To
             No work  on instrumental methods  of  determin-   complete  the  analysis  the  signal from  the ther-
             ing  chemical  composition  would  be  complete   mocouple  is  processed  by  a  micro-computer
             without mention of thermal analysis. This is the   which  calculates  and  prints  out  the  required
             name  applied  to  techniques  where  a  sample  is   analysis.
             heated  or  cooled  while  some physical  property   Figures  15.17-15.22  show  other  applications
             of  the  sample is recorded  as a  function  of  tem-   of  thermogravimetry  and  derivative  thermo-
             perature. The main purpose in making such meas-   gravimetry to commercial samples and are largely
             urements  is  most  often  not  to  evaluate  the   self-explanatory.
             variation  of  the  physical  property  itself  but  to   In  commercia1 thermal  analysis  instruments:
             use  the  thermal  analysis  record  to  study  both   the sample is heated  at a uniform  rate while its
             the  physical  and chemical changes  occurring  in   temperature  and  one  or  more  of  its  physical
             the sample on heating.                   properties  are measured  and recorded. A typical
               There are three main divisions of the technique   arrangement  is  shown  in  Figure  15.22(a). The
             depending on the type of parameter recorded  on   measuring  unit  has  a  holder  to  fix the position
             the  thermal  analysis  curve.  This  can  be  (a) the   of  the  sample  in  the  furnace,  a  means  of
             absolute value of the measured property, such as   controlling  the  atmosphere  around  the  sample,
             sample weight,  (b) the  difference between  some   a  thermocouple  for  measuring  the  sample  tern-
             property  of  the  sample  and that  of  a  standard   perature  and  the  sensor  for  the  property  to  be
             material, e.&., their temperature difference (these   measured,  e.&.,  a balance  for measuring  weight.
             are differential measurements), and (c) the rate at   The  design  of  the  property  sensor  has  to  be
             which the property is changing with temperature   such that it will function accurately over a wide
             or time, e.g., the weight loss (these are derivative   temperature  range  and  it  is  most  important  to
             measurements).                           ensure  that  the  atmosphere  around  the  sample
               A  convention has  grown  up for thermal  ana-   remains  fixed, be  it  an inert gas, a reactive gas,
             lysis  nomenclature,  and  recommendations  of   or a vacuum.
             the  International  Confederation  for  Thermal   The  temperature  control  unit  consists  of  a
             Analysis  are that  the  term  “thermogravimetry”   furnace  and  a  programming  unit,  the  function
             (TG)  be  used  for  measuring  sample  weight,   of  which  is  to  alter  the  sample  temperature
             “derivative  thermogravimetry”  (DTG)  for  rate   (not  the  furnace  temperature)  in  a  predeter-
             of weight loss, and “differential thermal analysis”   mined  manner.  The  recording  unit  receives sig-
             (DTA) for measuring the temperature difference   nals  from  the  property  sensor  and  the  sample
             between  sample  and  standard.  There  are  also   thermocouple,  amplifies  them,  and  displays
             many  other terms  relating  to  specific heat  mea-   them  as  a  thermal  analysis  curve.  Figure
             surement. magnetic susceptibility, evolved gases,   15.22(b)  shows  arrangements  for  differential
             etc.                                     instruments  where  the  sample  material  and  a
               During  the  past  20  years,  a  wide  choice  of   reference  material  are  placed  in  identical  envir-
             commercially  available  equipment  has  become   onments  with  sensors to  measure  the  difference
             available, and thermal analysis is now widely used   in one of  their properties. The differential signal
             as a tool in research and product control.   is amplified and recorded as in the basic system.
               One particular  application  is  to the composi-   In  derivative  instruments  (Figure  1522(c))  a
             tion  of  cast  iron  in terms  of  its carbon, silicon,   derivative  generator,  such  as  an  electro-optical
             and  phosphorus  content,  which  can  be  calcu-   device  or  an  electronic unit,  is  incorporated  to
             lated  from the  temperatures  at which it freezes.   compute  the  derivative of  an input  signal. Gen-
             As  it  is  an  alloy,  the  freezing  occurs  at  two   erally, both  the  derivative signal and  the  signal
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