Page 329 - Instrumentation Reference Book 3E
P. 329
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