Thermal analysis

Itransm'"ed\

5.7 Thermal analysis

164 Modern Physical Metallurgy and Materials Engineering depth, is based upon the well-known phenomenon of sputtering. The target surface is bombarded with a focused beam of primary ions that has been accelerated with a potential of 1 - 3 0 kV within a high-vacuum chamber ( 1 0 - 5 - 1 0 -l~ torr). These ions generate a series of collision cascades in a shallow surface layer, 0.5-5 nm deep, causing neutral atoms and, to a much smaller extent, secondary ions to be ejected (sputtered) from the specimen surface. Thus, a metallic oxide (MO) sample may act as a source of M, O, M +, O +, M - , O-, MO + and M O - species. The secondary ions, which are thus either monatomic or clustered, positive or negative, are directed into a mass spectrometer (analyser), wherein they are sorted and identified according to their mass/charge ratio. Exceptionally high elemental sensitivities, expressed in parts per million and even parts per billion, are achievable. All elements in the Periodic Table can be analysed and it is possible to distinguish between individual isotopes.

Studies of the self-diffusion of oxygen and nitrogen have been hindered because these light elements have no isotopes that can be used as radioactive tracers.

SIMS based on the stable isotope 180 provides a rapid method for determining self-diffusion coefficients. The physical process whereby ions are ejected is difficult to express in rigorous theoretical terms, consequently SIMS is usually semiquantitative, with dependence upon calibration with standard samples of known composition. SIMS is a valuable complement to other methods of surface analysis.

The available range of beam diameter is 1 lam to several millimetres. Although various types of ion beam are available (e.g. Ar-,

3202+, 160-,

Cs +, etc.) positively-charged beams are a common choice. How- ever, if the sample is insulating, positive charge tends to accumulate in the bombarded region, changing the effective value of the beam voltage and degrading the quality of signals. One partial remedy, applicable at low beam voltages, is to 'flood' the ion-bombarded area with a high-intensity electron beam. In some variants of SIMS laser beams are used instead of ion beams.

Of the large and growing variety of methods covered by the term SIMS, the dynamic, static and imaging modes are especially useful. Materials being investigated include metals, ceramics, polymers, catalysts, semiconductors and composites. Dynamic SIMS, which uses a relatively high beam current, is an important method for determining the distribution and very low concentration of dopants in semiconductors.

The beam scans a raster, 100-500 ~tm in size, and slowly erodes the surface of the sample. Secondary ions from the central region of the crater are analysed to produce a precise depth profile of concentration.

Static SIMS uses a much smaller beam current and the final spectra tend to be more informative, providing chemical data on the top few atomic layers of the sample surface. Little surface damage occurs and the method has been applied to polymers. The imaging version of SIMS has a resolution comparable to SEM

and provide 'maps' that show the lateral distribution of elements at grain boundaries and precipitated particles and hydrogen segregation in alloys. Imaging SIMS has been applied to transverse sections through the complex scale layers which form when alloys are exposed to hot oxidizing gases (e.g. 02, COz).

Its sensitivity is greater than that obtainable with conventional EDX in SEM analysis and has provided a better understanding of growth mechanisms and the special role of trace elements such as yttrium.

The characterization of materials 165

M, !thod

(a)

TGA

(b)

Physical parameter

utilized

....

Mass m

DTA Tsar. ^{-- Trel}

=AT

(c)

DSC dH

dt

Apparatus

f

Atmos

@= -- ~ ==" ---4

Ner

differ

Graph

T

Area

~

AHm

d~H 1 Endothermic d"~ Exothermic l

. . . . .

T --~

Figure 5.46 Basic methods of thermal analysis. (a) Thermogravimetric analysis (TGA). (b) differential thermal analysis (DTA) and (c) differential scanning calorimetry (DSC).

the graphical trace in a procedure known as derivative thermogravimetric analysis (DTGA).

5.7.3 Differential thermal analysis

DTA l reveals changes during the heating of a sample which involve evolution or absorption of energy. As shown diagrammatically in Figure 5.46b, a sample S and a chemically and thermally inert reference material R (sintered alumina or precipitated silica) are mounted in a recessed heating block and slowly heated. The thermocouples in S and R are connected in opposi- tion; their temperature difference AT is amplified and plotted against temperature. Peak area on this trace is a function of the change in enthalpy (AH) as well as the mass and thermal characteristics of the sample S. Small samples can be used to give sharper, narrower peaks, provided that they are fully representative of the source I Usually accredited to H. Le Chatelier (1887): improved version and forerunner of modern DTA used by W. C. Roberts-Austen (1899) in metallurgical studies of alloys.

material. Ideally, the specific heat capacities of S and R should be similar. DTA is generally regarded as a semi-quantitative or qualitative method. It has been used in studies of devitrification in oxide glasses and the glass transition in polymers. Figure 5.47b shows a comparison of the thermal response of high-alumina cement (HAC) and Portland cement. The amount of an undesirable weakening phase can be derived from the relative lengths of the ordinates X and Y in the HAC trace.

5.7.4 Differential scanning calorimetry In this method, unlike DTA, the sample and reference body have separate resistive heaters (Figure 5.46c).

When a difference in temperature develops between sample S and reference R, an automatic control loop heats the cooler of the two until the difference is eliminated. The electrical power needed to accomplish this equalizer is plotted against temperature. An endothermic change signifies that an enthalpy increase has occurred in S; accordingly, its peak is plotted upwards (unlike DTA traces). Differences in thermal conductivity and specific heat capacity have no effect

166 Modern Physical Metallurgy and Materials Engineering

Oil

_L_ L_

^Carbon

_-N ^black

i-- N2 Atmosphere ..-. Air atmosphere -

x.___

100 , ,

o 26o 46o 6oo

Temperature (~

cement Portland cement

. . . .

100 2()0

³⁶⁰ ⁴⁶⁰

500 - 6()0

Sample temperature (~

(a) (b)

..-.

_o

t~ |

-!-

"~T~a ~

fusion

/ ~Degradation

C ~

Crystallization

, c , oo

_sothermal

_J.~' .~ _{Jt ~} Isothermal

Temperature

(c)

Figure 5.47 Examples of thermal analysis (a) TGA curve for decomposition of rubber, showing decomposition of oil and polymer in N2 up to 600~ and oxidation of carbon black in air above 600~ (Hill and Nicholas, 1989), (b) DTA curve for high-alumina cement and Portland cement (Hill and Nicholas, 1989) and (c) DTA curve for a quenched glassy polymer (Hay, 1982).

and peak areas can be expressed as energy per unit mass. DSC has proved particularly valuable in polymer research, often being used in combination with other techniques, such as evolved gas analysis (EGA). DSC has been used in studies of the curing characteristics of rubbers and thermoset resins, transitions in liquid crystals and isothermal crystallization rates in thermoplastics. Figure 5.47c is a trace obtained for a quenched amorphous polymer. DSC has also been used in studies of the exothermic behaviour of cold-worked metals as they release 'stored energy' during annealing, energy absorption during eutectic melting of alloys, precipitation in aluminium-based alloys, relaxation transformations in metallic glasses and drying/firing transitions in clay minerals.

Itransm'&#34;ed\