TMA instruments

TMA instruments are also available commercially from Stanton Redcroft Ltd. and others. The Stanton Redcroft Model 791 can be used in the tempera-... 

A schematic of a commercial TMA instrument is shown in Fig. 16.33. The instrument consists of a dimensionally stable (with ternperamre) sample holder and measuring probe, a programmable furnace, a linear variable displacement transducer (LVDT) to measure the change in length, a means of applying force (load) to the sample via the probe (core rod, push rod), and a temperature sensor (usually a thermocouple). 

Half way between conventional thermomechanical analysis and dynamic mechanical analysis is the technique of dynamic force (or load) TMA. This method uses a standard TMA instrument but the force is changed between two values in a stepwise (or sometimes sinusoidal) fashion. The dimensional changes of the specimen are monitored as a function of time (and temperature) but no attempt is made to determine the modulus and damping properties of the material. 

When large numbers of samples are routinely measured, for example in quality control, an automated sample supplier can be fitted to the TA unit. Samples are handled and placed in the instrument by a robot arm, and removed after measurements have been completed. At present, robot arms are commercially available for DSC, TG-DTA and TMA instruments. An automated sample supplier is shown in Figure 2.13. 

TMA instrument, which employs a balance beam mechanism, in compression mode (courtesy of Ulvac Sinku-Riko)... 

Thermal analysis. A DuPont 943 TMA instrument was used for the measurement of mechanical properties of resist films of 2.0-0.5 pm thickness. A DuPont 910 DSC instrument was used for differential scanning calorimetry. 

TMA instruments must be calibrated for both temperature and dimensional motion. The temperature is calibrated using the same type of melting point standards used for DSC. High-purity In,... 

A wide variety of commercial instrumentation is available. Table 4.1 lists some of the current TMA instruments and their manufacturers, including contact information. 

Figure 4.3 is a simple diagram of a typical vertical design TMA instrument. Almost all TMA instruments have similar components a sample platform surrounded by a furnace (for controlled heating and cooling), a sample probe (one end of which touches the sample while the other end is connected to the... 

Figure 43. Schematic diagram of a typical vertical design TMA instrument (see www. anasys.co.uk).

Table 4.2 lists some of the current TMA instruments and their specifications such as temperature and force ranges. Common to all instrument offerings are four probe types (expansion, penetration, three-point bending, and tension) and purge systems that accommodate common gases like nitrogen, helium, and air. As noted, some TMA systems can operate in vacuum or reactive gas environments. Data were collected from current instrument catalogs. 

TABLE 4.2 Some Available TMA Instruments and Their Specifications... 

The TMA instrument must be calibrated in height (or position), force, eigen-deformation (calibration of the probe when large forces are applied), temperature, and expansion (cell constant). As in any thermal technique, the purpose of these calibrations is to minimize the difference between the measured values of these parameters and their true values. These calibrations are instrument-specific, and they are described in detail in the respective manufactures instruction manuals. 

During the position calibration procedure, the displacement transducer used to measure position is calibrated against a height displacement standard (usually made from a hard material such as sapphire) with a precisely known height. Current TMA instruments can measure probe position changes as small as lOnm (see discussion of LVDT in Section 4.3.1). 

There are a number of approaches to temperature calibration. The accuracy of the temperature values in TMA, as in other thermal analysis techniques, is especially important. The next section describes temperature calibration of TMA instruments. 

The purpose of temperature calibration is to match the thermocouple readings to the true temperature. In a TMA instrument, a single thermocouple is used to control and measure the temperature of the sample in the furnace. The conditions at which the temperature calibration is carried out [i.e., the measurement mode (compression, tension, etc.), heating rate, flow rate of the purge gas, the purge gas itself, etc.] should be identical to those of the measurements on actual samples. Also, the geometry of the calibration standard should be as similar as possible to the geometry of the samples. This means that for optimum calibration, film standards must be used for measurements on films, pellets for pellets, and fibers for fibers (Lotti and Canevarolo 1998 Mano and Cahon 2004). 

ASTM E1363, test method for Temperature Calibration ofThemomechani-cal Analyzers, provides a standard method for calibrating TMA instruments (Seyler and Earnest 1991 and 1992). In the recommended ASTM method, three standard materials (phenoxybenzene, Pb, and In) are used. 

The expansion response of the TMA instrument can be checked with a standard reference material of defined thermal expansion. The Polymer Handbook (Brandrup 1999) and the Handbook of Chemistry and Physics (Lide 1998) provide tables of CLTEs versus temperature for numerous materials. Prime (1997) also lists CLTE values for three calibration standards (lead, aluminum, and copper) in the temperature range from -100 C to 180 °C. ASTM E831 describes the standard test method for measuring CLTE of solid materials by TMA. 

It should be noted that, in addition to traditional TMA, other techniques for measuring CLTE include capacitance change, laser interferometry, dielectric analysis, and elipsometry. These techniques, summarized by Menczel et al. (1997), are especially useful for very thin films, where the traditional TMA instruments reach their sensitivity limits. 

A feature of the instrument is a rotating analysis head, which can be oriented through 180° to adjust the analysis configuration for different test types and sample geometries. In addition to operation in the dynamic mechanical mode, the DMA 8000 operates in a constant-force (TMA) mode versus time or temperature. Applications such as thermal expansion coefficient, softening and penetration, or extension or contraction in the tension geometry provide data equivalent to those obtained by many conunerdal standalone TMA instruments. 

A flexure analysis accessory is available on TMA instruments, which allows the determination of the deflection (distortion) temperatures of polymers at selected temperatures and sample loading forces [plots of temperatnre versus flexure (mm)]. 

Thermomechanical Analysis (TMA) measures unidirectional dimensional changes in materials as functions of time, temperature and applied force. The TMA measurements are coefficient of linear thermal expansion (CLTE), glass transition temperatures (Tg) and softening points (Ts). Newer applications of TMA include elasticity, melt viscosity, and heat deflection temperature. In addition to traditional TMA instruments, many modem dynamic mechanical thermal analysis (DMTA) instruments can operate in a TMA (static force) mode. The main differences between the two types of instruments are the size of the specimens and the materials used to fabricate the measurement fixtures (stage, probe, clamps, etc.). Most TMA instruments use quartz, while DMTA instruments use larger steel components. The specimens used in these experiments are... 

A major source of temperature error is nonlinear temperature response, as was observed by both Seyler and Earnest [i] and Matsumori et al. [2], That is, a straight line cannot describe the differences between the literature melting points of known materials and the temperature reported by the TMA over large temperature intervals. This phenomenon limits the linear temperature corrections of E 1363 to a smaller temperature range. To achieve accurate temperatures over wider ranges, it is critical to calibrate the TMA instrument at relatively small intervals over the range of interest. 

---