Sample crucible

It can be concluded from equations 12.11 and 12.12 that the small deviation of the zero line relative to the isothermal baseline under the same scanning conditions is proportional to the heating rate and the difference in heat capacities of the two empty crucibles. This deviation can be positive (as in figure 12.4) or negative, depending on the magnitude of the intrinsic thermal asymmetry of the system under scanning conditions and the relative masses of the two crucibles. When the sample is introduced in the sample crucible,... 

First, the zero line is recorded using two empty crucibles. Next, a calibrant substance (usually alumina, i.e., synthetic sapphire) is placed in the sample crucible and the temperature program is repeated. Finally, the calibrant is replaced by the sample under study (keeping the crucible) and the temperature program run a third time. Based on equations 12.20 and 12.21, it can be concluded that the ordinate difference between the traces of the calibrant curve and of the zero line obtained for a given time t leads to the corresponding value of fcp ... 

The rate of heating of the sample influences volatile matter values and makes it necessary to calibrate equipment to achieve a satisfactory and reproducible heating rate. This calibration can be accomplished by using either a manual or an automatic mechanical device that lowers the sample crucible into the electrically heated furnace at a reproducible rate. 

Another measurement principle is the DSC, after Boersma [8]. In this case, no compensation heating is used and a temperature difference is allowed between sample crucible and reference crucible (Figure 4.5). This temperature difference is recorded and plotted as a function of time or temperature. The instrument must be calibrated in order to identify the relation between heat release rate and temperature difference. Usually this calibration is by using the melting enthalpy of reference substances. This allows both a temperature calibration and a calorimetric calibration. In fact, the DSC after Boersma works following the isoperibolic operating mode (see Section 4.2.2). Nevertheless, the sample size is so small (3 to 20 mg) that it is close to ideal flux. 

A series of isothermal DSC experiments were performed on a sample. The samples were contained in pressure-resistant tight gold plated crucibles. The oven of the DSC was previously heated to the desired temperature with the reference in place. At time zero, the sample crucible was placed in the oven. The maximum heat release rate and the time at which it appeared were measured. The results are summarized in Table 12.2. 

Figure 3.1 is a schematic of the differential thermal analyzer (DTA) design. The device measures the difference in temperature between a sample and reference which are exposed to the same heating schedule via symmetric placement with respect to the furnace. The reference material is any substance, with about the same thermal mass as the sample, which undergoes no transformations in the temperature range of interest. The temperature difference between sample and reference is measured by a differential thermocouple in which one junction is in contact with the underside of the sample crucible, and the other is in contact with the underside of the reference crucible.1 The sample temperature is measured via the voltage across the appropriate screw terminals (Vt,) and similarly for the reference temperature (Vrr) generally only one or the other is recorded (see section 3.5.1). Sample and reference... 

Figure 3.10 Effect of choice of x-axis temperature. Top x-axis corresponding to reference temperature. Middle idealized case for 2-axis corresponding to sample temperature (immersed thermocouple junction). Bottom x-axis corresponding to sample temperature (thermocouple junction in contact with underside of sample crucible).

The linear drop and exponential recovery shape of these transformations also appear in power-compensated DSC traces, but for different reasons. The temperature measuring device (RTD) measures its own temperature, which is influenced by all substances in the chamber, the housing, the sample crucible, as well as the melting sample. The device adds power to the sample side as needed to compensate for the cooling effect on the chamber due to sample melting. This energy requirement increases lineaxly since the setpoint sample temperature increases linearly. When melting is over, the need for extra heat flow to the sample chamber side drops exponentially as the chamber temperature quickly catches up to the setpoint. 

To simulate conditions existing in a slagging coal gasifier, all tests were carried out in a reducing atmosphere of 20% H2 and 80% N2. The gas mixture is injected into the furnace at a flow rate of 500 cc/ min through an alumina tube that extends to within 1 inch of the top of the sample crucible. 

Since measured viscosities will vary depending on the dimensions of and the materials employed for the sample crucible and the rotating bob, measured values are related to absolute viscosities by means of an instrument factor. In these studies, the instrument factor was determined by tests with National Bureau of Standards glass viscosity standards whose viscosities are precisely defined and similar to those of the slags over the temperature range of interest. 

As previously noted, reactions with the sample crucible tended to result in depletion of iron oxide species (carbon-crucible tests) or enrichment in AI2O3 (alumina-crucible tests), so that in some cases the composition of the slags was significantly different from that of the original coal ash. 

The experiments were carried out in a Netzsch STA 409 C (Simultaneous Thermal Analysis - STA) in the TGA/DSC configuration. The STA has a vertical san le carrier with a reference and a sample crucible, and in order to account for buoyancy effects, a correction curve with empty crucibles was first conducted and then subtracted from the actual experiments. Platinum/Rhodium crucibles were used in order to get the best possible heat transfer. The thermocouple for each crucible was positioned Just below and in contact with the crucible. The ten rerature obtained from the measurement is the temperature in the reference side. This temperature is converted to the temperature in the sample side by using the DSC-signal in pV and a temperature-voltage table for the thermocouple. The product gases were swept away by lOO Nml/inin nitrogen which exited the top of the STA, The STA was calibrated for temperature and sensitivity (DSC) with metal standards at each heating rate. 

Mass of sample, crucible, and cover after first heating... 

To obtain an aqueous solution of the analyte for analysis, it was necessary to dry ash the sample in air to convert its organic matrix to carbon dioxide and water. This process involved heating each crucible and sample cautiously over an open flame until the sample stopped smoking. The crucible was then placed in a furnace and heated at 555°C for 2 hours. Dry ashing served to free the analyte from organic material and convert it to arsenic pentoxide. The dry solid in each sample crucible was then dissolved in dilute HCl, which converted the AS2O5 to soluble H3ASO4. 

Desiccator A container that provides a dry atmosphere for the storage of samples, crucibles, and precipitates. 

The theory involved in the operation of a hot-stage/DSC accessory is presented. The material under investigation is placed into a transparent crucible and then into the hot-stage accessory, which has a DTA/DSC sensor. A second transparent crucible, which is similar to the sample crucible, is positioned in the hot stage for use as a reference. Upon initiation of the heating program, the sample equilibrates for a moment to allow the crucibles to reach the temperature of the furnace. Once the... 

The effect of sample packing on the DTA curve has been illustrated by Gruver (95) as shown in Figure 5.34. In curve (X), the kaolin sample was placed in the sample crucible and settled by a slight tapping action in curve (jB), the sample was tamped in place by use of a small glass rod. The curves obtained turned out to be identical. Admittedly, this is rather a crude method of testing this effect. 

Microscope objective Sapphire sample crucible Heat protection fitter Metal plate with heating wires Sapphire reference crucible... 

Sample crucibles are generally metallic (Al, Pt) or ceramic (silica) and may or may not have a lid. Many metal pans with lids have the lid crimped on using a special tool. Best results are obtained when the area of contact between the sample and the pan or crucible is maximized. Samples are generally in the 1-10 mg range for analytical applications. 

The useful acronym SCRAM (sample-crucible-rate of heating-fltmosphere-mass) will enable the analyst to obtain good, reproducible results for most thermal methods provided that the following details are recorded for each run ... 

In this case the sample thermocouple is placed above the sample crucible. There are two purge gas systems. One flow passes over the... 

In many TG experiments, the temperature of the furnace is raised at a constant rate. This type of experiment is referred to as non-isothermal, scanning or rising temperature. An alternative experimental technique is available, and is often used in kinetic studies. Instead of raising the temperature at a constant rate, the temperature is held constant and the mass loss (or mass gain) observed at this fixed temperature. The results are then presented as mass loss against time, t. In practice the sample has to be placed on the thermobalance and the furnace at first left away from the sample. The furnace is then run up to the required temperature and left to stabilise. When the furnace temperature is constant at the required value, the furnace has to be moved quickly around the sample. There are a number of difficulties with this technique. The sample, crucible, thermocouple and cradle have to move rapidly from room temperature to the experimental temperature. They all have a finite thermal capacity, so cannot heat instantaneously. There is a thermal lag while the sample temperature rises. The first part of this rise does not matter, because the reaction being studied will not occur rapidly at lower temperatures. However, as the reaction temperature is approached, some reaction will... 

How reproducible is the instrument signal when a sample crucible is removed and subsequently replaced in the apparatus ... 

The sample to be studied is placed in the sample holder or crucible, which is mounted on (or suspended from) the weighing arm of the microbalance. A variety of crucible sizes, shapes and materials used (Figure 4.4). The melting point of the crucible should be at least 100 K greater than the temperature range of the experiment and there must be no chemical reaction between the crucible and the sample. Crucibles are typically made from platinum, aluminium, quartz or alumina (a ceramic), but crucibles made from other materials are available. The crucible should transfer heat as uniformly and as efficiently as possible to the sample. The shape, thermal conductivity and thermal mass of... 

Model IV Effusion from the Face Surface of Sample/Crucible is r.d.s. and p = = constant If effusion from the sample s face surface (A,. = 2 cm ) would... 

An ideal DSC curve showing the change of heat flow process is in Fig. 6.42. Exothermic and endothermic peaks are marked as EX and EN. Due to the imbalance in the thermal capacities of the sample crucible, its contents etc., offset 0 is observed. The base line of the curve (B) is decided by the heat capacity of the sample. For a precise measurement, the baseline corrections can be made by comparing the empty and with the sample loaded pan data. 

An apparent mass loss is caused by the upper flowing stream of gas in the vicinity of the sample holder. The upper flowing stream can be reduced by changing the configuration of the sample holder (see Section 2.2.1). It is also important that the size and shape of the sample crucible are adjusted appropriately. In some cases, it is recommended that a small hole is made in the crucible (Figure 3.6). 

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