Phase-transition calibration materials

A more suitable method to calibrate the temperature behavior of an NMR probe involves the characterization of temperature-dependent melting points and phase transitions. Table 1 is a list of materials that can be applied for the calibration of variable-temperature NMR probes in the temperature range of 279-791 K. 

Another way to calibrate temperatures in NMR spectroscopy consists of investigating materials that lead to signals with temperature-dependent chemical shifts (shift thermometers). For the development of shift thermometers, the temperature-dependent chemical shift is compared with the occurrence of melting and phase transitions, allowing a temperature calibration with high accuracy over a broad temperature range. 

Melting Points and Phase Transition Temperatures of Materials Used for the Calibration of NMR Probes at Elevated Temperatures (8)... 

For materials applications, the chemical shifts of methanol and ethylene glycol can be monitored in the liquid state to follow temperature [Hawl]. The most sensitive ehemical shift is the Co resonance of aqueous Co(CN)e with a sensitivity of 0.05 K at 7 T and 0.2 K at 2T [Dorl]. Furthermore, dibromomethane dissolved in a liquid crystal is a temperature sensitive NMR compound [Hed 1 ], and known phase-transition temperatures can be exploited to calibrate the temperature control unit [Hawl J. In temperature imaging of fluids, temperature can be determined from the temperature dependence of the selfdiffusion coefficient but convective motion may arise in temperature gradients [Hedl]. In the solid state, the longitudinal relaxation time of quadrupolar nuclei like Br is a temperature sensitive parameter [Suil, Sui2]. In elastomers, both T2 and Ti depend on temperature (Fig. 7.1.13). In filled SBR, T2 is the more sensitive parameter with a temperature coefficient of about 30 xs/K [Haul]. 

Of course, lattice parameters can also be used to study the effects of post-synthesis treatment (e.g. ion exchange, calcination, dealumination, sorption, etc.), to estimate Si/Al ratios in well-calibrated systems such as faujasite, to monitor a phase transition as a function of temperature, or to begin the structural characterization of a new material. Indexing a pattern can also serve to establish whether or not a phase is pure. If all lines can be indexed on a single unit cell, there is probably only one crystalline phase present. However, if there are unindexed lines, either the indexing is incorrect or there is a crystalline impurity present. 

Charrier et al. accurately measured the equilibrium temperature of the solid-solid phase transition of high purity caesium nitrate by stepwise heating and by the method of extrapolation to zero heating rate. The authors reported a mean value of 154.3 0.1 C, that was obtained by using two different heat-flux DSC instruments. Because the transition temperature of caesium nitrate is close to that of the fusion of indium, indium can be used as a single-point calibrant. This approach decreases the error that may arise when the calibration value for the material of interest is determined by interpolation between two-point or multiple-point calibrations [66],... 

Temperature calibration is achieved using standard reference materials whose transition temperatures are well characterized (Appendices 2.1 and 2.2) and in the same temperature range as the transition in the sample. The transition temperature can be determined by DTA, but the enthalpy of transition is difficult to measure because of non-uniform temperature gradients in the sample due to the strueture of the sample holder, which are difficult to quantify. This type of DTA instrument is rarely used as an independent apparatus and is generally coupled to another analytical instrument for simultaneous measurement of the phase transitions of metals and inorganic substances at temperatures greater than 1300 K. 

The main developments in experimental techniques for measuring high pressure to obtain reliable pressure sensors are extensively discussed by Decker et al. [42]. These include (1) the establishment of a primary pressure scale using a free piston gauge (2) the selection and precise measurement of identifiable phase transitions as fixed pressure points and (3) the use of interpolation and extrapolation techniques for continuous-pressure calibration based on changes in resistance, volume, or optical spectra (based on an equation of state). An alternative method of estimating absolute pressure in isotropically compressed materials is based on measurements of ultrasonic velocity [43, 44]. 

As noted previously, these considerations apply strictly to the steady state only - hence by no means to ordinary chemical or physical reactions that generate fast changes of the heat flow to the sample thus disturbing the steady state (a steady state develops, for example, when a long-lived radioactive material serves as the sample). The proportionality coefficients (calibration factor) that apply to the non-steady state must be determined in calibration tests. Under certain circumstances, they depend on the temperature course Ts(t) during the reaction (see Hbhne, Hemminger, and Flammersheim, 2003). It becomes obvious again that the temperature-time profile of these calibration experiments must match that of the sample reaction as closely as possible to ensure an accurate calibration for the particular non-steady state. Enthalpy standards with fast phase transitions may yield a different calibration factor to standards with slower delivery of heat. 

The pressure generated in a sample chamber is first calibrated against the supplied load in reference to the pressure fixed points listed in Table 5. This pressure calibration is carried out at room temperature by detecting changes in the electric resistance of these standard materials as they transform to their denser phases. For accuracy of temperature and pressure at elevated temperatures, an additional cahbration is done utilizing melting of gold, silver, copper, and various solid-state transitions. 

We adopt 652 3 k as the transition temperature of stoichiometric NiS from rhombohedral (B) to hexagonal (a) form base on the phase diagram of Kullerud and Yund (8). The temperature of this transition is very dependent on the exact stoichiometry of the material (8 ). The transition enthalpy has been measured via a DTA technique by Conard et al. (7) and we adopt their value of 1.54 0.1 kcal mol". This is considerably higher than an older value of 0.63 kcal mol measured by Biltz et al. (9, 5) but should be much more accurate due to the calibration technique used. Mah and Pankrantz ( 4) estimated 0.7 kcal mol. This transition enthalpy cannot be measured by drop calorimetry since the a form does not revert to the B form on cooling. 

We examine first the high pressure and temperature phase boundary between the tetrahedrally coordinated (coesite) and the octahedrally coordinated (stishovite) Si02 polymorphs. The phase diagram of Si02 is the focus of intense interest, because it has been shown recently that stishovite is the hardest known quenehable oxide and is a member of an emerging new family of superhard materials based on the AO2 formula (A = Si, Os, Ru, Mn, Sn, Ge, Pb) [51,52] the coesite-stishovite transition is used for pressure calibration in multianvil devices. However, there... 

Now, it has been shown for materials such as poly(propylene diol) (wherein both the absorption maximum for loss shear modulus and loss permittivity overlap near the frequency of IHz) that their normalized curves perfectly superimpose over their frequency band width. - As shown in Figure 9.15, the lower frequency loss shear modulus curves uniquely overlap with the loss permittivity data at higher frequency. As such the former is melded to calibrate the loss permittivity data to obtain a coarse estimate of the elastic modulus values. This provides an independent demonstration of the mechanic il resonance near 3 kHz and also allows reference to the 5 MHz dielectric relaxation as a mechanical resonance. Thus, as the folding and assembly of the elastic protein-based polymers proceed through the phase (inverse temperature) transition, the pentamers wrap up into a structurally repeating helical arrangement like that represented in Figure 9.17. 

JOSEPH D. MENCZEL PhD. a recognized expert in thermal analysis of polymers with some thirty years of industrial and academic experience, is Assistant Technical Director at Alcon Laboratories. He has researched more than 120 polymeric systems in which he studied calibration of DSCs, glass transition, nucleation, crystallization, melting, stability, mechanical and micromechanical properties of polymers, and polymer-water interactions. Dr. Menczel holds six patents and is the author of seventy scholarly papers. He is the author of two chapters in the book Thermal Characterization of Polymeric Materials In conducting DSC experiments, Dr. Menczel found a crystal/amorphous interface in semicrystalline polymers, which later became known as the rigid amorphous phase. He is also credited with developing the temperature calibration of DSCs for cooling experiments,... 

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