M. J. Tello

New experimental device for infrared spectral directional emissivity measurements in a controlled environment

A new experimental device for infrared spectral directional emissivity measurements in a controlled atmosphere is presented. The sample holder, which permits to measure spectral directional emissivity up to 1050K, is placed inside a stainless steel sample chamber that can be evacuated or filled with different gases. The signal detection is carried out by means of a Fourier transform infrared spectrometer. The experimental results focus on the capability of the device to perform emissivity measurements as a function of temperature, emission angle, and in situ surface state evolution. A careful study of the sample temperature homogeneity and the measurement method has been done, including the background radiation, the apparatus response function, and temperature differences between the sample and the blackbody radiator. As a consequence, a compact expression for the sample emissivity that generalizes those previously obtained for the direct radiometric measurement method is found. The error assessment shows...

Radiation thermometry applied to temperature measurement in the cutting process

Temperature measurement of cutting tools used in machining processes has great technological importance, and it is interesting in a large number of industrial applications because wear is directly related to this variable. The influence of emissivity on the temperature measurement using radiation thermometers and the dependence of the measured temperature on the emissivity as a function of the surface roughness and the oxidation state is studied in this paper. Emissivity is measured using the direct radiometric method for uncoated P10 tungsten carbide inserts. Theoretical temperature shifts produced by changes in emissivity are estimated for several types of radiation thermometers, and these shifts are compared to the experimental temperature measurements carried out in the orthogonal turning process of cylindrical samples of 42CrMo4 steel with different machinability grades.

A new method for measuring the ferroelectric behaviour of crystals at low frequencies

Reports a method for investigating ferroelectric crystals which permits the simultaneous measurement of three parameters, polarisation, dielectric susceptibility and conductivity, from a unique I=I(V) hysteresis loop for each temperature, frequency and applied field. Using a triangular signal, problems related to compensation of hysteresis loops and elimination of spurious contributions can be clarified. Therefore, the measuring method and temperature control proposed permit a detailed study of the critical behaviour. As an example, TGS measurements are carried out.

Influence of the carbon chain and metal ions on the structural phase transitions in chloride perovskite layer structures

Abstract Calorimetric results about the sequence of Structural Phase Transitions G0 → G1 → G2 in (CnH2n+1NH3)2MCl4 compounds with n=1 and 2 and M= Mn and Cd show a very low entropy for the G0 → G1 change in both Mn and Cd compounds. For the G1 → G2 change the results show a first order character as well as a very different nature between the compounds with n=1 and n=2. A first calorimetric study about the phase transitions in (NH3-(Ch2)n-NH3)CdCl4 compounds is also presented.

Diffraction symmetry of incommensurate structures

The symmetry of the diffusion pattern for an incommensurate displacive structure is analysed in a general context. The rotational symmetry and systematic extinctions in the diffraction diagram of this type of structure are shown to be directly based on a particular transformation property of the amplitudes of the atomic modulation waves intervening in the distortion for a subgroup of the basic structure space group. Hence a general three-dimensional framework can be established, where diffraction symmetry can straight-forwardly be used to restrict and relate the amplitudes corresponding to the atomic modulation waves of the distortion. It is shown how a full direct exploitation of diffraction symmetry can therefore be accomplished to determine the characteristics of the incommensurate distortion, without referring to the superspace group formalism. The equivalence of the present simpler approach with the latter is then demonstrated. Finally a concept of atomic scattering modulation factors is proposed to reduce the expression of the structure factor for these phases to a very simple form analogous to those used in commensurate cases.

Combined standard uncertainty in direct emissivity measurements

In order to design a device to carry out direct emissivity measurements, a key point is the analysis of all the uncertainty components that give rise to the combined standard uncertainty. This will permit to choose the most appropriate measurement method to minimize the uncertainty, and also to identify the sources of the largest errors. If the experimental device is already in use, the complete uncertainty characterization, in addition to the emissivity uncertainty calculation, will permit the improvement of the device capabilities. Thus, a guideline to the experimentalists working in this subject is provided. In this work, a complete study of the uncertainty components in direct emissivity determination is carried out. First of all, the emissivity measurement method and the uncertainty estimation methodology are introduced. After that, the influence of the uncertainties of each of the magnitudes used to obtain the emissivity is analyzed theoretically. The most important error sources depending on the me...

Analysis of calibration methods for direct emissivity measurements

We analyze two important points related to the experimental emissivity measurements in this paper: the radiometer calibration accuracy and its stability to determine the required frequency of calibration. The usual two-temperature calibration method is compared to a more accurate method, which uses the measurement of blackbody radiation at several temperatures. Additionally, the suitability of the two-temperature method is studied as a function of the gap between both temperatures. Differences higher than 200°C are needed to obtain an acceptable calibration. The temporal stability of the calibration and the influence of the environmental conditions are also analyzed.

Dielectric and optical studies near the chiral smectic C-smectic A transition of (S)-2-hydroxy-4-decyloxybenzylidene-4′-amino-2p″-methylbutylcinnamate

Abstract The complex dielectric permittivity of HDOBAMBC has been measured in the vicinity of the chiral smectic C-smectic A transition as a function of temperature and frequency. The Goldstone mode contribution has been clearly identified and its relaxation frequency has been determined over the whole S*C range. High accuracy tilt angle measurements have also been performed by using a new method which allows its determination in a direct and virtually continuous way. These experimental results together with additional measurements of the polarization and helical pitch have been compared with those of DOBAMBC and analized in the framework of the generalized Landau theory. As for DOBAMBC a reentrant behaviour S*C-SC-S*C has been observed under an electric field near the S*C-SA transition.

Thermal hysteresis and memory effect in the ferroelectric incommensurate tetramethylammonium tetrachloro cobaltate

Abstract Dielectric susceptibility measurements checking thermal hysteresis and memory effects in (N(CH 3 ) 4 ) 2 CoCl 4 crystals in the temperature range 273–298K are reported. The differences between the values measured in the heating and cooling curves include a definite difference between the dielectric susceptibility maxima when entering and exiting the ferroelectric phase. Thermal hysteresis loops constructed in one of the incommensurate phases differ Considerably from those previously reported in Rb 2 ZnCl 4 . Characteristic memory effects could also be observed with the same order of magnitude as those found in other incommensurate materials.

Critical study of the commensurate-incommensurate phase transition in the ferroelectric Rb2ZnCl4

Abstract Specific heat measurements confirm the existence of two successive phase transformations in Rb2ZnCl4 in the 48 K to 330 K temperature range. The critical exponents near Ti show that the critical behaviour for the paraelectric-incommensurate phase transition is that of the XY model.