Mihai Chirtoc

Assessment of calibration procedures for accurate determination of thermal parameters of liquids and their temperature dependence using the photopyroelectric method

We present a systematic theoretical and experimental investigation on the accuracy of thermal diffusivity α and thermal effusivity e of liquids measured by the photopyroelectric (PPE) method in back-detection configuration (BPPE). Special cases corresponding to different cell structures are analyzed in terms of error determination of α and e for water and ethylene glycol. We propose a new normalization procedure allowing for estimation of these parameters with accuracy of 2% on α and 5% on e over extended frequency range. The normalization eliminates the frequency-dependent influence of the transducer impedance and associated electronics, reduces the errors due to coupling fluid between cell components, and reduces the number of temperature-dependent parameters that must be known in order to characterize the sample. Technical solutions for improving the performances are suggested. Another goal of the study was to demonstrate the possibility of the BPPE method to yield small variations of thermal parameter...

Evidencing Molecular Associations in Binary Liquid Mixtures via Photothermal Measurements of Thermophysical Parameters

Abstract The back photopyroelectric (PPE) configuration, with opaque sample and thermally thick sample and sensor, was applied in order to obtain room temperature values of the thermal diffusivity of some liquid mixtures. The methodology is based on a samples thickness scan, and not on a frequency scan as is usually the case. The possibility of precisely controlling the samples thickness variation leads to very accurate and reproducible values for the thermal diffusivity. The method is proposed as an alternative for evidencing molecular associations in binary liquid mixtures.

Direct Determination of Thermophysical Parameter √ Kpc in Mayonnaise, Shortening, and Edible Oil

Inverse photopyroelectric (IPPE) spectroscopy, a novel variant among the photothermal spectroscopies, has been used for direct determination of (Kρc)½ in a variety of food samples. The method appears capable of thermal characterization, and when combined with more mature techniques may prove useful in food products industry and storage.

Operation theory of pyroelectric detectors

A new physical model of the pyroelectric detectors of radiation is presented and the expressions for the voltage response are derived in a unitary manner under limiting operating conditions, for every modulation function of the radiation. It is found that the complete set of operation modes is described by transfer functions which are either linear, or contain an integral or a differential of the input function. The operation mode is completely determined by three parameters: the duration of the irradiation sequence, and the thermal and electrical time constants of the detector.

Quantitative scanning thermal microscopy based on determination of thermal probe dynamic resistance

Resistive thermal probes used in scanning thermal microscopy provide high spatial resolution of measurement accompanied with high sensitivity to temperature changes. At the same time their sensitivity to variations of thermal conductivity of a sample is relatively low. In typical dc operation mode the static resistance of the thermal probe is measured. It is shown both analytically and experimentally that the sensitivity of measurement can be improved by a factor of three by measuring the dynamic resistance of a dc biased probe superimposed with small ac current. The dynamic resistance can be treated as a complex value. Its amplitude represents the slope of the static voltage-current U-I characteristic for a given I while its phase describes the delay between the measured ac voltage and applied ac current component in the probe. The phase signal also reveals dependence on the sample thermal conductivity. Signal changes are relatively small but very repeatable. In contrast, the difference between dynamic and static resistance has higher sensitivity (the same maximum value as that of the 2nd and 3rd harmonics), and also much higher amplitude than higher harmonics. The proposed dc + ac excitation scheme combines the benefits of dc excitation (mechanical stability of probe-sample contact, average temperature control) with those of ac excitation (base-line stability, rejection of ambient temperature influence, high sensitivity, lock-in signal processing), when the experimental conditions prohibit large ac excitation.

Temperature-dependent quantitative 3ω scanning thermal microscopy: Local thermal conductivity changes in NiTi microstructures induced by martensite-austenite phase transition

We develop the theoretical description of 3omega signals from the resistive Wollaston thermal probe (ThP) of a scanning thermal microscope (SThM) in terms of an equivalent low-pass filter. The normalized amplitude and phase frequency spectra are completely characterized by a single parameter, the crossover frequency f(c)(k) depending on the sample thermal conductivity k. The application concerns polycrystalline NiTi shape memory alloy microstructured by focused Ga ion beam milling and implantation. The calibration of the ThP combined with a novel two-step normalization procedure allowed quantitative exploitation of 3omega signal variations as small as -1.75% in amplitude and 0.60 degrees in phase upon heating the sample from room temperature to 100 degrees C. This corresponds to k increase of 23.9% that is consistent with the expected thermal conductivity variation due to martensite-austenite structural phase transition. To our knowledge this is for the first time that SThM 3omega phase information is used quantitatively as well. The static, calibrated 3omega measurements are complementary to 3omega SThM images of the patterned sample surface. The local SThM measurement of temperature-dependent thermal conductivity opens the possibility to imaging structural phase transitions at submicron scale.

INFRARED TRANSIENT THERMOGRAPHY FOR NON-CONTACT, NONDESTRUCTIVE INSPECTION OF WHOLE AND DISSECTED APPLES AND OF CHERRY TOMATOES AT DIFFERENT MATURITY STAGES

Introduction Fruits and vegetables used in various processed products, as well as in fresh market sales, are expected to comply with the certain quality standards. However. quality is everything but a single parameter state and, therefore, a variety of subjective and objective measurements are usually performed. In particular, the firmness and maturity are considered important attributes to the quality of fruits and vegetable crops. Presently. these parameters are usually determined either destructively (by a pressure tester) or by means of ultrasound; a development of alternative methods has been strongly encouraged in the recent years.

Photopyroelectric spectroscopy: absorption, transmission or reflectance?

A unitary theoretical interpretation is given to various PPE spectroscopic experimental data containing spectral inversions and anomalies. The discussion is made in terms of newly introduced PPE sensitivity factors.

Spectrally resolved modulated infrared radiometry of photothermal, photocarrier, and photoluminescence response of CdSe crystals: Determination of optical, thermal, and electronic transport parameters

Spectrally resolved modulated infrared radiometry (SR-MIRR) with super-band gap photoexcitation is introduced as a self-consistent method for semiconductor characterization (CdSe crystals grown under different conditions). Starting from a theoretical model combining the contributions of the photothermal (PT) and photocarrier (PC) signal components, an expression is derived for the thermal-to-plasma wave transition frequency ftc which is found to be wavelength-independent. The deviation of the PC component from the model at high frequency is quantitatively explained by a quasi-continuous distribution of carrier recombination lifetimes. The integral, broad frequency band (0.1u2009Hz–1u2009MHz) MIRR measurements simultaneously yielded the thermal diffusivity a, the effective IR optical absorption coefficient βeff, and the bulk carrier lifetime τc. Spectrally resolved frequency scans were conducted with interchangeable IR bandpass filters (2.2–11.3u2009μm) in front of the detector. The perfect spectral match of the PT an...

Thermal Conductivity Profile Determination in Proton-Irradiated ZrC by Spatial and Frequency Scanning Thermal Wave Methods

Using complementary thermal wave methods, the irradiation damaged region of zirconium carbide (ZrC) is characterized by quantifiably profiling the thermophysical property degradation. The ZrC sample was irradiated by a 2.6u2009MeV proton beam at 600u2009°C to a dose of 1.75 displacements per atom. Spatial scanning techniques including scanning thermal microscopy (SThM), lock-in infrared thermography (lock-in IRT), and photothermal radiometry (PTR) were used to directly map the in-depth profile of thermal conductivity on a cross section of the ZrC sample. The advantages and limitations of each system are discussed and compared, finding consistent results from all techniques. SThM provides the best resolution finding a very uniform thermal conductivity envelope in the damaged region measuring ∼52u2009±u20092u2009μm deep. Frequency-based scanning PTR provides quantification of the thermal parameters of the sample using the SThM measured profile to provide validation of a heating model. Measured irradiated and virgin thermal con...