Investigation of the dielectric properties of BaTiO3 single crystals of different qualities by the thermal noise method

Comparative dielectric investigations of bulk barium titanate crystals grown using different techniques have been performed by the thermal noise and traditional methods. The temperature dependences of the permittivity have been used to determine the phase transition temperatures and the Curie-Weiss constants. The investigation performed using the thermal noise method has revealed a number of phenomena that complicate the separation of the noise associated with the change in the impedance of the sample. Possible approaches to the elimination or minimization of contributions from different effects to the total noise voltage have been described. The piezoelectric resonances that also contribute to the total noise signal have been found for the first time and investigated by the thermal noise method. The possible nature of the obtained temperature dependences of the piezoelectric resonance frequencies has been discussed.     

The influence of heat exchange between a sensitive element and its surroundings on the specific detectivity of pyroelectric detectors

The specific detectivity of pyroelectric sensors at low frequencies is mainly influenced by the thermal conditions within the sensor. The temperature noise caused by heat exchange between the sensitive element and its surroundings is transformed into an increase of the dielectric loss due to electrothermal coupling. The complex normalized current responsivity is used to calculate the influence of thermal conditions. While the absolute value ¦T_R¦ gives the frequency response of the sensitivity, the additional dielectric loss tan δ_T is deduced from the imaginary part. The loss due to electrothermal coupling tan δ_T exceeds the dielectric intrinsic loss tan δ_i for most sensor structures within the frequency range up to 100 Hz. Thus, the maximum attainable specific detectivity of a pyroelectric sensor at normal operation frequencies is dependent on its construction rather than on the material parameter dielectric intrinsic loss tan δ_i which is frequently referred to. The effect of electrothermal coupling can be employed in sensors with reduced sensitivity at low frequencies as used for FTIR-devices.     

Three-dimensional tracking of small spheres in focused laser beams: influence of the detection angular aperture

Back-focal-plane interferometry is a method capable of determining the three-dimensional position of a particle with high precision (< 3 nm) at high sampling rates (1 MHz). We investigated theoretically the performance of such a system for dielectric spheres with diameters D = 0.53-3 microm and for metallic spheres with D < or = 300 nm. Good sensitivity and linearity were achieved for a detection angular aperture sin(alpha) of no more than 0.5. A value of sin(alpha) > 0.7 should be used only for dielectric spheres with diameters approximately equal to the laser wavelength. Harmonic optical traps can be calibrated by measurement of the thermal motion of the sphere. We performed Brownian dynamics simulations and subsequent thermal noise analyses to prove that the wrong sin(alpha) incorrectly suggests an increased and nonharmonic axial trapping potential.     

Concrete bridge-borne low-frequency noise simulation based on train–track–bridge dynamic interaction

Both the vibration of a railway bridge under a moving train and the associated bridge-borne noise are time-varying in nature. The former is commonly predicted in the time domain to take its time-varying and nonlinear properties into account, whereas acoustic computation is generally conducted in the frequency domain to obtain steady responses. This paper presents a general procedure for obtaining various characteristics of concrete bridge-borne low-frequency noise by bridging the gap between time-domain bridge vibration computation and frequency-domain bridge-borne noise simulation. The finite element method (FEM) is first used to solve the transient train–track–bridge dynamic interaction problem, with an emphasis on the local vibration of the bridge. The boundary element method (BEM) is then applied to find the frequency-dependent modal acoustic transfer vectors (MATVs). The time-domain sound pressure is finally obtained with the help of time–frequency transforms. The proposed procedure is applied to a real urban rail transit U-shaped concrete bridge to compute the bridge acceleration and bridge-borne noise, and these results are compared with the field measurement results. Both sets of results show the proposed procedure to be feasible and accurate and the dominant frequencies of concrete bridge-borne noise to range from 32 Hz to 100 Hz.     

A study of thermal, dielectric and magnetic properties of strontium malonate crystals

Crystals of strontium malonate (SrC₃H₂O₄) were grown in silica gel by the single diffusion technique. The thermo gravimetric (TG), differential thermal analysis (DTA) and differential scanning calorimetric (DSC) studies were carried out to investigate the thermal stability of the crystal. The dielectric behavior of the title compound crystal was investigated by measuring the dielectric parameters - dielectric constant, dielectric loss and AC conductivity as a function of four frequencies −1 kHz, 10 kHz, 100 kHz and 1 MHz at temperatures ranging from 50 to 170 °C. Results indicate that the title compound is thermally stable up to about 409 °C and is a promising low ε_r-value dielectric material. The magnetic behavior of the crystal was also explored using a vibrating sample magnetometer.     

The evidence of the existing and the parameters of the second region of water dipole relaxation

In order to explain experimental data of dielectric properties of water in the microwave region at temperature from 0 up to 90 °C and at frequencies from 9.5 to 75.4 GHz, we propose a new method. This method is based on normalizing all considered experimental data and presenting them in a sole Cole–Cole plot. The normalizing relation is derived with supposition that there exists a sole Debye dispersion region. As the result of this normalization, we present the data of various experimental groups made at various temperatures and frequencies, and, hence, we get the possibility to process at the same time various experimental data. The analysis carried out in wide frequency range from ωτ1 to ωτ≈10 showed that dielectric permittivity of water cannot be completely explained in the model of a sole Debye relaxation region. It was shown that it is enough to suppose two relaxation Debye regions. The processing of experimental data allowed us to determine the parameters of these regions.     

Synthesis and Characterization of Fluorinated PBO With High Thermal Stability and Low Dielectric Constant

A series of novel fluorinated benzoxazole polymers (6FPBO) with high thermal stability and low dielectric constant were synthesized by copolymerization of 1,3-diamino-4, 6-dihydroxybenzene dihydrochloride (DAR), purified terephthalic acid (PTA) and various amount of 4′4-(hexafluoroisopropylidene) bis(benzoic acid) (BIS-B-AF) in the medium of polyphosphoric acid (PPA). 6FPBO fibers were then obtained via dry-jet wet-spinning technique and characterized by Fourier transform infrared (FTIR) spectra, thermogravimetric analysis (TGA), broadband dielectric spectrometer, single fiber tensile testing machine, and scanning electron microscopy (SEM). The FTIR spectrum of 6FPBO fibers indicated that the fluorine groups were incorporated into PBO molecular chains successfully. TGA curves revealed that 6FPBO fibers possessed high thermal stability, just as pure PBO fibers. Moreover, the dielectric constant spectra of 6FPBO showed that the polymers had low dielectric constant, especially in the range of high-frequency.     

Influence of thermal degradation and saline exposure on dielectric permittivity of polyimide

This paper studies the relationship between dielectric permittivity and mechanisms of thermal degradation of polyimide (PI) film in air and saline exposure. The real permittivity and loss factor of dried PI were measured at temperatures from −140 to 180 °C and frequencies from 1 Hz to 1 MHz. Two peaks in the temperature-dependent loss factor revealed the occurrence of β- and γ-relaxations in PI. Following thermal degradation at 475 °C for 3 h, the real permittivity of PI was observed to increase by 14% because of the formation of free radicals. The intensity of β-relaxation was also greatly increased after thermal degradation due to scission of chemical bonding in imide groups. Activation energy obtained from the Arrhenius law for γ-relaxation did not change after thermal degradation. For samples exposed to either distilled water or salt water, the variation in salinity did not significantly influence the dielectric permittivity.     

Growth,structural, mechanical,spectral and dielectric characterization of NaCl-added Triglycine sulfate single crystals

Pure and sodium chloride (NaCl)-added Triglycine sulfate (TGS) crystals were grown from aqueous solutions by slow evaporation technique. The values of concentration of dopants in the mother solution were 0.2, 0.6 and 1 mol%. The solubility of the grown samples have been found out at various temperatures. The determination of unit cell parameters was carried out by single crystal XRD method and found that the grown crystals crystallize in monoclinic structure. The dielectric characterization for the pure and NaCl-doped TGS crystals was performed by measuring the dielectric parameters like dielectric constant and dielectric loss with various frequencies in the range 10²–10⁶ Hz and with the temperatures ranging from 30 to 70 °C and this study reveals an increase of dielectric constant and loss with the increase of NaCl concentration. Studies on mechanical properties like microhardness and density of the grown pure and NaCl-doped TGS crystals were carried out. UV–Visible transmittance studies were carried out for the grown samples. A sharp fall in the transmittance is observed at 228 nm for pure and NaCl-doped TGS crystals. Atomic absorption spectroscopic (AAS) study was done on the NaCl-doped TGS crystals to ascertain the presence of Na⁺ ions in the lattice.     

Noise in the Helical Edge Channel Anisotropically Coupled to a Local Spin

We calculate the frequency-dependent shot noise in the edge states of a two-dimensional topological insulator coupled to a magnetic impurity with spin S = 1/2 of arbitrary anisotropy. If the anisotropy is absent, the noise is purely thermal at low frequencies, but tends to the Poisson noise of the full current I at high frequencies. If the interaction only flips the impurity spin but conserves those of electrons, the noise at high voltages eV ≫ T is frequency-independent. Both the noise and the backscattering current I_bs saturate at voltageindependent values. Finally, if the Hamiltonian contains all types of non-spin-conserving scattering, the noise at high voltages becomes frequency-dependent again. At low frequencies, its ratio to 2eI_bs is larger than 1 and may reach 2 in the limit I_bs→0. At high frequencies, it tends to 1.

     

Experimental investigation on the sound pressure level for a high thermal capacity burner during a running cycle

In many research or technical expertise studies the maximum noise level of a boiler is associated with the maximum thermal load of the burner. However, this type of air injected burners presents a complex running cycle with different functioning periods, where different parts (engine, fan, flame) of the burner are running separately or in the same time. In this study we are focused on the analysis, by experimental measurements, of the entire functioning cycle of a boiler by pointing out the noise differences and their importance when doing an experimental acoustical investigation. The entire 1/3 octave spectrum of the sound pressure level (SPL) was recorded during a complete running cycle by means of logging software associated to the sound meter. The sound equivalent level was calculated for each period of the running cycle and compared to the norms and with two theoretical prediction models that take into account the heating power and the boiler room volume. It was found that the most accurate data are obtained when the measurements are done in one-third octave. The maximum noise level was established to be not for the maximum thermal load period, but for the ventilation period of the boiler (before gas injection) with 82.1 dB at 125 Hz. A shut down delay was detected at the end of the cycle with 13 s for higher frequencies, due to the vibration of the boiler parts. Two 3D graphical representations point out the most important frequencies characterizing each running state of the burner. Compared to the noise curve (NC85) the minimum differences between the admissible values and the ones produced by the burner were found to be around 5.5 dB and therefore no acoustical treatment was needed. The results of the SPL prediction models matched the experimental data only for some of the boiler cycle periods and for only some of the frequencies. This type of detailed experiment investigation of the burner noise highlights the periods of the running cycle and the frequencies where the noise level requires acoustical treatment.     

Studies on growth and characterization of l-alanine strontium chloride trihydrate single crystals for optical applications

The optically transparent nonlinear optical single crystals of l-alanine strontium chloride trihydrate (LASCT) were grown by slow evaporation solution growth method using water as solvent. The purity of the crystals was increased by the method of recrystallization. The grown crystals were analyzed by the single crystal X-ray diffraction pattern which proved that LASCT belongs to monoclinic crystal system. The presence of various functional groups and modes of vibrations were identified by FTIR spectroscopy. The optical absorption study confirms the suitability of the crystal for device applications. The thermal strength and the decomposition of the grown crystals were studied using TG/DTA analyses. The dielectric constant and dielectric loss measurements of the grown crystals at different frequencies of the applied field were measured and reported. The mechanical strength of the crystal is estimated by Vicker's hardness test. The nonlinear optical properties of the grown crystals were confirmed by second harmonic generation test which shows the suitability of NLO applications.     

The effect of rubber dampers on engine’s NVH and thermal performance

Fins as extended surfaces are attached to the internal combustion engine surfaces for enhancing the heat transfer. However, these fins vibrate at various frequencies, which produce undesirable radiated noise. To mitigate this effect, automobile industry inserts rubber dampers between these fins. These rubber dampers reduce the fins’ amplitude of vibration and thus reduce the radiated noise from the fin surfaces. Investigations on the effect of rubber dampers on the engine’s NVH (Noise–Vibration–Harshness) and thermal performance using numerical (FEM and CFD) and experimental measurement have been presented in this paper. Experiments were conducted in the semi-anechoic chamber on an engine with and without rubber dampers to measure the radiated noise from the fins. It was found that rubber dampers assist in reducing engine high frequency noise signals at higher engine speeds. Modal and harmonic response analysis was carried out on various designs for NVH characteristics improvement. Prototypes of the final design was made and tested for the NVH performance. Computational fluid dynamics (CFD) simulations were performed on engine with and without rubber dampers to investigate the thermal performance. It was found that rubber dampers increase engine temperature by about 10%. Effect of rubbers dampers on the cost and environmental impact has also been discussed. This paper provides a systematic procedure to investigate the effect of rubber dampers and a method to eliminate these dampers from the engines with the same NVH and better thermal performances.     

Optimization method based on Generalized Pattern Search Algorithm to identify bridge parameters indirectly by a passing vehicle

Generalized Pattern Search Algorithm (GPSA) has rarely been investigated for structural health monitoring, but may have potential application in civil engineering, because it does not require any gradient information of the objective function. Meanwhile, indirect identification is an attractive concept that recognizes the bridge parameters by the vehicle responses. This paper proposes a theoretical indirect identification method based on optimization method, and the implementation is performed by the GPSA. Firstly, the GPSA theory is investigated, and a simple example is employed to describe the process of the algorithm. Secondly, a theoretical indirect identification method is proposed, based on the optimization method rather than the conventional transforms from time domain to frequency domain. The proposed method can identify the parameters of the vehicle–bridge system, including the bridge stiffness and the 1st frequency. Based on the optimization method, the feasibility and accuracy of GPSA are demonstrated with 0.06% of errors. The GPSA shows good robustness in the identifications with various noise levels, and the maximum error is about 3.30% and can be accepted for the engineering application even with a SNR 5 noise level. The computation time relies only on the function evaluation times, and is not positively related to the noise level. Thirdly, the performance of GPSA is compared with that of Genetic Algorithm (GA). The accuracy of GPSA and GA are approximately equivalent with various noise levels. Compared with GA, GPSA needs fewer iterations and much fewer evaluations, therefore is more efficient in the identification with an almost consistent accuracy with various noise levels.     

A Method for Determining the Dielectric Constant of Microwave PCB Substrates

This paper presents a simple method for determining the dielectric constant of microwave PCB substrates. In the presented method, a bandpass microstrip filter designed on the PCB substrate with a user-predicted dielectric constant value is implemented for a given center frequency. The simulation results of the designed bandpass filter are obtained by the help of microwave design software; XFDTD?. Experimental results regarding the filter frequency characteristic are accomplished by means of a vector network analyzer. The simulation results of the designed filter are modified to overlap with the experimental ones by varying the dielectric constant value. When the simulation and experimental results are overlapped, the value of dielectric constant is accurately selected. In order to illustrate the validity of proposed method, the dielectric constant values of flame resistant-4 (FR4) substrates are acquired at IEEE 802.11b/g and IEEE 802.11a wireless local area network (WLAN) application frequencies. The results obtained by using the presented method agree with the previous studies in the literature.     

Synthesis,spectral, thermal,optical and dielectric studies on 4-dimethylaminopyridinium picrate crystals

Single crystal of 4-dimethylaminopyridinium picrate was grown by slow evaporation solution growth method. The optical properties of the crystal were studied by using UV–vis absorption and transmittance studies. The emission spectrum indicates that the crystal shows green and red fluorescence emissions. The band gap energy of the crystal was calculated and it is found to be 2.05 eV. The thermal stability of the crystal was studied using thermogravimetry-differential thermal (TG-DTA) analyses. The thermal anomalies observed in the differential scanning calorimetry (DSC) heating and cooling cycles indicate the occurrence of a first order phase transition. FTIR spectrum was used to confirm the presence of various functional groups in the crystal. The synthesized crystal shows SHG efficiency 32 times greater than that of potassium dihydrogen phosphate (KDP). The dielectric constant and dielectric loss of the crystal decreases with increases in frequency.     

High Thermally Stable and Organosoluble New Poly(amide-imide)s Derived from Bis(4-trimellitimido phenoxy)phenyl Triptycene

A triptycene-containing dicarboxylic acid monomer, bis(4-trimellitimido phenoxy)phenyl triptycene, was successfully synthesized by refluxing the diamine, bis(4-aminophenoxy)phenyl triptycene with trimellitic anhydride in glacial acetic anhydride. A series of aromatic poly(amide-imide)s were prepared by condensation polymerization of the dicarboxylic acid monomer and seven different commercially available diamines. The resulting poly(amide-imide)s were soluble in various solvents, such as N-methyl-2-pyrrolidone, N,N-dimethylacetamide, N,N-dimethylformamide, dimethyl sulfoxide, and pyridine. The polymers were amorphous and had inherent viscosities in the range 0.40–0.67?dL/g. These polymers had low dielectric constants ranging from 2.09 to 2.18. The glass transition temperatures of these polymers were observed between 229°C and 277°C. These polymers showed good thermal stability without significant weight loss up to 500°C. The temperatures at 10% weight loss ranged from 512°C to 570°C in nitrogen. The UV-Vis absorption spectra revealed that most of the polymers had absorption maxima around 312–348 nm, indicating the conjugation between the aromatic rings and nitrogen atoms. Solvent cast films had tensile strengths from 75 to 108?Mpa, elongates at break of 7%–9% and tensile moduli of 1.7–2.0?Gpa, indicating they were strong materials.     

Dielectric behavior and transport properties of ZnO nanorods

Highly optical, good crystalline and randomly aligned ZnO nanorods were synthesized by the hydrothermal method. The dielectric properties of ZnO nanorods were attributed to the interfacial polarization at low frequencies (below 10 kHz) and orientational polarization at higher frequencies. The observed ω^(n?1) dependence of dielectric loss was discussed on the basis of the Universal model of dielectric response. Dielectric loss peak was composed of the Debye like loss peak at higher frequencies and interfacial loss peak at lower frequencies. Charge transport through the grain and grain boundary region was investigated by impedance spectroscopy. At higher temperatures the conductivity of the nanorod was mainly through the grain interior and the overall impedance was contributed by the grain boundary region. The activation energy of nanorod was calculated as 0.078 eV, which is slightly higher than the reported bulk value.     

Confinement-induced differences between dielectric normal modes and segmental modes of an ion-conducting polymer

Dielectric measurement in the range 0.1 Hz to 1 MHz were used to study the motions of polymers and ions in an ion-conducting polymer, polypropylene oxide containing small quantities (on the order of 1%) of lithium ions (LiClO₄), confined as a sandwich of uniform thickness between parallel insulating mica surfaces. In the dielectric loss spectrum, we observed three peaks; they originated from the normal mode of the polymer, segmental mode of the polymer, and ion motions. With decreasing film thickness, the peak frequencies corresponding to the normal mode and ion motion shifted to lower frequencies, indicating retardation due to confinement above 30 nm. This was accompanied by diminished intensity of the dielectric normal-mode relaxation, suggesting that confinement diminished the fluctuations of the end-to-end vector of the chain dipole in the direction between the confining surfaces. On the contrary, the segmental mode was not affected at that thickness. Finally, significant retardation of the segmental mode was observed only for the thinnest film (14 nm). The different dynamical modes of the polymer (segmental and slowest normal modes) respond with different thickness and temperature dependence to confinement. Received 31 August 2001 and Received in final form 30 October 2001     

Dielectric behaviour of polycrystalline lead germanate

The dielectric behaviour of ferroelectric Lead Germanate (Pb₅Ge₃O₁₁) is reported in the frequency region 60 Hz-100 KHz for various particle size ranges from 40 to 300 μ m. The dielectric constant and conductivity decrease with decreasing particle size. The dielectric anomaly in small particle size unsintered samples disappears at lower frequencies, while at 100 KHz a broad diffuse phase transition is observed for all the samples, irrespective of particle size. A strong dielectric dispersion has been observed in the frequency range 60 Hz–100 KHz. The dielectric constant of sintered samples for the same panicle size range, and the dielectric anomaly which was absent at lower frequencies for unsintered samples re-appears in sintered samples. These results have been explained by assuming a high resistivity surface layer having a lower dielectric constant than the bulk at the boundary of particles.