Analysis of noise temperature sensitivity for the design of a broadband thermal noise primary standard

A broadband primary standard for thermal noise measurements is presented and its thermal and electromagnetic behaviour is analysed by means of a novel hybrid analytical?numerical simulation methodology. The standard consists of a broadband termination connected to a 3.5mm coaxial airline partially immersed in liquid nitrogen and is designed in order to obtain a low reflectivity and a low uncertainty in the noise temperature. A detailed sensitivity analysis is made in order to highlight the critical characteristics that mostly affect the uncertainty in the noise temperature, and also to determine the manufacturing and operation tolerances for a proper performance in the range 10MHz to 26.5 GHz. Aspects such as the thermal bead design, the level of liquid nitrogen or the uncertainties associated with the temperatures, the physical properties of the materials in the standard and the simulation techniques are discussed

Thermal Analog to AFM Force-Displacement Measurements for Nanoscale Interfacial Contact Resistance

Thermal diffusion measurements on polymethylmethacrylate-coated Si substrates using heated atomic force microscopy tips were performed to determine the contact resistance between an organic thin film and Si. The measurement methodology presented demonstrates how the thermal contrast signal obtained during a force-displacement ramp is used to quantify the resistance to heat transfer through an internal interface. The results also delineate the interrogation thickness beyond which thermal diffusion in the organic thin film is not affected appreciably by the underlying substrate

Effect of Texture on Temperature-Dependent Properties of K0.5Na0.5NbO3 Modified Bi1/2Na1/2TiO3-xBaTiO(3)

Textured (1-x-y)Bi1/2Na1/2TiO3-xBaTiO(3)-yK(0.5)Na(0.5)NbO(3) (BNT-100xBT-100yKNN) ceramics with a {001} pseudocubic (pc) orientation were fabricated by templated grain growth using Bi1/2Na1/2TiO3 templates. Temperature-dependent electromechanical results demonstrate that the strain response of templated BNT-xBT-yKNN ceramics is stable from room temperature (RT) to 125 degrees C. The temperature-dependent strain and polarization response are compared to randomly oriented ceramics, for BNT-100xBT-2KNN (0.05 <= x <= 0.07). Textured BNT-7BT-2KNN reached a maximum 0.47% strain response at 5 kV/mm, an almost 50% increase compared to randomly oriented BNT-7BT-2KNN. Over the temperature range RT-125 degrees C, the strain response of templated BNT-6BT-2KNN degraded from 0.38% to 0.22% (-42.1%) compared to 0.37% to 0.18% (-51.4%) for randomly oriented ceramics. The temperature-dependent strain response suggests that templated BNT-100xBT-100yKNN ceramics are well suited for elevated temperature applications.close0

Domain switching mechanisms in polycrystalline ferroelectrics with asymmetric hysteretic behavior

A numerical method is presented to predict the effect of microstructure on the local polarization switching of bulk ferroelectric ceramics. The model shows that a built-in electromechanical field develops in a ferroelectric material as a result of the spatial coupling of the grains and the direct physical coupling between the thermomechanical and electromechanical properties of a bulk ceramic material. The built-in fields that result from the thermomechanically induced grain-grain electromechanical interactions result in the appearance of four microstructural switching mechanisms: (1) simple switching, where the c-axes of ferroelectric domains will align with the direction of the applied macroscopic electric field by starting from the core of each grain; (2) grain boundary induced switching, where the domain's switching response will initiate at grain corners and boundaries as a result of the polarization and stress that is locally generated from the strong anisotropy of the dielectric permittivity and the local piezoelectric contributions to polarization from the surrounding material; (3) negative poling, where abutting ferroelectric domains of opposite polarity actively oppose domain switching by increasing their degree of tetragonality by interacting with the surrounding domains that have already switched to align with the applied electrostatic field. Finally, (4) domain reswitching mechanism is observed at very large applied electric fields, and is characterized by the appearance of polarization domain reversals events in the direction of their originally unswitched state. This mechanism is a consequence of the competition between the macroscopic applied electric field, and the induced electric field that results from the neighboring domains (or grains) interactions. The model shows that these built-in electromechanical fields and mesoscale mechanisms contribute to the asymmetry of the macroscopic hysteretic behavior in poled samples. Furthermore, below a material-dependent operating temperature, the predicted built-in electric fields can potentially drive the aging and electrical fatigue of the system to further skew the shape of the hysteresis loops

Stress, temperature and electric field effects in the lead-free (Ba,Ca)(Ti,Zr)O3 piezoelectric system

The large signal strain response as a function of uniaxial compressive stress, electric field and temperature is investigated for compositions across the morphotropic phase boundary in the (Ba,Ca)(Ti,Zr)O3 ferroelectric system. The largest piezoelectric coefficient in terms of unipolar strain divided by the maximum applied field, Su/EmaxSu/Emax, is 1540 pm V−1, which clearly exceeds the piezoelectric response of most lead zirconate titanate materials. The extraordinarily large piezoelectric properties occur in the vicinity of the morphotropic phase boundary region on the rhombohedral side of the phase diagram. In this material, an electric threshold field is observed that is required to overcome the stress-induced domain clamping and obtain a measurable strain response. Moreover, the study reveals that careful selection of composition, stress and field amplitude allow for large signal piezoelectric coefficients of over 740 pm V−1 in the temperature range of 25–75 °C. The extraordinarily large unipolar strain response can be assigned to an electric field-controlled regime, in which the unipolar compressive stress induces non-180° domain switching perpendicular to the applied electric field. During electrical loading, the electric field can realign these domains back into the parallel direction, maximizing non-180° domain switching and enhancing unipolar strain

The equilibrium crystal shape of strontium titanate and its relationship to the grain boundary plane distribution

In this study, the equilibrium crystal shape (ECS) of a model system, strontium titanate, is compared with the grain boundary plane distribution (GBPD) as a function of temperature. Strontium titanate has a pronounced surface energy anisotropy and a grain growth anomaly, with the grain growth rate decreasing by orders of magnitude with increasing temperature. The ECS was determined from the shape of small intragranular pores and the GBPD was determined from orientation measurements on surfaces, with the relative areas of grain boundary planes in a polycrystal correlated to the surface energy of both adjacent crystal planes. The grain boundary energy has been previously proposed to be the sum of the surface energy of the adjacent grains less a binding energy that is assumed to be constant. While much experimental evidence exists for this assumption at a fixed temperature, the influence of temperature is not known. While the anisotropy of the ECS was found to decrease with temperature, the anisotropy of the GBPD increased with temperature. These findings indicate that changes in the binding energy with temperature must be considered, as the binding energy links the surface energy to the grain boundary energy. The results are discussed with respect to the grain growth anomaly of strontium titanate, in which the grain growth decreases with increasing temperature