Zirconia toughened ceramics

The objectives for the thesis were to generate tough ceramics utiising the toughening mechanisms inherent to zirconia materials. The aims have been realised with the successful fabrication of hot pressed silicon nitride / zirconia composite ceramics. The zirconia was prestabilised with two different types of dopant additives, yttria and ceria, with the intention of understanding the chemical compatibility with the silicon nitride matrix and the overall effect on the subsequent mechanical properties. The volume fraction of added zirconia was also varied. The increased toughness over silicon nitride materials alone was attributed to the toughening agents inherent to zirconia which existed either in the form of the tetragonal polymorph or the monoclinic variant. The toughening modes were dependent on initial chemistry of the composite system. When the zirconia was prestabilised with yttria the tetragonal polymorph was retained within the composite. The enhanced toughness was attributed to a transformation toughening mechanism. However, when the zirconia was prestabiised with ceria the depletion of Ce from solid solution with the zirconia during processing resulted in the formation of the unstabiised monoclinic variant. The enhanced toughness was attributed, in this case, to a microcrack type energy absorption mechanism, similar to several ZTA composite ceramics. Additionally, an experiment using ultrasound non-destructive testing, indicated that Tetragonal Zirconia Polycrystals (TZP) is ferroelastic and, as such, can provide a significant contribution to enhanced levels of fracture toughness in these materials or composites containing the same. Further work has been conducted to actually observe, as a function of applied unia.xial stress, the crystallographic changes occurring within the bulk of a 3Y-TZP ceramic via neutron elastic scattering at the ILL, Grenoble, France. This experiment has provided clear direct proof of the ferroelastic nature of zirconia. A similar experiment will be carried out at the Rutherford Laboratory, though with significantly improved statistics. An approach to improve the high temperature properties of TZP via the chemical alteration of the grain boundary phase was also considered. As a preliminary step the grain boundary volume was increased through controlled additions of the grain boundary composition in the form of both a premilled and a premelted glass. Poor fired densities were attained, however, due to the solute additive partitioning from the generation of an enhanced grain boundary phase to overstabilisation of the zirconia resulting in the formation of cubic stabilised zirconia. Furthermore, the incorporation of nitrogen within the grain boundary phase, via sintering TZP with sole additions of A1N, resulted in the attainment of poor fired densities and hence was not considered a suitable method for grain boundary modification

Ferroelectricity in Dion–Jacobson ABiNb2O7(A = Rb, Cs) compounds

The ferroelectric properties of 2-layer Dion–Jacobson compounds ABiNb2O7 (A = Rb and Cs) were studied. Ferroelectricity and piezoelectricity of CsBiNb2O7 were demonstrated for the first time. The ferroelectric domain structure of Dion–Jacobson compounds were imaged using PFM. The Curie points of RbBiNb2O7 and CsBiNb2O7 are 1098 ± 5 and 1033 ± 5 °C, respectively. The piezoelectric constant of RbBiNb2O7 and CsBiNb2O7 are approximately 5 and 8 pC N−1. Thermal depoling was also studied to confirm the Curie temperature and the stability of the piezoelectricit

Room-temperature multiferroic behavior in layer-structured Aurivillius phase ceramics

Multiferroics that simultaneously exhibit ferroelectricity and ferromagnetism have recently attracted great attention due to their potential application in next generation electronic devices. However, only a few single-phase multiferroic materials exhibit ferroelectric and ferromagnetic orders at room temperature. Recently, some bismuth layer-structured Aurivillius compounds were reported as multiferroics at room temperature, but the origin of their magnetic property is still under debate because the net magnetization may originate from the presence of secondary phases that are not easily detected by laboratory XRD diffractometers. Here, textured Aurivillius phase Bi5.25La0.75FeCoTi3O18 ceramics were prepared by Spark Plasma Sintering. The ferromagnetic character of the ceramics was indicated by the magnetic field-induced reversible intensity changes of a certain set of crystalline planes belonging to the Aurivillius phase, as measured by in situ neutron diffraction under the applied magnetic field. The first principles calculations indicate that the ferromagnetism originates from double exchange interactions Fe3þ–O–Fe3þ, Co3þ–O–Co3þ, and Fe3þ–O–Co3þ in the ferro-toroidal main phase. The magnetic-controlled ferroelectric domain switching was observed by piezoelectric force microscopy at room temperature. The prepared Aurivillius phase ceramics, with Co/Fe contributing to magnetization and polarization at the same time, can be considered an intrinsic room-temperature multiferroic

A liquid crystalline copper phthalocyanine derivative for high performance organic thin film transistors

This journal is © The Royal Society of Chemistry 2012Bottom-gate, bottom-contact organic thin film transistors (OTFTs) were fabricated using solvent soluble copper 1,4,8,11,15,18,22,25-octakis(hexyl)phthalocyanine as the active semiconductor layer. The compound was deposited as 70 nm thick spin-coated films onto gold source–drain electrodes supported on octadecyltrichlorosilane treated 250 nm thick SiO2 gate insulators. The performance of the OTFTs was optimised by investigating the effects of vacuum annealing of the films at temperatures between 50 0C and 200 0C, a range that included the thermotropic mesophase of the bulk material. These effects were monitored by ultraviolet-visible absorption spectroscopy, atomic force microscopy and XRD measurements. Device performance was shown to be dependent upon the annealing temperature due to structural changes of the film. Devices heat treated at 100 0C under vacuum (≥10-7 mbar) were found to exhibit the highest field-effect mobility, 0.7 cm2 V^-1 s^-1, with an on–off current modulation ratio of~107, a reduced threshold voltage of 2.0 V and a sub-threshold swing of 1.11 V per decade.UK Technology Strategy Board (Project no: TP/6/EPH/6/S/K2536J) and UK National Measurement System (Project IRD C02 ‘‘Plastic Electronics’’, 2008–2011)

Simultaneous large optical and piezoelectric effects induced by domain reconfiguration related to ferroelectric phase transitions

Electrical switching of ferroelectric domains and subsequent domain wall motion promotes strong piezoelectric activity; however, light scatters at refractive index discontinuities such as those found at domain wall boundaries. Thus, simultaneously achieving large piezoelectric effect and high optical transmissivity is generally deemed infeasible. Here, it is demonstrated that the ferroelectric domains in perovskite Pb(In1/2Nb1/2)O3 Pb(Mg1/3Nb2/3)O3-PbTiO3 domain-engineered crystals can be manipulated by electrical field and mechanical stress to reversibly and repeatably, with small hysteresis, transform the opaque poly-domain structure into a highly transparent mono-domain state. This control of optical properties can be achieved at very low electric fields (less than 1.5 kV cm−1) and is accompanied by a large (>10000 pm V−1) piezoelectric coefficient that is superior to that of linear state-of-the-art materials by a factor of three or more. The coexistence of tunable optical transmissivity and high piezoelectricity paves the way for a new class of photonic devices

Quantification of electromechanical coupling measured with Piezoresponse Force Microscopy

Here we study the piezoresponse of epitaxial ferroelectric samples excited through top electrode structures with conductive tips in the global excitation mode and compare these results to displacement values obtained using artifact-free laser Doppler vibrometry (LDV) measurements. Substrate bending modes are studied using finite element simulations and LDV measurements, and found to be negligible for top electrode diameters below 100 μm. The effect of electrostatic forces on the piezoresponse measurements is analyzed and methods for minimizing these are discussed. Using a resistive tip-electrode contact model the piezoresponse measurements are found to be in good agreement with values obtained from calibrations, providing a link between nanometer scale piezoresponse measurements and quantitative LDV measurements

6MPI0301 Losses in piezoelectric materials

The losses associated with piezoelectric behaviour have been investigated, by measurements at frequencies and fields where extrinsic contributions play a significant role. The study concentrated on measurements in the quasi static frequency range 0.1 to 1kHz, rather than through resonance methods. The piezoelectric loss was measured using a standard Berlincourt type setup modified to enable the phase angle between the applied stress and charge output to be analysed. The mechanical loss was also investigated by measuring the phase angle between applied stress, and induced strain in a strain gauged piezoceramic sample. The piezoelectric loss showed an increase with increasing AC amplitude for hard and soft. However, the increase in loss seen at low frequencies was seen in all materials including Lithium Niobate, and it was concluded that this was an experimental artefact. The elastic loss measurements were more difficult to quantify because of bending of the sample, giving rise to arbitrary initial phase angles. (author)Includes bibliographical referencesSIGLEAvailable from British Library Document Supply Centre- DSC:6180. 5139(126) / BLDSC - British Library Document Supply CentreGBUnited Kingdo

Polarization dynamics and non-equilibrium processes in ferroelectric switching

Time and thermal dependent effects are critical for the operation of non-volatile memories based on ferroelectrics. In this paper we assume a domain nucleation process of the polarization reversal and we discuss the polarization dynamics in the framework of a non-equilibrium statistical approach. This approach yields analytical expressions, which can be used to explain a wide range of time and thermal dependent effects in ferroelectrics. Experimental P-E loops data obtained for soft ferroelectrics at various temperatures are well predicted by the theory

Flextensional ultrasonic piezoelectric micro-motor

This paper presents the experimental design, construction, and operational characteristics of a new type of standing wave piezoelectric ultrasonic micro-motor. The motor uses a composite stator, consisting of a metallic flextensional mode converter, or "cymbal," bonded to a 2-mm-square piezoelectric plate. The cymbal converts contour-mode vibrations of the plate into oscillations in the cymbal, perpendicular to the stator plane. These are further converted into rotational movement in a rotor pressed against the cymbal by means of an elastic-fin friction drive to produce the required rotary actuation. The motor operates on a single-phase electrical supply, and direct control of the output speed and torque can be achieved by adjusting the amplitude and frequency of the supply voltage. Noncontact optical techniques were used to assess the performance of the developed micro-motor. The operational characteristics were developed from the acceleration and deceleration characteristics. No-load output speed (11 rev s-1) and stall torque (27 nNm) were derived using high-speed imaging and image analysis. Maximum efficiency was 0.6