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In this paper, the field-induced residual ferroelectricity in antiferroelectric lead zirconate thin films has been studied by impedance measurements together with a hyperbolic law analysis, which permits us to extract the different contributions to the material’s complex permittivity. By measuring the Rayleigh coefficient αr, it appears that the residual ferroelectricity is considerably enhanced when the sample has been previously exposed to an electric field close to the antiferroelectric to ferroelectric transition field. This indicates that a part of the material remains ferroelectric after the antiferroelectric–ferroelectric backward transition, which constitutes an additional contribution to polarization. Consequently, a higher domain wall density and mobility can be observed. Measurements after exposition to thermal treatment show that this ferroelectric response is metastable.

International audience; The fabrication of low-resistance and thermally stable Ohmic contacts is essential for the realization of reliable GaN power devices. In the particular case of p-type GaN, a thin Ni/Au bilayer is commonly used for Ohmic contacts. However, Au metal contacts are quite expensive, are incompatible with the complementary metal oxide–semiconductor foundries, and also have poor thermal stability. Thus, seeking an alternative that is affordable and thermally stable is crucial. In the present study, we investigate Au-free Ohmic contact formation on p-type GaN using a bilayer Ni/Al-doped ZnO (AZO) thin film. Careful studies were focused on identifying the role of process parameters such as annealing parameters: temperature, time, and atmosphere in order to obtain an excellent Ohmic contact on p-GaN. Our results show that the contact resistance can be significantly reduced using a Ni/AZO bilayer with a suitable rapid thermal process. We demonstrate that the specific contact resistance for Ni/AZO on p-GaN can reach the lowest value of 1.85 × 10–4 Ω·cm2 for a sample with a 5 nm Ni layer annealed at 500 °C in air for 5 min. Our work demonstrates that the bilayer Ni/AZO contact could be suitable for efficient GaN power diodes or transistors.

International audience; The weak ferroelectric contribution to the polarization of antiferroelectric lead zirconate (PZO) thin films has been investigated using PUND (Positive Up Negative Down) pulse measurements and hysterons decomposition by First Order Reversal Curves (FORC) technique. The PUND allows decomposing the measured current, obtained from the polarization-electric field loop measurements, into a switching and a non-switching contribution. We show that the weak ferroelectric phase is enhanced when a large electric field has been previously applied to the material, in order to switch from the antiferroelectric to the ferroelectric phase. Using the PUND measurement at fields below the antiferroelectric to ferroelectric phase transition, the polarization loop corresponding only to the ferroelectric switching contribution has been determined revealing that this contribution to the overall polarization is small. FORC measurements, however, indicate that the ferroelectric phase is present at different fields. At low fields, a quite homogeneous distribution of hysterons exists and at large field, a high concentration of hysterons at a field near to the antiferroelectric to ferroelectric phase transition can be seen. Moreover, when changing the delay between pulses of the PUND and the FORC measurements, we show that this weak ferroelectricity contribution is metastable and decreases with time.

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International audience; Sintering is a very important process in materials science and technological applications. Despite breakthroughs in achieving optimized piezoelectric properties, fundamentals of K$_{0.5}$Na$_{0.5}$NbO$_3$ (KNN) sintering are not yet fully understood, facing densification versus grain growth competition. At present, microscale events during KNN sintering under reducing atmospheres are real-time monitored using a High Temperature-Environmental Scanning Electron Microscope. A two contacting KNN particles model satisfying the Kingery and Berg's bulk diffusion model is reported. Dynamic events like individual grain growth and grain elimination process are explored through a postanalysis of recorded image series. The diffusion coefficient for oxygen vacancies of 10$^{−8}$ cm$^2$ s$^{−1}$ and average boundary mobility of 10$^{−9}$ cm$^4$ J$^{−1}$ s$^{−1}$ are reported for the KNN ceramics. Moreover, the local pore shrinkage is consistent with the Kingery and François's concept of pore stability except that pore curvatures are not all concave, convex or flat due to anisotropic grain-boundary energies. The global grain growth kinetics are described using parabolic and/or cubic laws. The effect of atmospheres and microstructure evolution on the intrinsic and extrinsic contributions to the dielectric response using Rayleigh's law is also explored. These results bring a new breath for KNN sintering studies in order to adapt the sintering process.

ZnO nanowires (NWs) are very attractive for a wide range of nanotechnological applications owing to their tunable electron concentration

In this paper, the nanolamination of a ZrO2 insulator by Al2O3 for metal insulator metal capacitor applications has been studied. The insulating layers (ZrO2 and Al2O3) were deposited by atomic layer deposition and the electrodes were made of TiN. Different configurations of ZrO2 and Al2O3 alternations were studied, including 1 to 16 Al2O3 inclusions in the ZrO2 layer. X-ray diffraction of the insulator configurations showed that with four or more Al2O3 inclusions, the structure loses its crystalline orientation and becomes amorphous. Electrical characterizations have been conducted to study the capacitance, breakdown field, and leakage current for every insulator configuration. The capacitance density significantly decreases as the number of Al2O3 layers increases, except when an amorphous transition occurs; at this point, a local maximum of 17 nF/mm2 was found. A 19% increase of the breakdown field of samples with two or more Al2O3 inclusions has been correlated with an increase of leakage current explained by the emergence of the Fowler–Nordeim conduction mechanism at electrical fields higher than 4 MV/cm.

In this study, First-Order Reversal Curves (FORC) measurements and impedance spectroscopy coupled to hyperbolic law analysis have been used in order to characterize lead zirconate antiferroelectric thin films elaborated by a sol-gel process. The presence of a weak residual ferroelectric behavior has been shown even if this contribution is not clearly visible on the polarization-electric field loops. Moreover, FORC measurements indicate that the weak ferroelectric phase switch only when the antiferroelectric phase state of the cell is modified. On the other hand, impedance spectroscopy reveals a fairly good distribution of small residual ferroelectric clusters in the material as no coalescence of ferroelectric domains has been observed at low fields. This study aims to show that FORC distribution measurements and impedance spectroscopy coupled to the hyperbolic law analysis are very sensitive and complementary methods for the understanding of the polarization switching dynamics in antiferroelectric materials.

The origin of low thermal conductivity in niobium-containing perovskites was investigated. Therefore, dense NaNbO3 and KNbO3 ceramics were sintered by spark plasma sintering. The grain size was in the 1–10 μm range. NaNbO3 ceramic has shown a stable thermal conductivity circa 2.6 W m−1 K−1 from 373 to 1000 K and KNbO3, values from 2.5 to a plateau at 1.2 W m−1 K−1 since 673 K. This value is the lowest thermal conductivity reported for an oxide presenting a perovskite structure. The origin of such low thermal conductivities is indicated to be related with cation deficiency on the A-site of the niobium-containing perovskites.

AlN nucleation layers are the basement of GaN-on-Si structures grown for light-emitting diodes, high frequency telecommunication and power switching systems. In this context, our work aims to understand the origin of propagation losses in GaN-on-Si High Electron Mobility Transistors at microwaves frequencies, which are critical for efficient devices and circuits. AlN/Si structures are grown by Metalorganic Vapor Phase Epitaxy. Acceptor dopant in-diffusion (Al and Ga) into the Si substrate is studied by Secondary Ion Mass Spectroscopy and is mainly located in the first 200 nm beneath the interface. In this region, an acceptor concentration of a few 1018 cm-3 is estimated from Capacitance–Voltage (C–V) measurements while the volume hole concentration of several 1017 cm-3 is deduced from sheet resistance. Furthermore, the combination of scanning capacitance microscopy and scanning spreading resistance microscopy enables the 2D profiling of both the p-type conductive channel and the space charge region beneath the AlN/Si interface. We demonstrate that samples grown at lower temperature exhibit a p-doped conductive channel over a shallower depth which explains lower propagation losses in comparison with those synthesized at higher temperature. Our work highlights that this p-type channel can increase the propagation losses in the high-frequency devices but also that a memory effect associated with the previous sample growths with GaN can noticeably affect the physical properties in absence of proper reactor preparation. Hence, monitoring the acceptor dopant in-diffusion beneath the AlN/Si interface is crucial for achieving efficient GaN-on-Si microwave power devices.

The miniaturization of electronic devices and power systems requires the fabrication of functional components in the form of micrometer-sized thick films. A major challenge is the integration of functional ceramics with metals, which are considered incompatible with high-temperature ceramic processing. To overcome the integration barrier, an aerosol deposition (AD) spray-coating method based on room temperature deposition can be used. By employing the AD method, we were able to deposit relaxor-ferroelectric 0.65Pb(Mg1/3Nb2/3)O3–0.35PbTiO3 ceramic thick films on low-cost stainless-steel substrates. The as-deposited films were dense, with ∼97% of the theoretical density. Moreover, the post-deposition annealing at 500 °C did not result in any microstructural changes. Compared to the as-deposited films, the annealed films exhibit improved energy storage and electromechanical properties. The annealed thick films achieve a recoverable energy density of 15.1 J⋅cm−3 at an electric field of 1350 kV⋅cm−1 and an electric-field cycling stability of 5 million cycles. A piezoelectric response was detected through the entire film thickness by piezoelectric force microscopy. Macroscopic displacement measurements revealed a maximum relative strain of 0.38% at 1000 kV⋅cm−1, corresponding to inverse effective piezoelectric coefficient of ∼40 pm⋅V−1. In this study, we overcame the integration challenges and demonstrated the multifunctionalization of future ceramic-metal structures, as the deposited thick films on stainless steel exhibit energy storage capability and piezoelectric properties.

In this work, the effects of thermal annealing at 500 °C on aerosol-deposited 0.65Pb(Mg1/3Nb2/3)O3-0.35PbTiO3 thick films on stainless-steel substrates are investigated using two complementary methods at high and low applied external electric fields. The first one is the positive up negative down method, which allows us to obtain information about the switching and non-switching contributions to the polarization. It shows that the as-deposited film is ferroelectric before annealing, since it has a switching contribution to the polarization. After annealing, both the switching and non-switching contributions to polarization increased by a factor of 1.6 and 2.33, respectively, indicating a stronger ferroelectric behavior. The second method is based on impedance spectroscopy coupled with Rayleigh analysis. The results show that post-deposition thermal annealing increases the reversible domain wall contribution to the dielectric permittivity by a factor of 11 while keeping the threshold field similar. This indicates that, after annealing, domain wall density is larger while domain wall mobility remains similar. These two complementary characterization methods show that annealing increases the ferroelectric behavior of the thick film by increasing the domain wall density, and its influence is visible both on polarization vs electric field loop and dielectric permittivity.

In the area of membrane separations the development of powerful on-line diagnostic tools allowing the reliability insurance of both membranes and associated processes becomes a major concern. In this context, Acoustic Emission (AE) appears as a potentially attractive method to ensure i) quality control for membrane production lines and ii) on-line monitoring of their evolution when implemented in integrated industrial systems requiring high level of safety/security. In this work, simultaneous gas permeation measurements and acoustic emission technique have been coupled in order to characterize series of porous membranes operating under various conditions. AE events resulting from different gas transport mechanisms through porous membranes series have been identified and classified thanks to a statistical post-treatment of the recorded acoustic signals. A close relationship between AE signal characteristics, physicochemical properties of the porous membranes and associated gas transport mechanisms was established. These promising results constitute a key step in the development of an innovative tool for non-invasive on-line diagnosis dedicated to the characterization and control of porous ceramic membranes.

K 0.5 Na 0.5 Nb 1-x Ta x O 3 (KNNT) (with x = 0.00, 0.05, 0.10, 0.20, 0.30, 0.50 and 1) ceramics are prepared by ball milling and two calcinations at 830 °C for 5 h. Subsequent sintering of centimeter size pellets, 1–2 mm thick, is studied using conventional and spark plasma sintering techniques with various conditions. X-Ray diffraction and Raman spectroscopy phase identification reveal orthorhombic to tetragonal phase transitions occurring at about x = 0.50, associated to chemical disorder. Scanning electron microscope observations and associated energy dispersive X-ray spectroscopy analysis reveal some composite aspect of the ceramics. Substitution of niobium by tantalum, corresponding to x increase, decreases significantly the grain size but also the densification of the ceramics sintered by conventional sintering, while, enhancement of the piezoelectric properties is observed for both sintering techniques. Thanks to parameters optimization of the spark plasma sintering process, temperature-time-pressure, significant improvement of the relative density over 96%, is obtained for all the compositions sintered between 920 and 960 °C, under 50 MPa, for 5–10 min with heating rates of 100 °C/min. High relative permittivity (e r = 1027), piezoelectric charge coefficient (d 33 = 160 pC/N) and piezoelectric coupling factor (k p = 46%) are obtained in spark plasma sintered K 0.5 Na 0.5 Nb 1-x Ta x O 3 composite ceramics, for x ranging between 0.10 and 0.30 and for some specific spark plasma sintering conditions. Thus, tantalum single element substitution on niobium site, combined with spark plasma sintering, is revealed to be a powerful combination for the optimization and the reliability of piezoelectric properties in KNN system.

International audience; The temperature-dependence of the piezoelectric properties of trigonal -GeO2 single-crystals obtained by the high-temperature flux method was measured by the resonance technique of the electrical impedance in the 20°C-600°C range. To approach the values of the two independent piezoelectric coefficients d11 and d14, we first measured as a function of temperature the elastic coefficients S11, S14 and S66 and the dielectric permittivity 11 which are involved in the coupling coefficient k of both the thickness shear mode and the transverse mode. A Y-cut plate with a simple +45°-rotation ((YXtwl) +45°/0°/0°) was used to measure the coupling coefficient of the thickness shear mode, and two X-turned plates ((XYtwl) +45°/0°/0° and (XYtwl)-45°/0°/0°) were prepared to characterize the coupling coefficient of two transverse modes. From the whole experimental measurements, the piezoelectric coefficients of -GeO2 were calculated up to 600 °C. They show that this crystal is one of the most efficient in regard of the -quartz-like family at room temperature, and that its thermal comportment retains large piezoelectric properties up to 600°C.

International audience; In this study, two different methods have been used in order to characterize leadzirconate anti-ferroelectric thin film elaborated by a modified sol-gel process: First-Order Reversal Curves (FORC) measurements and impedance spectroscopy coupled to hyperbolic law analysis. Approaches at low and high applied electric fields allow concluding on the presence of a weak residual ferroelectric behavior even if this contribution is not visible on the polarization-electric field loops. Moreover, the weak ferroelectric phase seems to switch only when the phase of the antiferroelectric cells is modified and no coalescence of ferroelectric domains at low field occurs due to a well distribution of small residual ferroelectric clusters in the material. The main goal of this paper is to show that FORC distribution measurements and impedance spectroscopy coupled to the hyperbolic law analysis are very sensitive and complementary methods.

Lead zirconate titanate (PZT) ceramics are the dominant piezoelectric elements for non-destructive evaluation (NDE) and ultrasonic transducers devices. However, the presence of lead content may impose the scientific community to develop lead-free ceramics, concerning human health and environmental safety. During the past ten years, many contributions have highlighted the potential properties of complex compositions like LiNbO3, LiTaO3 and LiSbO3 in the lead-free (K0.5Na0.5)NbO3 KNN system. In this context, for the first time, the practical applications and the effectiveness of simply undoped (K0.5Na0.5)NbO3 (KNN) ceramics are investigated. KNN powder is prepared by conventional solid state mixed oxide route. Ceramics of this material are prepared using conventional sintering (CS) and spark plasma sintering (SPS). Thickness coupling factor k t of 44–46%, planar coupling factor k p of 29–45%, relative permittivity at constant strain e 33 , r S of 125–243 and acoustic impedance Z of 23–30 MRay are obtained for these two kinds of undoped KNN ceramics. Both ceramics are used to build single-element ultrasonic transducers. Relative bandwidth of 49–78% and insertion loss of −27 and −51 dB are obtained for SPS and CS transducers, respectively. These results are suitable for use in non-destructive evaluation. The effectiveness of undoped KNN is evaluated using the KLM model, and compared to standard PZT based probe. Finally, chemical aging test of undoped KNN has demonstrated its stability in water.

Lead free piezoelectric K 0.5 Na 0.5 NbO 3 (KNN) is synthesised by the conventional solid state mixed oxide route using dry ball milling. Spark plasma sintering technique (SPS) is an original and fast method which enables the grain growth to be controlled and allows to decrease a potential alkali volatilisation. Undoped KNN are sintered by SPS in the 920–975 °C temperature range, for dwell times of 5–15 min. The density range is 93–97%. Density, grain size and composition of samples are comparatively analysed. Electromechanical performances are measured and related to the microstructure. High planar coupling coefficient k p of 48%, thickness coupling factor k t of 45%, dielectric constant at constant strain e 33 S / e 0 of 328, and low mechanical losses δ m of 2% are reached for undoped KNN. These results are promising and suitable for transducer applications.

International audience; The floating zone method (FZM) is used to grow large crystal ingots of lead-free (K0.5Na0.5)NbO3 (KNN), (K0.5Na0.5)(Nb0.95Mn0.05)O3 (KNN-Mn), and (K0.5Na0.5)(Nb0.95Ta0.05)O3 (KNN-Ta). FZM allows high quality crystal to be obtained without using a crucible. However, the KNN molten zone is instable at high temperature (∼1200 °C) due to its incongruent melting and alkali segregation. The challenge of this work is to control the stability and the viscosity of the molten zone which allows long growth in order to achieve large volume single crystals. Our work reveals that the use of a slight overpressure of gas (O2 or N2) and doping KNN with Mn and Ta allows the molten zone to be maintained over 50 h. In such conditions, the as-grown crystals are preferentially oriented along the [011] direction and display piezoelectric properties such as kt of 0.48, d33 of 65–70 pC/N, ε33,rS of 123–148, δe of 0.03–0.07, and δm of 0.11–0.21. The ferroelectric measurements show saturated hysteresis loops with a decrease in EC and Pr when attaining single crystal after several hours of growth. High resolution transmission electron microscopy (HRTEM) investigations show complex domain structures that are mainly attributed to the accommodation of the stresses occurring during the crystal growth in the high thermal gradient and in the phase transition from tetragonal to orthorhombic phases during cooling.

International audience; Recently, many techniques have been used to grow large K0.5Na0.5NbO3 (KNN) based single crystals. However, most of them required the use of a crucible, fluxes in the melt at high temperature and long process times that could lead to alkali volatilization or inclusion of impurities in the crystals. In this study, the floating zone method, which is especially suitable for compounds that melt incongruently or present volatile elements, is employed to study the microstructure of [011] oriented KNN crystals. Then, the domain structure and their relationships with piezoelectric properties in KNN are investigated using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and piezo force microscopy (PFM). This study reveals self-organized and hierarchized domain structures on various length scales ranging from micrometer to nanometer scale in KNN crystals. First, parallel stripe-like microdomains of 6–7 μm wide, which contain themselves parallel stripes (1–4 μm wide), are observed using SEM. These domains have been confirmed by TEM. TEM observations have also shown the presence of additional zigzag sub-microdomains with 90 and 120° orientations, which are arranged inside these observed stripes. Moreover, high resolution TEM highlighted the presence of regular antiphase boundaries at the location of the domain walls, which are caused by the small misfits between the parameters of the different structures. Split spots on the FFT image attests of these phenomena. Additionally, PFM images showed also parallel stripe-like microdomains (3–4 μm wide), which contain sub-microdomains. These sub-microdomains consist of parallel stripes of 200–400 nm and zigzag nanodomains with 60, 90, 120 and 180° orientations. The local polarization switching test by PFM emphasized that the observed domain structures correspond well with ferroelectric domains, confirming through microstructural observations the ferroelectricity of KNN. The combination of SEM, TEM and PFM observations of orthorhombic KNN crystals have shown the correlation between microstructure, sub-microstructure and domain structures at different scales. The [011] oriented KNN crystal exhibited interesting piezoelectric properties such as kt of 0.47, d33 ∼ 60–80 pm V−1, εSr,33 of 123, dielectric losses (δe) of 0.07 but also high mechanical losses (δm) of 0.21 that can be induced by 90° domain walls moving under the influence of an electric field during poling.

Conventional solid state mixed oxide route using manual and ball milling is investigated for the preparation of K0·5Na0·5NbO3 (KNN) ceramics. Microstructure engineering was made using two milling methods and sintering techniques, and the crystal growth; then electromechanical properties were investigated as a function of sintering temperature, densification and grain size. The sintering conditions were set at 920°C/5 min for spark plasma sintering and 1090–1120°C/10 and 48 h for classical sintering. KNN crystal was grown using floating zone technique under nitrogen gas, where the translation and rotation speeds were fixed at 3 mm h−1 and 20 rev min−1 respectively. Piezoelectric and dielectric performances were measured and related to the microstructure. High kt (33 to 48%), kp of 18 to 48% and d33 of 127–140 pC N−1 were reached for relative densities of 84 to 96%. KNN ceramics are now available for the design of ultrasonic sensors.

K0.5Na0.5NbO3 (KNN) was manufactured by spark plasma sintering (SPS), which is a fast sintering method allowing to control the grain growth. Different samples of KNN are sintered with SPS at 920°C under 50 MPa for 5 minutes. High densities over than 97% are achieved. In order to make domain engineering, KNN crystals are grown by floating zone method. Stable molten zone is reached when oxygen or nitrogen gas flux is used, leading up to 50 mm length of crystals. High electromechanical coupling factor kt about 46 %, kp around 45 % and ε33S/ε0 of 253 are achieved for KNN ceramics poled at optimum electric field about 3 kV / mm. KNN crystal boule exhibits kt about 40 % against 34 % for KNN ceramic, both poled at 1 kV / mm. These results are promising to replace PZT for transducers applications.

The aim of this work is to take control on the grain size and densification of undoped K0.5Na0.5NbO3 (KNN), from micrometer to millimeter grain sizes. For this purpose, ceramics are prepared by spark plasma sintering (SPS) and conventional sintering. Their piezoelectric properties are comparatively studied. After full structural and microstructural characterizations, high electromechanical properties (kt = 45 %, kp = 30 %, Z = 20 MRay) for conventional sintering and (kt = 45 %, kp = 48 %, Z = 30 MRay) for SPS sintering are obtained. Otherwise, millimeter grains are produced by the floating zone method used for crystal growth, and their electromechanical properties are determined and compared to their homologous ceramics. Moreover, using the characteristics of the three materials, simulated electroacoustic responses of single-element transducers are compared. The results show that KNN is suitable for transducer applications.