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Energy efficiency

Tunable capacitors

Contact : wolfman@univ-tours.fr / beatrice.negulescu@univ-tours.fr / jo.sakai@univ-tours.fr 

Tunable RF capacitors with high dielectric response can be used for impedance matching between the signal generator and the antenna in mobile phone. Matching the impedance reduces reflected signal and so helps saving the battery. Using the combinatorial PLD technique we search for an alternative to (Ba,Sr)TiO3 which is now widely used for tunable capacitor applications.

Liu and collaborators (PRL 1003, 257602 (2009)) have shown that a morphotropic phase boundary (MPB) exists in the (1-x)Ba(Ti0.8Zr0.2)O3-x(Ba0.7Ca0.3)TiO3 solid solution for x = 0.5 with enhanced piezoelectric properties. We decided to explore the BaTiO3-BaZrO3-CaTiO3 ternary phase diagram to see whether we could spot some other MPB and if it boosts the tunability. Part of the phase diagram was studied by combinatorial deposition of 8 samples following the lines of constant Ca composition presented below.  

Fig. 1: Screening of the ternary phase diagram. Combinatorial deposition done with 3 targets. Each line represents one sample with variable composition.

In each sample 600 capacitors were measured in order to obtain a statistical distribution of the dielectric constants. Finally, a distribution map of the dielectric characteristics can be constructed. Two zones of interest for the tunability are identified: first for low amount of Ca (~1-2%) and the second around 10% Ca. Zone two has better performances in terms of dielectric loss. 

Part of these results are published in :Journal of Applied Physics 119, 094107 (2016); doi: 10.1063/1.4942924.


 

Fig. 2: Dielectric properties as a function of composition for one sample.

12 capacitance are measured for each composition.


Fig. 3: Tunability (%) map superposed on the phase diagram of the (Ba,Ca)(Ti,Zr)O3 

Functionally Graded Material (FGM)

Contact : wolfman@univ-tours.fr / beatrice.negulescu@univ-tours.fr / jo.sakai@univ-tours.fr 

(Ba, Sr)TiO3 epitaxial films crystallinity and dielectric properties are very sensitive to the strain exerted by the substrate. Using dual-PLD system, we prepared SrTiO3 (STO) to Ba0.6Sr0.4TiO3 (BST06) out-of-plane composition-graded films on STO (100) substrates.  These coherently- strained FGM films played the role of improving the crystallinity of BST06 top layers. FGM films that bridge compositions of substrate and top layer materials can be useful buffer layers for epitaxial growth of lattice-mismatched oxide films.

 

Fig. 1: Ba0.6Sr0.4TiO3 (BST06) top layer on SrTiO3 (001) substrate with a composition-graded (from SrTiO3 to BST06) buffer layer : Schematic (left) and a reciprocal space map (right).

For more information see our publication: Journal of Crystal Growth 380, 106 (2013).


Insulator-metal (I-M) Transitions

Contact : wolfman@univ-tours.fr / beatrice.negulescu@univ-tours.fr / jo.sakai@univ-tours.fr 

Materials exhibiting an I-M transition around room temperature with a large hysteresis could be used as the active part of a Resistive RAM (RRAM), a non-volatile memory type. We particularly study vanadium oxides for that purpose :
 
  • V2O3

V2O3 thin films on C- or R-plane sapphire (Al2O3) substrates by PLD.  Depending on the deposition conditions, c/a ratios at room temperature of (0001)- oriented V2O3 films widely ranged from 2.79 to 2.88. Their electrical properties were closely related with their c/a ratios. The R-plane films showed more abrupt M-I transitions at ~ 150 K compared to C-plane films.

 

Fig. 1: (Left) I-M and M-I transition temperatures and resistivity ratio between 50 and 300 K of V2O3 films as functions of c/a ratio.  (Right) Temperature dependence of resistivity of V2O3 films grown on C- and R-plane sapphire substrates.

These results are published in : Applied Physics Letters 107, 241901 (2015) ; doi: 10.1063/1.4937456
 

  • VO2

Our fundamental studies on pulsed laser-deposited VO2 thin films includes i) the evaluation of crystallinity and I-M transition properties of VO2 on Pt bottom layers, and ii) the effect of thermal expansion coefficient of the substrates on the transition temperature of VO2 films.

Fig. 2: (a) Raman spectra of a VO2 film on a Pt-Si substrate at various temperatures.  (b)(c) Deconvolution of Raman spectra at 70˚C and 75˚C, respectively, into monoclinic and rutile phases.  (d) Temperature dependence of the monoclinic phase ratio m / (m + r) deduced from the deconvolution.  (e) Transition temperatures of VO2 films on various substrates as functions of thermal expansion of the substrates.

For more information, see Journal of Applied Physics 113, 123503 (2013); doi: 10.1063/1.4795813 and 116, 123510 (2014); doi: 10.1063/1.4896500


Tuneable emissivity ceramics and coating in the infrared range

Contact : patrick.laffez@univ-tours.fr

Transition metal oxide materials are finding many applications in electronic and optical devices due to their wide range of electrical, magnetic, and optical properties. Among these interesting properties, the optimization of thermochromic switching in the infrared range is challenging.

One of the potential applications of thermochromic IR coating is their use in the field of the IR discretion or furtivity. The concept of thermal furtivity is the ideal situation when an object or a material is undetectable whatever its temperature, comparatively to the observed background, which is supposed to be at constant temperature. In other words, the object and the background have ideally the same emittance whatever the temperature of the object, at least in the detection band of the camera.

This situation can be approached if one considers an adaptative material with partial emissivity in band II or III decreasing with temperature. Thus, materials with a low temperature, semiconductor like behavior, and a high temperature metallic state, with a transition temperature adjustable around room temperature, are good candidates for such applications.

The manganite system
In recent work [Appl. Phys. Lett. 89, 081909 (2006), Appl. Phys. Lett. 93, 151910 (2008)], we showed that electron doped (Sm1−x,Cax)MnO3 perovskite manganite are potential infrared thermochromic materials and we have measured the thermal emittance changes at the charge ordering transition of (Sm0.35Ca0.65)MnO3. This property was evidenced in bulk ceramics [Appl. Phys. Lett. 89, 081909 (2006), Appl. Phys. Lett. 93, 151910 (2008)] and more recently in Sm0.5Ca0.5MnO3 films [J. Appl. Phys. 111, 113517 (2012)].

 
 
Figure 1 : Absorptance spectrum at different temperature for Sm0.35Ca0.65MnO3 ceramic. [Appl. Phys. Lett. 89, 081909 (2006), Appl. Phys. Lett. 93, 151910 (2008)]. Figure 2 : Relative integrated intensity in the range 8–14 μm showing the evolution with temperature of the sample opacity of Sm-Ca-Mn bulk ceramics. [Appl. Phys. Lett. 89, 081909 (2006), Appl. Phys. Lett. 93, 151910 (2008)].

Figure 3 : Evolution of the infrared transmittance at different temperatures 77 K to 420 K for the Sm0.5Ca0.5MnO3 coating annealed at 1070 K. [J. Appl. Phys. 111, 113517 (2012)]. Figure 4 : Average intensity versus temperature of the Sm0.5Ca0.5MnO3 films annealed at 1070 K for each transparency band of the atmosphere: BI (1-2.8 µm), BII (3-5 µm) and B III (8-14 µm) ref [J. Appl. Phys. 111, 113517 (2012)].

The nickelate system
The rare earth nickelate perovskite ReNiO3 has been strongly investigated in the past due to its metal insulator transition easily controllable by changing the rare earth nature. Such compound are however difficult to synthesize, an often need high oxygen pressure for the stabilisation of Ni3+. 

In recent work, [Journal of solid state chemistry, 183 (2010) 1663-1669] by using polymeric precursor associated with moderate pressure annealing, we showed that it is possible to obtain rare earth nickelates with metal insulator transition. The perovskite structure was confirmed by electron diffraction and high resolution electon microscopy. The thermal infrared characterization confirmed the metal insulator transition.
Figure 5 : Electron diffraction patterns of the Nd0.3Sm0.7NiO3 samples synthesised at 20bar (after [Journal of solid state chemistry, 183 (2010) 1663-1669])

 
Figure 6 : Slight contrast differences can be observed in the [0 1 0] oriented images for the Nd0.3Sm0.7NiO3 prepared at 20 bar. Figure 7 : Thermochromic behavior of Nd0.3Sm0.7NiO3 samples annealed under 20 and 175bar at 278 and 373K.

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