G. Vélu - Academia.edu (original) (raw)

Papers by G. Vélu

IEEE Microwave and Wireless Components Letters, 2006

ABSTRACT Ferro- and para-electric BaSrTiO3 (εr∼350 and tgδ∼5×10-2 at 0V) thin films were deposite... more ABSTRACT Ferro- and para-electric BaSrTiO3 (εr∼350 and tgδ∼5×10-2 at 0V) thin films were deposited by low-cost sol-gel techniques. Subsequently, the films were used for fabricating coplanar waveguide phaseshifters using tunable finger-shaped capacitors. A 310° phaseshift was obtained at 30GHz and 35V of tuning voltage with 3.6dB of insertion loss yielding a figure of merit of 85°/dB.

IEEE Transactions on Dielectrics and Electrical Insulation

In the past few years there has been a growing trend in the use of ceramic coated high temperatur... more In the past few years there has been a growing trend in the use of ceramic coated high temperature resistant wires for applications up to 500 °C. The main drawback in developing new materials has been the relatively poor dielectric characteristics at high temperature. One solution is to strengthen the ceramic layer by covering it with boron oxide and this additional layer improves the dielectric characteristics of the ceramic wire. Partial discharge inception voltage threshold is greatly increased compared to non-impregnated wires and this has been observed over a large temperature range.

IEEE Transactions on Dielectrics and Electrical Insulation

The purpose of this paper is to understand the pros and cons of aluminum conductors insulated wit... more The purpose of this paper is to understand the pros and cons of aluminum conductors insulated with anodized aluminum for rotating machines used in aircraft. The investigation is focused on the breakdown voltage and partial discharge inception voltage of simple systems (crossed pairs, simplified coils). The results show that both breakdown voltage and partial discharge inception voltage are very sensitive to curvature radius and lower than those achieved using conventional copper conductors insulated with polymeric enamels. However, the operating temperature of the insulation (600 °C) is extremely appealing. Therefore, it could be interesting to devise high temperature fillers and suitable manufacturing procedures to exploit this insulation system in specific applications, where the weight of the machine is the key factor for material selection.

2011 Annual Report Conference on Electrical Insulation and Dielectric Phenomena, 2011

The presented paper focuses on monitoring of turn-to-turn insulation quality. It is based on the ... more The presented paper focuses on monitoring of turn-to-turn insulation quality. It is based on the measure of the variation of the winding turn-to-turn capacitance and the Partial Discharge Inception Voltage (PDIV). The test methods is defined and described. Twisted pairs of three different diameters of enameled wires were tested. The PDIV is measured with an adequate system properly calibrated to minimize background noise to an acceptable value. Finally, it is shown an increased capacitance and a reduction PDIV between aging cycles. The correlation of these two parameters opens perspectives for preventive maintenance applications.

2014 IEEE Electrical Insulation Conference (EIC), 2014

ABSTRACT This paper discuss about the advantages of thermo-bonded coils in rotating machines fed ... more ABSTRACT This paper discuss about the advantages of thermo-bonded coils in rotating machines fed by PWM inverters, relatively to the classical impregnation methods, for aircraft applications. Measurements made on three stators built with different technologies show that thermo-bonding methods give higher Partial Discharge Inception Voltages (PDIV) for the turn-to-turn insulation. A simple Finite Element model of the insulation of two adjacent enameled wires associated with the Paschen law explains the physical causes to these advantages. The model proposes also a method for evaluating the effect of the bonding layer thickness.

2005 European Microwave Conference, 2005

ABSTRACT BaSrTiO3 thin films were deposited by low-cost sol-gel techniques and exhibit relative p... more ABSTRACT BaSrTiO3 thin films were deposited by low-cost sol-gel techniques and exhibit relative permittivity of 300 and loss tangent of 0.012 with a roll off above 5 GHz. Subsequently the films were used for fabricating coplanar waveguide phaseshifters using tunable finger-shaped capacitors. A 310° phaseshift was thus obtained at 30 GHz and 35 V of tuning voltage with 3.6 dB of insertion loss yielding a figure of merit of 85°/dB.

Sensors and Actuators A: Physical, 1999

The investigation of piezoelectric properties of materials in the thin layer form has become an i... more The investigation of piezoelectric properties of materials in the thin layer form has become an important task because of the increased range of their applications as actuators and sensors. The sensor magnitude, is a direct function of the e31 piezoelectric constant. Pb(Zr,Ti)O3 thin films and the modified compositions have attracted great attention in recent years as promising for use in

Microelectronics Reliability, 1999

Lead zirconate titanate (PZT) thin films were deposited by radio-frequency (r.f.) magnetron sputt... more Lead zirconate titanate (PZT) thin films were deposited by radio-frequency (r.f.) magnetron sputtering with stoichiometric single oxide target on silicon substrates. The influence of the growth conditions (with or without substrate heating) and the post-annealing treatments (conventional or rapid thermal annealing) on the film properties in terms of structure and microstructure was systematically evaluated. The electrical properties such as the dielectric constant, the loss factor, the leakage current and the ferroelectric properties have been characterized.

Journal of the European Ceramic Society, 1999

ABSTRACT PZT (54/46) thin films were deposited by r.f. magnetron sputtering followed by a post-an... more ABSTRACT PZT (54/46) thin films were deposited by r.f. magnetron sputtering followed by a post-annealing treatment on silicon substrates. The crucial role of a Ti adhesion layer on the Ti/Pt bottom electrode is presented. The deposition conditions and the thickness of Ti have dominant effects on the interactions between Ti and Pt during the annealing treatment. The Pt layer, whatever its thickness, did not act as a barrier against Ti-out diffusion. The stability of the bottom electrode was achieved by using a TiOx layer instead of a pure metallic Ti adhesion layer. The electrical properties of PZT films in terms of dielectric and ferroelectric performance were evaluated, particularly as a function of the PZT film thickness.

Journal of Applied Physics, 2011

Journal of Alloys and Compounds, 2005

The present work concerns the preparation and dielectric characterization of some lead-free ceram... more The present work concerns the preparation and dielectric characterization of some lead-free ceramics with perovskite compositions derived from BaTiO 3 (BT). Here we report detailed dielectric studies carried out on barium strontium titanate Ba x Sr 1−x TiO 3 (BST) (80:20) and (60:40) compositions. We also present data on room temperature X-ray diffraction and dielectric permittivity measurements made in the temperature range from 80 to 600 K with frequencies from 0.1 to 200 kHz, respectively. The dielectric properties were studied as a function of frequency and temperature and well-defined ferroelectric behaviour of first order transition was observed. It follows the Curie-Weiss law above the transition temperature in the paraelectric region.

Integrated Ferroelectrics, 2007

ABSTRACT Paraelectric Ba0.5Sr0.5TiO3 films 0.3 μm thick have been deposited by sol-gel on c-axis ... more ABSTRACT Paraelectric Ba0.5Sr0.5TiO3 films 0.3 μm thick have been deposited by sol-gel on c-axis sapphire substrates. They have been investigated from 1 kHz to 60 GHz using coplanar waveguide transmission lines and interdigitated capacitors. The dielectric constant ϵr is around 300 and the loss tangent is 0.16 at 50 GHz. The tunability is constant with frequency with a mean value of 42%. Analog phase shifters were subsequently fabricated. A 180° phase shift was obtained at 60 GHz with a 17 V bias. The maximum value of phase shift per decibel of insertion losses (at 0 V) is 13°/dB at 30 GHz with a bias of 30 V.

Ferroelectrics, 1999

The investigation of piezoelectric properties of materials in the thin layer form has become an i... more The investigation of piezoelectric properties of materials in the thin layer form has become an important task because of the increased range of their applications as actuators and sensors. Pb(Zr, Ti)O3 thin films and the modified compositions have attracted great attention in recent years as promising for use in micro-electro-mechanical systems. The actuator magnitude, is an indirect function of the

Ferroelectrics, 2003

We report on the deposition of Ba 0.7 Sr 0.3 TiO 3 thin films, BST (70/30), by a sol-gel process.... more We report on the deposition of Ba 0.7 Sr 0.3 TiO 3 thin films, BST (70/30), by a sol-gel process. After deposition by spin coating, the film is annealed at 750°C, one hour in air. BST films have been deposited on (111) silicon substrates (bare and platinum coated) and on (0001) sapphire. X-Ray diffration patterns show that the films are

Applied Physics Letters, 2001

The RoHS european directive has prohibited lead in electric and electronic applications since Jul... more The RoHS european directive has prohibited lead in electric and electronic applications since July 2006. In this view, the paper deals with a study of a lead free ferroelectric material Ba x Sr 1-x TiO 3 (BST). The aim is to obtain equal or better performances than those of materials containing lead like PbZr x Ti 1-x O 3 (PZT). In this study, we focus on a Ba 0.9 Sr 0.1 TiO 3 (BST 90/10) thin film deposited by a sol-gel process on commercial (100) silicon substrates with platinum bottom electrode. We have improved the deposition by optimizing the annealing temperature (950 °C) and duration (15 mn). In these conditions, the dielectric properties measured at 100 kHz are: e' = 890 and tgδ = 0.01. The measurement of the pyroelectric coefficient gives γ= 340 μC/m 2 /K. The piezoelectric properties of two films were measured at the nanoscale and the macroscopic scale. Using piezoelectric force microscopy, we have evidenced the existence of local piezoelectric hysteresis loops on a 400 nm thick film. We have measured the piezoelectric coefficient d 33 on the 1μm thick film. This film exhibit a piezoelectric constant to 19 pC/N.

Lead in electronic components is prohibited by the RoHS directive since july 2006. Nevertheless, ... more Lead in electronic components is prohibited by the RoHS directive since july 2006. Nevertheless, there are some exceptions for example PbZr X Ti 1-X O 3 (PZT) materials because they present exceptional piezoelectric properties. PZT is used as sensors and actuators in numerous applications such in sonar, echography, alarm, etc…Up to now, neither in ceramic form nor in film form, it have been surpassed by lead free materials. At present, many studies are devoted to find a substitute to PZT as it will be probably banned in five years or so. Our work is in this objective and concerns the study of the electrical properties at low frequency, from DC to 1MHz, of ferroelectric films with thickness inferior or equal to 1µm. The starting material is BaTiO 3 in which Ba is exchanged by Sr at different levels and also doped with various cations (monovalent, divalent, trivalent…). This permits us to improve the dielectric and ferroelectric properties: for example to decrease the losses and to increase the tunability. Another method to optimize the electrical properties is to study the annealing conditions: very high temperature applied a short time (950°C@15mn) should be favoured. This leads to both higher dielectric constant and polarization. The pyroelectric and piezoelectric coefficients have also been improved but as concerns the *

IEEE Microwave and Wireless Components Letters, 2006

ABSTRACT Ferro- and para-electric BaSrTiO3 (εr∼350 and tgδ∼5×10-2 at 0V) thin films were deposite... more ABSTRACT Ferro- and para-electric BaSrTiO3 (εr∼350 and tgδ∼5×10-2 at 0V) thin films were deposited by low-cost sol-gel techniques. Subsequently, the films were used for fabricating coplanar waveguide phaseshifters using tunable finger-shaped capacitors. A 310° phaseshift was obtained at 30GHz and 35V of tuning voltage with 3.6dB of insertion loss yielding a figure of merit of 85°/dB.

IEEE Transactions on Dielectrics and Electrical Insulation

In the past few years there has been a growing trend in the use of ceramic coated high temperatur... more In the past few years there has been a growing trend in the use of ceramic coated high temperature resistant wires for applications up to 500 °C. The main drawback in developing new materials has been the relatively poor dielectric characteristics at high temperature. One solution is to strengthen the ceramic layer by covering it with boron oxide and this additional layer improves the dielectric characteristics of the ceramic wire. Partial discharge inception voltage threshold is greatly increased compared to non-impregnated wires and this has been observed over a large temperature range.

IEEE Transactions on Dielectrics and Electrical Insulation

The purpose of this paper is to understand the pros and cons of aluminum conductors insulated wit... more The purpose of this paper is to understand the pros and cons of aluminum conductors insulated with anodized aluminum for rotating machines used in aircraft. The investigation is focused on the breakdown voltage and partial discharge inception voltage of simple systems (crossed pairs, simplified coils). The results show that both breakdown voltage and partial discharge inception voltage are very sensitive to curvature radius and lower than those achieved using conventional copper conductors insulated with polymeric enamels. However, the operating temperature of the insulation (600 °C) is extremely appealing. Therefore, it could be interesting to devise high temperature fillers and suitable manufacturing procedures to exploit this insulation system in specific applications, where the weight of the machine is the key factor for material selection.

2011 Annual Report Conference on Electrical Insulation and Dielectric Phenomena, 2011

The presented paper focuses on monitoring of turn-to-turn insulation quality. It is based on the ... more The presented paper focuses on monitoring of turn-to-turn insulation quality. It is based on the measure of the variation of the winding turn-to-turn capacitance and the Partial Discharge Inception Voltage (PDIV). The test methods is defined and described. Twisted pairs of three different diameters of enameled wires were tested. The PDIV is measured with an adequate system properly calibrated to minimize background noise to an acceptable value. Finally, it is shown an increased capacitance and a reduction PDIV between aging cycles. The correlation of these two parameters opens perspectives for preventive maintenance applications.

2014 IEEE Electrical Insulation Conference (EIC), 2014

ABSTRACT This paper discuss about the advantages of thermo-bonded coils in rotating machines fed ... more ABSTRACT This paper discuss about the advantages of thermo-bonded coils in rotating machines fed by PWM inverters, relatively to the classical impregnation methods, for aircraft applications. Measurements made on three stators built with different technologies show that thermo-bonding methods give higher Partial Discharge Inception Voltages (PDIV) for the turn-to-turn insulation. A simple Finite Element model of the insulation of two adjacent enameled wires associated with the Paschen law explains the physical causes to these advantages. The model proposes also a method for evaluating the effect of the bonding layer thickness.

2005 European Microwave Conference, 2005

ABSTRACT BaSrTiO3 thin films were deposited by low-cost sol-gel techniques and exhibit relative p... more ABSTRACT BaSrTiO3 thin films were deposited by low-cost sol-gel techniques and exhibit relative permittivity of 300 and loss tangent of 0.012 with a roll off above 5 GHz. Subsequently the films were used for fabricating coplanar waveguide phaseshifters using tunable finger-shaped capacitors. A 310° phaseshift was thus obtained at 30 GHz and 35 V of tuning voltage with 3.6 dB of insertion loss yielding a figure of merit of 85°/dB.

Sensors and Actuators A: Physical, 1999

The investigation of piezoelectric properties of materials in the thin layer form has become an i... more The investigation of piezoelectric properties of materials in the thin layer form has become an important task because of the increased range of their applications as actuators and sensors. The sensor magnitude, is a direct function of the e31 piezoelectric constant. Pb(Zr,Ti)O3 thin films and the modified compositions have attracted great attention in recent years as promising for use in

Microelectronics Reliability, 1999

Lead zirconate titanate (PZT) thin films were deposited by radio-frequency (r.f.) magnetron sputt... more Lead zirconate titanate (PZT) thin films were deposited by radio-frequency (r.f.) magnetron sputtering with stoichiometric single oxide target on silicon substrates. The influence of the growth conditions (with or without substrate heating) and the post-annealing treatments (conventional or rapid thermal annealing) on the film properties in terms of structure and microstructure was systematically evaluated. The electrical properties such as the dielectric constant, the loss factor, the leakage current and the ferroelectric properties have been characterized.

Journal of the European Ceramic Society, 1999

ABSTRACT PZT (54/46) thin films were deposited by r.f. magnetron sputtering followed by a post-an... more ABSTRACT PZT (54/46) thin films were deposited by r.f. magnetron sputtering followed by a post-annealing treatment on silicon substrates. The crucial role of a Ti adhesion layer on the Ti/Pt bottom electrode is presented. The deposition conditions and the thickness of Ti have dominant effects on the interactions between Ti and Pt during the annealing treatment. The Pt layer, whatever its thickness, did not act as a barrier against Ti-out diffusion. The stability of the bottom electrode was achieved by using a TiOx layer instead of a pure metallic Ti adhesion layer. The electrical properties of PZT films in terms of dielectric and ferroelectric performance were evaluated, particularly as a function of the PZT film thickness.

Journal of Applied Physics, 2011

Journal of Alloys and Compounds, 2005

The present work concerns the preparation and dielectric characterization of some lead-free ceram... more The present work concerns the preparation and dielectric characterization of some lead-free ceramics with perovskite compositions derived from BaTiO 3 (BT). Here we report detailed dielectric studies carried out on barium strontium titanate Ba x Sr 1−x TiO 3 (BST) (80:20) and (60:40) compositions. We also present data on room temperature X-ray diffraction and dielectric permittivity measurements made in the temperature range from 80 to 600 K with frequencies from 0.1 to 200 kHz, respectively. The dielectric properties were studied as a function of frequency and temperature and well-defined ferroelectric behaviour of first order transition was observed. It follows the Curie-Weiss law above the transition temperature in the paraelectric region.

Integrated Ferroelectrics, 2007

ABSTRACT Paraelectric Ba0.5Sr0.5TiO3 films 0.3 μm thick have been deposited by sol-gel on c-axis ... more ABSTRACT Paraelectric Ba0.5Sr0.5TiO3 films 0.3 μm thick have been deposited by sol-gel on c-axis sapphire substrates. They have been investigated from 1 kHz to 60 GHz using coplanar waveguide transmission lines and interdigitated capacitors. The dielectric constant ϵr is around 300 and the loss tangent is 0.16 at 50 GHz. The tunability is constant with frequency with a mean value of 42%. Analog phase shifters were subsequently fabricated. A 180° phase shift was obtained at 60 GHz with a 17 V bias. The maximum value of phase shift per decibel of insertion losses (at 0 V) is 13°/dB at 30 GHz with a bias of 30 V.

Ferroelectrics, 1999

The investigation of piezoelectric properties of materials in the thin layer form has become an i... more The investigation of piezoelectric properties of materials in the thin layer form has become an important task because of the increased range of their applications as actuators and sensors. Pb(Zr, Ti)O3 thin films and the modified compositions have attracted great attention in recent years as promising for use in micro-electro-mechanical systems. The actuator magnitude, is an indirect function of the

Ferroelectrics, 2003

We report on the deposition of Ba 0.7 Sr 0.3 TiO 3 thin films, BST (70/30), by a sol-gel process.... more We report on the deposition of Ba 0.7 Sr 0.3 TiO 3 thin films, BST (70/30), by a sol-gel process. After deposition by spin coating, the film is annealed at 750°C, one hour in air. BST films have been deposited on (111) silicon substrates (bare and platinum coated) and on (0001) sapphire. X-Ray diffration patterns show that the films are

Applied Physics Letters, 2001

The RoHS european directive has prohibited lead in electric and electronic applications since Jul... more The RoHS european directive has prohibited lead in electric and electronic applications since July 2006. In this view, the paper deals with a study of a lead free ferroelectric material Ba x Sr 1-x TiO 3 (BST). The aim is to obtain equal or better performances than those of materials containing lead like PbZr x Ti 1-x O 3 (PZT). In this study, we focus on a Ba 0.9 Sr 0.1 TiO 3 (BST 90/10) thin film deposited by a sol-gel process on commercial (100) silicon substrates with platinum bottom electrode. We have improved the deposition by optimizing the annealing temperature (950 °C) and duration (15 mn). In these conditions, the dielectric properties measured at 100 kHz are: e' = 890 and tgδ = 0.01. The measurement of the pyroelectric coefficient gives γ= 340 μC/m 2 /K. The piezoelectric properties of two films were measured at the nanoscale and the macroscopic scale. Using piezoelectric force microscopy, we have evidenced the existence of local piezoelectric hysteresis loops on a 400 nm thick film. We have measured the piezoelectric coefficient d 33 on the 1μm thick film. This film exhibit a piezoelectric constant to 19 pC/N.

Lead in electronic components is prohibited by the RoHS directive since july 2006. Nevertheless, ... more Lead in electronic components is prohibited by the RoHS directive since july 2006. Nevertheless, there are some exceptions for example PbZr X Ti 1-X O 3 (PZT) materials because they present exceptional piezoelectric properties. PZT is used as sensors and actuators in numerous applications such in sonar, echography, alarm, etc…Up to now, neither in ceramic form nor in film form, it have been surpassed by lead free materials. At present, many studies are devoted to find a substitute to PZT as it will be probably banned in five years or so. Our work is in this objective and concerns the study of the electrical properties at low frequency, from DC to 1MHz, of ferroelectric films with thickness inferior or equal to 1µm. The starting material is BaTiO 3 in which Ba is exchanged by Sr at different levels and also doped with various cations (monovalent, divalent, trivalent…). This permits us to improve the dielectric and ferroelectric properties: for example to decrease the losses and to increase the tunability. Another method to optimize the electrical properties is to study the annealing conditions: very high temperature applied a short time (950°C@15mn) should be favoured. This leads to both higher dielectric constant and polarization. The pyroelectric and piezoelectric coefficients have also been improved but as concerns the *