Reliability investigation of a carbon nanotube array thermal interface material

As feature density increases within microelectronics, so does the dissipated power density, which puts an increased demand on thermal management. Thermal interface materials (TIMs) are used at the interface between contacting surfaces to reduce the thermal resistance, and is a critical component within many electronics systems. Arrays of carbon nanotubes (CNTs) have gained significant interest for application as TIMs, due to the high thermal conductivity, no internal thermal contact resistances and an excellent conformability. While studies show excellent thermal performance, there has to date been no investigation into the reliability of CNT array TIMs. In this study, CNT array TIMs bonded with polymer to close a Si-Cu interface were subjected to thermal cycling. Thermal interface resistance measurements showed a large degradation of the thermal performance of the interface within the first 100 cycles. More detailed thermal investigation of the interface components showed that the connection between CNTs and catalyst substrate degrades during thermal cycling even in the absence of thermal expansion mismatch, and the nature of this degradation was further analyzed using X-ray photoelectron spectroscopy. This study indicates that the reliability will be an important consideration for further development and commercialization of CNT array TIMs

Texturation of lead-free BaTiO₃ based piezoelectric ceramics: [Invited]

oral invitéInternational audiencePiezoelectric ceramics have been integrated for a long time in a widerange of devices, particularly in ultrasonic applications (sonar systems,medical imaging...) and most of them use Pb(Zr,Ti)O3 (PZT) materials.However, due to health care and environmental problems, lead contentmust be reduced in such applications. Among the few lead-freematerials families which can be considered for the replacement of PZT,BaTiO3 appear as interesting because of its piezoelectric properties atroom temperature and capacity to be modulated by doping, even ifCurie temperature is not very high (120°C).However, ceramics are generally limited by their isotropic nature. Forthis reason, texturing process was developed in order toimprove/optimize their electromechanical properties. The aim of thepresent study is thus to obtain textured BaTiO3 based materials byusing the templated grain growth process (TGG) and to measure theirpiezoelectric properties.Doped and undoped BaTiO3 powders were prepared by classical solidstate route while BaTiO3 templates were elaborated by a molten saltsprocess. Green ceramics were then obtained by tapecasting of a slurrycontaining templates and matrix particles dispersed in theappropriated solvent. After drying, green sheet was cut, stacked,pressed and then sintered at the appropriated temperature, in order toobtain thick or thin samples. This process allowed obtaining highlyorientedmaterials (texturation degree 70% to 90%). Piezoelectricproperties were investigated, for doped and undoped samples, and fordifferent sintering parameters. Some samples appears as veryinteresting for piezoelectric applications with d33* higher than 300 pC/N, against 180 pC/N for BaTiO3 ceramics obtained by classical way

Thermal Reliability Study of Polymer Bonded Carbon Nanotube Array Thermal Interface Materials

Following Moores law, the development of electronics has led to an exponential increase of transistor density over the last couple of decades. Unfortunately, this trend also gives an increased heat power density in active components. Thermal interface materials (TIMs) are used to decrease the thermal resistance in thermal packages by filling out air gaps that naturally form there. TIMs are at the same time identified as a bottleneck due to their relatively low thermal conductivity. Carbon nanotubes (CNTs) are proposed as a future material for TIMs due to their high thermal conductivity and conformable nature. However, no reliability studies for CNT array TIMs can be found in literature that would demonstrate how these types of interfaces would perform. This is to the authors best knowledge the first reported study on thermal reliability for a CNT array TIM, which will be an important step towards a market realisation

Highly textured lead-free piezoelectric polycrystals grown by the micro-pulling down freezing technique in the BaTiO3 –CaTiO3 system

International audienceHighly textured cm-sized (Ba1−xCax)TiO3 (x = 0.05) polycrystals at off-eutectic composition were grown by the micro-pulling down freezing technique from a BaTiO3–CaTiO3 system at a high pulling velocity of about 48 mm h−1. Textured polycrystals exhibiting submillimetre-sized dendrites along the (110)pc growth direction and geometrical grains along the (100)pc direction were revealed by a complementary analysis performed by Laue diffraction and EBSD techniques. An average Ca content of about x = 0.0435 was measured throughout the whole volume of the boule and led to an effective partition coefficient of Ca of about 0.87. LIBS measurements highlighted the local segregation of Ca and major impurity contents within dendrites, grains and at the grain boundaries. Dielectric and piezoelectric measurements performed on oriented polycrystalline textured samples led to Curie temperatures up to 116 °C and d33 up to 214 pC N−1, in very good agreement with literature values. Both chemical and physical results obtained in the BCT system make the μ-PD technique a promising and alternative way to improve the piezoelectric response of lead-free solid solution-based crystals through their texturing along preferential crystallographic directions

Spinodal Decomposition in Lead-free Piezoelectric BaTiO3 –CaTiO3 –BaZrO3 Crystals

Polycrystals and centimeter-sized BaTiO3-based single crystals were grown by top seeded solution growth from the BaTiO3–CaTiO3–BaZrO3 system. High effective partition coefficients of Zr ranging from 15 to 6 with small Zr content have been calculated from Castaing microprobe measurements, whereas those of Ca increase slightly from 0.45 to 0.7. A spinodal decomposition mechanism is emphasized during the growth leading to the emergence of two phases with close compositions. Chemical analysis displayed periodical Zr and Ca content fluctuations within the whole boules, and Rietveld measurements highlighted two phases belonging to perovskite structures with tetragonal P4mm and orthorhombic Amm2 space groups. Samples with various calcium and zirconium contents were characterized by means of dielectric and piezoelectric measurements. Most efficient samples are indistinctly polycrystals or oriented single crystals where electromechanical performances are compositional-dependent. Polycrystalline samples and single crystals oriented along (001)pc and (110)pc displayed Curie temperatures ranging from 50 to 111 °C. Electromechanical coupling factor up to 58% and piezoelectric charge coefficient d33 = 496 pC·N–1 were obtained at room temperature. The miscibility gap between the two perovskite solid solutions as well as Ca and Zr element content variation in single crystals lower crystal piezoelectric response that remains nonetheless of the same efficiency compared to that of ceramics of the same composition

Design and realization of characterization demonstrator to investigate thermal performance of vertically-aligned carbon nanotubes TIM for avionics and aerospace applications

In the joint project SMARTHERM the applicability of vertically-aligned carbon nanotubes (VACNT) is main subject of interest. Target is the implementation of a VACNT layer as functional thermal interface material into an RF package to prove this promising technology\u27s feasibility. This paper presents the approach the SMARTHERM consortium has taken so far. Beginning with a general motivation why to engage in this topic, the approach and the chosen demonstrator design are introduced. One focus lies on the description of the demonstrator and its components alongside with a comprehensive view into the assembling as main challenge. The choice of measurement techniques is discussed and the measures of success are defined. Finally, the results are presented, discussed and concluded and an outlook is provided how the findings influence further approaches in the project

High-Frequency Linear Array (20 MHz) based on Lead-Free BCTZ Crystal

International audienceCentimeter-sized BaTiO 3 -based crystals grown by top-seeded solution growth from the BaTiO 3 –CaTiO 3 –BaZrO 3 system were used to process a high-frequency (HF) lead-free linear array. Piezoelectric plates with (110)pc cut within 1° accuracy were used to manufacture two 1-3 piezo-composites with thicknesses of 270 and 78 μm for resonant frequencies in air of 10 and 30 MHz, respectively. The electromechanical characterization of the BCTZ crystal plates and the 10 MHz piezocomposite yielded thickness coupling factors of 40% and 50%, respectively. We quantified the electromechanical performance of the second piezocomposite (30 MHz) according to the reduction in the pillar sizes during the fabrication process. The dimensions of the piezocomposite at 30 MHz were sufficient for a 128-element array with a 70 μm element pitch and a 1.5 mm elevation aperture. The transducer stack (backing, matching layers, lens and electrical components) was tuned with the characteristics of the lead-free materials to deliver optimal bandwidth and sensitivity. The probe was connected to a real-time HF 128-channel echographic system for acoustic characterization (electroacoustic response, radiation pattern) and to acquire high-resolution in vivo images of human skin. The center frequency of the experimental probe was 20 MHz, and the fractional bandwidth at -6 dB was 41%. Skin images were compared against those obtained with a lead-based 20-MHz commercial imaging probe. Despite significant differences in sensitivity between elements, in vivo images obtained with a BCTZ-based probe convincingly demonstrated the potential of integrating this piezoelectric material in an imaging probe

Lead-free piezoelectric crystals grown by the micro-pulling down technique in the BaTiO3–CaTiO3–BaZrO3 system

BaTiO3-based crystal fibres with mm-sized grains were grown by the micro-pulling down technique from the BaTiO3–CaTiO3–BaZrO3 solid solution with pulling velocities of about 6, 9 and 15 mm h−1. The natural growth direction was identified as (001)pc. For the pulling velocities of about 15 mm h−1 and 9 mm h−1, effective partition coefficients have been calculated from Castaing micro-probe measurements, and gave, respectively, 1.3 and 2 for Zr, and 0.95 and 0.9 for Ca. Laser-induced breakdown spectroscopy measurements reveal a strong inhomogeneity and variations of Zr contents while Ca contents show an opposite variation trend with a more steady distribution. Coexistence of two crystallized perovskite solid solutions is suggested. Most efficient polycrystals with mm-sized grains and 0.5 mol% Zr and 11 mol% Ca as average contents exhibit Curie temperatures higher than 113 °C, electromechanical coupling factors kt up to 41% and piezoelectric charge coefficients d33 up to 242 pC N−1 at room temperature. These values are similar to piezoelectric coefficients reported in the literature for oriented flux-grown single crystals with close compositions. Both chemical and physical results obtained in the BCTZ system make the μ-PD technique a promising way to improve the piezoelectric response of lead-free solid solution-based single crystals