Sintering Of Cu Nanoparticles Research Articles

Enhanced shear strength and microstructure of Cu–Cu interconnection by low-temperature sintering of Cu nanoparticles

Enhanced shear strength and microstructure of Cu–Cu interconnection by low-temperature sintering of Cu nanoparticles

Laser sintering of Cu nanoparticles deposited on ceramic substrates: Experiments and modeling

Laser sintering of Cu nanoparticles deposited on ceramic substrates: Experiments and modeling

The formation of Cu-Cu joints by low temperature sintering Cu NPs with copper formate layer and its oxidation enhancement

Highly conductive and robust Cu-Cu jonts are achieved by the low temperature sintering of Cu nanoparticles (Cu NPs) with a formic acid treatment. When sintered at 300 °C, the Cu-Cu joints exhibited a low resistivity of 4.79 µΩ·cm with a shear strength larger than 30 MPa. In addition, unique oxidation-enhancing effect of Cu-Cu joints is discovered in thermal storage experiments. When thermal storage at 50 h, the strength of the joints increases to 39.25 MPa, while the resistivity is only increased by a factor of 5.78. This discovery provides a potential packaging enhancement process for practical application of electronic devices.

Silica-modulated Cu-ZnO-Al2O3 catalyst for efficient hydrogenation of CO2 to methanol

Cu/ZnO/Al2O3 catalyst has been widely studied in the hydrogenation of CO2 to methanol, but it still suffers from insufficient catalyst stability and methanol selectivity. Herein, we overcome these issues by introducing the silica promoter to the catalyst, achieving the Cu/ZnO/Al2O3/SiO2 (CZAS) catalysts with significantly improved methanol selectivity and catalyst durability. Characterizations of samples showed that the silica species efficiently hindered the sintering of Cu nanoparticles during the catalysis, exhibiting constant performances in the continuous tests. In addition, the silica also electronically modulated the Cu nanoparticles that result in abundant Cuδ+ species in the catalyst, which minimized the methanol decomposition to improve the methanol selectivity, compared with the silica-free Cu/ZnO/Al2O3 catalyst. As a result, the CZAS exhibited CO2 conversion at 12.6 % and methanol selectivity at 85.1 %, giving a one-pass methanol yield at 10.7 %. The strategy in this work might provide an efficient route for enhancing the current industrial catalysts.

Sorbitol-derived carbon overlayers encapsulated Cu nanoparticles on SiO2: Stable and efficient for the continuous hydrogenation of ethylene carbonate

SummaryAn ultrastable and efficient Cu@C/SiO2 nanocatalyst was fabricated for the hydrogenation of ethylene carbonate, in which Cu nanoparticles are encapsulated by sorbitol-derived graphitized carbon overlayers. During the calcination of Cu-sorbitol/SiO2 precursors under N2 atmosphere, sorbitol decomposed to CO and CO2. The in situ generated CO not only reduced Cu2+ to Cu0/Cu+, but also formed graphitized carbon overlayers on the Cu surface via the disproportionation of CO. The Cu@C/SiO2 catalyst exhibited superior catalytic performance (91% MeOH yield and 43.6 h−1 TOF) at a H2/EC molar ratio of 20. Of particular note, the Cu@C/SiO2 catalyst showed remarkable long-term stability during 736 h time-on-stream test without any deactivation. The graphitized carbon overlayers on the surface of Cu nanoparticles not only functioned synergistically with the surface Cu0/Cu+ sites to promote the EC hydrogenation but also suppressed the sintering of Cu nanoparticles. Furthermore, the interaction of Cu nanoparticles and graphitized carbon overlayers stabilized the surface Cu+/(Cu0+Cu+) ratio.

Ultra-high thermal stability of sputtering reconstructed Cu-based catalysts

The rational design of high-temperature endurable Cu-based catalysts is a long-sought goal since they are suffering from significant sintering. Establishing a barrier on the metal surface by the classical strong metal-support interaction (SMSI) is supposed to be an efficient way for immobilizing nanoparticles. However, Cu particles were regarded as impossible to form classical SMSI before irreversible sintering. Herein, we fabricate the SMSI between sputtering reconstructed Cu and flame-made LaTiO2 support at a mild reduction temperature, exhibiting an ultra-stable performance for more than 500 h at 600 °C. The sintering of Cu nanoparticles is effectively suppressed even at as high as 800 °C. The critical factors to success are reconstructing the electronic structure of Cu atoms in parallel with enhancing the support reducibility, which makes them adjustable by sputtering power or decorated supports. This strategy will extremely broaden the applications of Cu-based catalysts at more severe conditions and shed light on establishing SMSI on other metals.

Understanding the sintering and heat dissipation behaviours of Cu nanoparticles during low-temperature selective laser sintering process on flexible substrates

In this paper, a facile method was introduced to fabricate fine conductive patterns by low-temperature selective laser sintering of Cu nanoparticles. By virtue of a nanosecond-pulsed ultraviolet laser source, fine circuits with a thickness of ∼8 μm and a conductivity of 3–6.5 × 106 S m−1 was successfully fabricated from Cu nanoparticle paste with a high metal content >80 wt.% on a flexible substrate at a wide scan rate range of 5–500 mm s−1. The sintered circuits exhibited a special sandwich morphology, with fully melted features in the centre and thermally sintered neck-connected features on the edges. A twofold linear relationship between the width of thermally sintered region and the reciprocal of the laser scan rate was revealed, indicating a heat dissipation mode transition from metal layer dominated dissipation to substrate dominated dissipation with the decrease of the laser scan rate. Finite element simulations were carried out to study the evolutions of temperature field and heat dissipation with changing laser scan rate and power, and the results fitted well with experimental results. On this basis, a relational expression was further proposed to determine the optimal processing window for laser sintering of metal nanoparticle layers. Our method extends the producible thickness and improves the fabrication efficiency of laser-printed circuits with desirable conductivity. The findings of this study can provide a guidance for understanding and controlling the sintering and heat transfer process of printing methods with similar materials and techniques.

Reactive Sintering of Cu Nanoparticles at Ambient Conditions for Printed Electronics.

A new approach is presented to overcome the disadvantages of oxidation and harsh sintering conditions of Cu nanoparticle (Cu NP) conductive inks simultaneously. In this process, oleylamine (OAM) adsorbed on particles was effectively eliminated via the reactive desorption by formic acid in alcohols; meanwhile, Cu ion was generated on the surface. The desorption of OAM resulted in more severe surface oxidation of Cu NPs. The oxide (Cu2O) and Cu2+ distributed on the Cu NP surface could be reduced to Cu(0) by NaBH4 solution and take on the role of soldering flux to weld particles into a blocky structure. With the compact coalescence of particles without oxides, the resistivity of metal patterns could fall below 20 μΩ·cm and exhibit proper adhesion. Thanks to the sintering of Cu NPs at ambient conditions, the conductive patterns could be facilely formed on thermosensitive substrates. As the oxide state of Cu would be reduced during sintering, the partially oxidized Cu nanoparticles could be directly applied to conductive inks.

Laser sintering of Cu nanoparticles on PET polymer substrate for printed electronics at different wavelengths and process conditions

This study explores the feasibility of different laser systems to sinter screen-printed lines from nonconductive copper nanoparticles (Cu NPs) on polyethylene terephthalate polymer film. These materials are commonly used in manufacturing functional printed electronics for large-area applications. Here, optical and thermal characterization of the materials is conducted to identify suitable laser sources and process conditions. Direct diode (808 nm), Nd:YAG (1064 nm and second harmonic of 532 nm), and ytterbium fiber (1070 nm) lasers are explored. Optimal parameters for sintering the Cu NPs are identified for each laser system, which targets low resistivity and high processing speed. Finally, the quality of the sintered tracks is quantified, and the laser sintering mechanisms observed under different wavelengths are analyzed. Practical considerations are discussed to improve the laser sintering process of Cu NPs.

Laser sintering of Cu nanoparticles: analysis based on modified continuum-atomistic model

This paper reports on the use of molecular dynamics simulation in conjunction with a modified two-temperature model to elucidate the dynamic behavior of copper nanoparticles (Cu-NPs) undergoing laser sintering. Simulations were conducted using seven models of Cu-NP powders to investigate the means by which particle size (SP), incident laser fluence (I0), and stacking patterns (powders comprising Cu-NPs of either one or two sizes) affect the evolution of electronic temperature (Te), lattice temperature (Tl), pressure (P), and atomic density (dAN) in Cu-NP powders under the effects of laser sintering. We also studied the evolution of sintering configurations and the radial distribution function (g(r)) of Cu-NPs. In powders comprising Cu-NPs of uniform size (2 nm $$\le$$ Sp $$\le$$ 5 nm), the apparent density (d) initially increased (i.e., the powder densified) with I0 and then decreased (i.e., the powder coarsened) with a further increase in I0. Single-size powders composed of smaller Cu-NPs (Sp = 1 nm) underwent densification followed by coarsening followed by densification. Dual-size powders comprising Cu-NPs (Sp = 1 and 3 nm) underwent coarsening almost linearly with an increase in I0, due to the fact that in this model, the incident energy was sufficient to facilitate the atomic diffusion of smaller Cu particles toward larger Cu particles.

Pressureless die attach by transient liquid phase sintering of Cu nanoparticles and Sn-58Bi particles assisted by polyvinylpyrrolidone dispersant

Highly reliable bonding materials have attracted tremendous interest due to a growing demand for high-temperature electronics. We developed a fluxless and binder-free paste comprising Cu nanoparticles (NPs), Sn-58Bi (SnBi) particles, and polyvinylpyrrolidone (PVP) dispersing agent, which enables pressureless, low-temperature (190–250 °C) formation of robust joints (over 7 MPa) by means of transient liquid phase sintering (TLPS). Microstructural evolution of the joint was investigated under variations in PVP molecular weight (MW; 10,000, 55,000, 360,000, or 1,300,000) and bonding conditions including temperature and time. In a die-shear test, the joint formed with PVP MW 360,000 was the strongest due to its proper particle dispersion and the formation of intermetallic compounds (IMCs). Conditions of excessive PVP MW, bonding temperature, and time impeded the bonding characteristics of the TLPS joint, with formation of voids and increasing brittleness. TLPS bonding with the optimal dispersing agent enabled pressureless die attach without chip damage, demonstrating applicability as a simple, robust interconnection for high-temperature electronics.

Effect of Bed Temperature on the Laser Energy Required to Sinter Copper Nanoparticles

Copper nanoparticles (NPs), due to their high electrical conductivity, low cost, and easy availability, provide an excellent alternative to other metal NPs such as gold, silver, and aluminum in applications ranging from direct printing of conductive patterns on metal and flexible substrates for printed electronics applications to making three-dimensional freeform structures for interconnect fabrication for chip-packaging applications. Lack of research on identification of optimum sintering parameters such as fluence/irradiance requirements for sintering of Cu NPs serves as the primary motivation for this study. This article focuses on the identification of a good sintering irradiance window for Cu NPs on an aluminum substrate using a continuous wave (CW) laser. The study also includes the comparison of CW laser sintering irradiance windows obtained with substrates at different initial temperatures. The irradiance requirements for sintering of Cu NPs with the substrate at 150–200°C were found to be 5–17 times smaller than the irradiance requirements for sintering with the substrate at room temperature. These findings were also compared against the results obtained with a nanosecond (ns) laser and a femtosecond (fs) laser.

Molecular dynamics study on the coalescence kinetics and mechanical behavior of nanoporous structure formed by thermal sintering of Cu nanoparticles

A molecular dynamics (MD) simulation is performed on the coalescence kinetics and mechanical behavior of a thermally sintered nanoporous copper (Cu) nanoparticulate system. To investigate the effect of particle size and sintering temperature on the coalescence of the nanoparticulate system, particles with sizes of 4, 5, and 6 nm are sintered at temperatures of 300, 500, and 700 K. To determine the thermal sintering process at elevated temperatures and ambient pressure, bulk periodic nanoparticle unit cells consisting of a finite number of nanoparticles are equilibrated through isothermal–isobaric ensemble simulations. In thermally sintered configurations, uniaxial tension/compression and shearing simulations are applied at a constant strain rate to derive stress–strain curves. It is found that stacking faults are actively generated in smaller nanoparticles even at a low sintering temperature, while local amorphization and surface and grain boundary diffusion are rather prominent in larger nanoparticles. Even at the same sintering temperature, the density of the sintered nanoparticle increases as the size of the nanoparticle decreases. In elastic moduli, the same particle size dependency is observed, while no obvious difference is observed in tension and compression. On the other hand, the yield strengths of the sintered nanoparticles in tension are larger than those in compression. The asymmetric yield strength of the sintered systems is clarified by addressing the surface stress and surface equilibrium strain of atoms on the surface of nanopores by the evolution of atomic virial stress in tension and compression.

Effect of copper oxide shell thickness on flash light sintering of copper nanoparticle ink

In this study, the effect of the thickness of a copper oxide-shell on flash light sintering of Cu nanoparticles (NPs) was investigated.

Highly Conductive Cu-Cu Joint Formation by Low-Temperature Sintering of Formic Acid-Treated Cu Nanoparticles.

Highly conductive Cu-Cu interconnections of SiC die with Ti/Ni/Cu metallization and direct bonded copper substrate for high-power semiconductor devices are achieved by the low-temperature sintering of Cu nanoparticles with a formic acid treatment. The Cu-Cu joints formed via a long-range sintering process exhibited good electrical conductivity and high strength. When sintered at 260 °C, the Cu nanoparticle layer exhibited a low resistivity of 5.65 μΩ·cm and the joints displayed a high shear strength of 43.4 MPa. When sintered at 320 °C, the resistivity decreased to 3.16 μΩ·cm and the shear strength increased to 51.7 MPa. The microstructure analysis demonstrated that the formation of Cu-Cu joints was realized by metallurgical bonding at the contact interface between the Cu pad and the sintered Cu nanoparticle layer, and the densely sintered layer was composed of polycrystals with a size of hundreds of nanometers. In addition, high-density twins were found in the interior of the sintered layer, which contributed to the improvement of the performance of the Cu-Cu joints. This bonding technology is suitable for high-power devices operating under high temperatures.

Formation of closely packed Cu nanoparticle films by capillary immersion force for preparing low-resistivity Cu films at low temperature

Films made of closely packed Cu nanoparticles (NPs) were obtained by drop casting Cu NP inks. The capillary immersion force exerted during the drying of the inks caused the Cu NPs to attract each other, resulting in closely packed Cu NP films. The apparent density of the films was found to depend on the type of solvent in the ink because the capillary immersion force is affected by the solvent surface tension and dispersibility of Cu NPs in the solvent. The closely packed particulate structure facilitated the sintering of Cu NPs even at low temperature, leading to low-resistivity Cu films. The sintering was also enhanced with a decrease in the size of NPs used. We demonstrated that a closely packed particulate structure using Cu NPs with a mean diameter 61.7 nm showed lower resistivity (7.6 μΩ cm) than a traditionally made Cu NP film (162 μΩ cm) after heat treatment.

The low temperature exothermic sintering of formic acid treated Cu nanoparticles for conductive ink

In this study, we present a new simple method using formic acid pretreatment to achieve low temperature sintering of Cu nanoparticles, and the Cu nanoparticles have been successfully prepared with a modified polyol method. For formic acid pretreatment, the spherical Cu nanoparticles with an average size of 30 nm were saturated with a mixed solution of formic acid and absolute ethanol directly. The surface composition analysis indicated that the initial oxide on the surface of Cu nanoparticles was completely removed, and formate was generated as a by-product which could decompose at low temperature. According to the thermal analysis results, it was found that the sintering temperature of Cu nanoparticles with formic acid pretreatment was reduced to 160 °C. And the corresponding microstructures further demonstrated that the emerging small exothermic peak of thermal curve over the temperature range between 120 and 160 °C was caused by the formation of sintering necks. The formic acid treated Cu nanoparticle films sintered at 260 and 320 °C exhibited an electrical resistivity of 6.1 and 3.6 μΩ cm respectively.

Pressureless Bonding by Use of Cu and Sn Mixed Nanoparticles

Because of high thermal and electrical conductivity, high melting point, and low cost, bonding by sintering of Cu nanoparticles is promising as a new method to replace the Pb-rich solders currently used in high-temperature applications. However, it is difficult to achieve sufficient strength by using this method because, in the absence of applied pressure, oxidized surfaces inhibit sintering. In this study, we report pressureless bonding of Cu plates with Ni layers by use of Cu and Sn mixed nanoparticles. Bonding was achieved without pressure from 250°C to 350°C in hydrogen by use of pure Cu nanoparticles, and by using mixtures of Cu and Sn nanoparticles in which the amount of Sn varied from 10 to 50 wt.%. The highest strength bonds were obtained by use of Cu–10 wt.% Sn mixed nanoparticles, because the sinterability of the Cu nanoparticles is enhanced by diffusion of Sn into Cu to form an appropriate amount of Cu–Sn intermetallic compounds (IMC) and diminish microvoids. However, when the amount of Sn was greater than 10 wt.%, Cu–Sn IMC were formed to such an extent that the significant reduction of Cu-rich layers led to reduced strength. When the bonding temperature was 350°C, Sn diffused into Cu so much that microvoids were formed in the Sn-rich layer. Because the number of microvoids increased as the amount of Sn was increased, the shear strength could not be enhanced by bonding at higher temperature when the amount of Sn was greater than 30 wt.%.

Three-dimensional sponge-like architectured cupric oxides as high-power and long-life anode material for lithium rechargeable batteries

Abstract Cupric oxide (CuO) nanoparticles (NPs) with three-dimensional (3D) sponge structure are obtained through the sintering of Cu NPs at 360 °C. Their morphology is analyzed by transmission electron microscopy (TEM) and scanning electron microscopy (SEM), and their crystal structure is checked by X-ray diffraction. CuO NPs have a 3D porous structure. The NPs are assembled to form larger secondary particles with many empty spaces among them, and they have a CuO phase after the heat treatment. CuO NPs with this novel architecture exhibit good electrochemical performance as anode material. The anode material with a sponge-like structure is prepared at 360 °C, as the Li-ion battery exhibits a high electrochemical capacity of 674 mAh g −1 . When the sample is sintered at 360 °C, the charge/discharge capacities increase gradually and cycle up to 50 cycles at a C/10 rate, exhibiting excellent rate capability compared with earlier reported CuO/CuO-composite anodes. Electrochemical impedance spectroscopy (EIS) measurements suggest that the superior electrical conductivity of the sample sintered at 360 °C is the main factor responsible for the improved power capability.

Preparation and Low-Temperature Sintering of Cu Nanoparticles for High-Power Devices

One of the fundamental requirements for high-temperature electronic packaging is reliable silicon attach with low and stable electrical resistance. This paper presents a study conducted on Cu nanoparticles as an alternative lead-free interconnect material for high-temperature applications. Cu nanoparticles were prepared using pulsed wire evaporation technique in water medium. Pure Cu nanoparticles without any organic mixture were used in this paper. An economical approach to extract the nanoparticles from water was established. In situ Cu nanoparticles oxide reduction was successfully done using forming gas (N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -5%H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ). Cross-section analysis on bonded interface shows onset of Cu nanoparticles sintering at 400°C. We successfully demonstrated the possibilities of using Cu nanoparticles as silicon die attach material for high-temperature electronic devices.