Nucleation Of Whiskers Research Articles

Whisker Nucleation by Slip-Assisted Grain Rotation During Thermal Cycling

Whisker Nucleation by Slip-Assisted Grain Rotation During Thermal Cycling

Preparation of mullite whiskers from high alumina fly ash and its reinforced porous structure

Porous has been formed in samples when mullite whiskers are prepared by firing compacts of high-aluminum fly ash and ammonium aluminum sulfate hydrate with a small amount of sodium dihydrogen phosphate and SiO2 at high temperature for 4 h. The effects of the particle size of fly ash, the firing temperature, and the content of sodium dihydrogen phosphate on the porous structure and the formation of whiskers are discussed. The results show that the smaller the particle size of fly ash, the more uniform porous in sample after fired, which is more conducive to the nucleation of mullite whiskers. Additionally, the amount of whiskers formation increased significantly with the firing temperature increasing, nonetheless, the pore size of porous reduced and the aspect ratio of whiskers gradually increased by reducing the content of sodium dihydrogen phosphate; owing to the reduction of glass melt in the matrix, the Al2O3/SiO2 ratio of the formed mullite whiskers gradually decreased. Thus, in order to obtain mullite whiskers self-reinforcement porous ceramics with the relatively low cost and no pollution to the environment, through the above preparation process, the addition amount of sodium dihydrogen phosphate and calcination temperature should be properly controlled.

Molten salt synthesis of mullite whiskers entirely derived from fly ash for electronic packaging toughening ceramic applications

Preparation of mullite from high-alumina coal fly ash (CFA) has become one of the most important ways for high-value utilization. However, low Al2O3/SiO2 ratio, poor reaction activity and various impurities in fly ash led to poor quality of mullite products. Conventional sintering method requires addition of aluminum sources and sintering additives, and high synthesis temperature. This paper proposes a new technical route for preparation of mullite whiskers entirely from fly ash by molten salt method based on additive activation-acid leaching process. The growth process and synthesis mechanism of mullite whiskers in Na2SO4 molten salt from fly ash were systematically investigated, and the application of fly ash-derived mullite whiskers in electronic packaging toughening ceramic was also explored. The results show that leaching liquor containing Al3+, Na+, and SO42−, and amorphous silica slag can be separated and obtained after acid leaching of activated fly ash at 80 °C, and the fly ash-derived mullite whiskers with a length of about 1 μm can be synthesized from the mixture of crystalline products and silica slag by the proposed method under the optimal synthesis conditions. Using amorphous silica slag with higher dissolution activity as the silicon source, the nucleation of mullite whiskers with homogeneous nucleation process is promoted and highly monodispersed mullite whiskers derived from fly ash form. The fly ash-derived mullite whiskers can improve the mechanical strength, reduce the dielectric constant and dielectric loss of the composite ceramic substrates, and the dielectric constant and dielectric loss of the composite ceramic are the lowest 5.16 and 0.006, respectively, when the mullite addition content is 20 wt.%, indicating that they could be used as electronic packaging materials. This paper provides a novel pathway for synthesis of mullite whiskers and high-value utilization of fly ash.

Revealing the Pb Whisker Growth Mechanism from Al-Alloy Surface and Morphological Dependency on Material Stress and Growth Environment.

Spontaneous metallic Pb whisker formation from Pb and Bi containing Al-alloy’s surfaces is a newly discovered phenomenon. The whiskers display unique formations, growth and morphology, which give the opportunity to be applied for specialized sensor and electronics applications. Within this work, the impact of environmental conditions (gas composition and moisture) is investigated and correlated with the modification of whisker evolution and growth dynamics. Furthermore, the residual stress state of the aluminum matrix using deep cryogenic treatment is modified and used to further increase whisker nucleation and growth by up to three- and seven-fold, respectively, supported by quantitative results. The results of this paper indicate the possibility to manipulate the whisker not only in terms of their kinetics but also their morphology (optimal conditions are 20% O2 and 35% humidity). Such features allow the tailoring of the whisker structure and surface to volume ratio, which can be optimized for different applications. Finally, this research provides new insight into the growth dynamics of the whiskers through in situ and ex situ measurements, providing further evidence of the complex nucleation and growth mechanisms that dictate the spontaneous growth of Pb whiskers from Al-alloy 6026 surfaces with growth velocities up to 1.15 µm/s.

Resistive Switching Memristor: On the Direct Observation of Physical Nature of Parameter Variability.

Ion-based memristive switching has attracted widespread attention from industries owing to its outstanding advantages in storage and neuromorphic computing. Major issues for achieving brain-inspired computation of highly functional memory in redox-based ion devices are relatively large variability in their operating parameters and limited cycling endurance. In some devices, volatile and nonvolatile operations often replace each other without changing operating conditions. To address these issues, it is important to observe directly what is happening in repeated operations. Herein, we use a planar device that enables direct capturing of microscopic behaviors in the nucleation and growth of metal whiskers under repeated switching to verify the microscopic origin of the large parameter variability. We report direct observations that reveal the physical origin for the large cycle-to-cycle and device-to-device variability in memristive switching, which was achieved using planar polymer atomic switches with a gap >1 μm. We find that the deposition location of metal atoms is closely related to the crystallinity of the ion transport layer (solid polymer electrolyte, SPE). The filament variability (shape, position, quantity, etc.) during different cycles and devices is indeed the main reason for the observed variability in the operating characteristics. The results shed unique light on the complexity of the operation of the ion device, that is, the evolution of the dielectric layer and metal filament must be considered.

Analysis of Pressure-Induced Whisker Nucleation and Growth in Thin Sn Films

Analysis of Pressure-Induced Whisker Nucleation and Growth in Thin Sn Films

Extraordinary Nanocrystalline Pb Whisker Growth from Bi-Mg-Pb Pools in Aluminum Alloy 6026 Moderated through Oriented Attachment.

The elucidation of spontaneous growth of metal whiskers from metal surfaces is still ongoing, with the mainstream research conducted on Sn whiskers. This work reports on the discovery of Pb whisker growth from Bi-Mg-Pb solid pools found in common machinable aluminum alloy. The whiskers and hillocks display unique morphologies and complex growth that have not been documented beforehand. In contrast to typical understanding of whisker growth, the presented Pb whiskers show a clear nanocrystalline induced growth mechanism, which is a novel concept. Furthermore, the investigated whiskers are also found to be completely composed of nanocrystals throughout their entire length. The performed research gives new insight into nucleation and growth of metal whiskers, which raises new theoretical questions and challenges current theories of spontaneous metal whisker growth. Additionally, this work provides the first microscopic confirmation of recrystallization growth theory of whiskers that relates to oriented attachment of nanocrystals formed within an amorphous metallic matrix. The impact of mechanical stress, generated through Bi oxidation within the pools, is theoretically discussed with relation to the observed whisker and hillock growth. The newly discovered nanocrystalline growth provides a new step towards understanding spontaneous metal whisker growth and possibility of developing nanostructures for potential usage in sensing and electronics applications.

Long-term thermal aging of parylene conformal coating under high humidity and its effects on tin whisker mitigation

Usage of pure tin (Sn) or Sn-rich alloy in electronics industry is growing, which makes them more susceptible to the spontaneous growth of tin whiskers. The risk of having these whiskers can be dramatically mitigated by applying a conformal coating such as parylene C. The purpose of this paper is to investigate the degradation of parylene C coating (with two thicknesses, 12.5 μm and 25 μm) during thermal aging in high humidity environment (85 °C/85% relative humidity for up to 5000 h) and its effect on the tin whisker mitigation property. It was observed that under the high temperature and high humidity (HTHH) environment, the oxidation occurs at early times, followed by bond scission, resulting in the loss in strength and modulus of the parylene coating. After 2500 h of aging, due to the degradation of the silane agent at the tin/parylene interface, the adhesion of the parylene coating on the tin surface was significantly weakened, which resulted in extensive corrosion of the tin solderpads. The loss of adhesion of the coating, along with the corrosion of the tin surface facilitated tin whisker nucleation at the interface while the loss of mechanical strength and the brittleness made the growing whiskers penetrate easily through the coating. Tin whiskers with a smaller diameter were buckled and grew along the delaminated interfacial region while those with a larger diameter penetrated and continued to grow through the coating. Although the thicker parylene coating helped delay the complete penetration and increase the critical diameter of whiskers for complete penetration, it showed more tin whisker activities and easy penetration by large diameter whiskers due to poorer adhesion. Hence, the effectiveness of the parylene coating on tin whisker mitigation will depend on its degradation kinetics of mechanical properties and adhesion under such harsh conditions.

A comparative study on the growth behaviors of Sn whiskers and hillocks in a Sn-Al alloy coating under different environments

In this study, the effects of factors such as temperature, solution and surface roughness on the growth of whiskers in a 100 μm thick Sn-Al alloy coating were comparatively investigated under different environments. The results indicated that whiskers did not grow after nucleation at 30 °C in oil. With the increase in aging temperature, whiskers started to grow after nucleation. The morphologies of whiskers showed the filament-type at 50 °C and the hillock-type at 100 °C. The residual compressive stress in the coating gradually decreased with the growth of whiskers and hillocks. Whisker nucleation was not found on the coating kept in water solution. The residual stress in the coating was rapidly released through the micro-cracks at the coating/substrate interface instead of growing whiskers. The oxide layer on the coatings with rough surfaces was easier to crack due to the higher thermal stress. The thermal stress on the oxide layer was in directly proportional to the depth of indentation and inversely proportional to the grain size. Whiskers preferentially grew on the (001)-oriented Sn grains in the coating. The finite element simulation results indicated that the grains with c-axis perpendicular to the coating surface had higher thermal strain and tended to grow whiskers. During creep deformation, the stress exponent increased from 7.0 to 11.5 with the increase in temperature. The mechanism for whisker growth was dislocation slip. Grain boundaries were observed in the hillocks. The power-law breakdown was the mechanism for the growth of hillocks.

(Invited) In-Situ Microscopy and Spectroscopy Probing of Li Dendrite Nucleation and Growth

Lithium (Li) metal has very low standard electrochemical redox potential and very high theoretical specific capacity, making it an ultimate anode material for rechargeable batteries. However, its application has been impeded by the formation of Li whiskers, which not only considerably consumes electrolyte and active Li, but also may lead to short-circuit of the battery. Tackling of these obstacles is limited by the elusive understanding on the intrinsic formation mechanisms of the Li whiskers and their growth behavior under the constraints from the separator. Here, by coupling an atomic force microscopy cantilever into a solid open-cell setup in environmental transmission electron microscopy, we directly captured the nucleation and growth behavior of Li whiskers under elastic constraint that mimics the effect of a separator. We show that Li deposition is initiated by a sluggish nucleation of a single crystalline Li particle with no preferential growth directions. Dramatically, we discovered that carbonate species in the initial solid electrolyte interphase that is very adjacent to the developing Li metal plays a decisive role in the subsequent formation of Li with a whisker morphology. Characteristically, the growing Li whisker, depending on both intrinsic and extrinsic conditions, can yield, buckle, kink, or stop (axial) growth under elastic constraint from the separator. These findings reveal the intrinsic cause and give a clear process on the formation and behavior of dendritic Li under stress, providing the much-needed insights for solving the Li whisker formation from the root cause rather than as currently containing it, and therefore potentially leading to the safe operation of Li metal anode in batteries.

Whisker growth under a controlled driving force: Pressure induced whisker nucleation and growth

We investigate whisker nucleation and growth kinetics in Sn coatings induced by the application of mechanical pressure. The pressure was applied to the samples using a clamping fixture that enables a constant pressure to be maintained over long duration of time. The fixture is small enough so that it could be transferred inside a Scanning Electron Microscope (SEM) chamber to observe the stress driven whisker nucleation and growth in real-time while the applied pressure is maintained on the Sn films.

Origin of lithium whisker formation and growth under stress.

Lithium metal has the lowest standard electrochemical redox potential and very high theoretical specific capacity, making it the ultimate anode material for rechargeable batteries. However, its application in batteries has been impeded by the formation of Li whiskers, which consume the electrolyte, deplete active Li and may lead to short-circuit of the battery. Tackling these issues successfully is dependent on acquiring sufficient understanding of the formation mechanisms and growth of Li whiskers under the mechanical constraints of a separator. Here, by coupling an atomic force microscopy cantilever into a solid open-cell set-up in environmental transmission electron microscopy, we directly capture the nucleation and growth behaviour of Li whiskers under elastic constraint. We show that Li deposition is initiated by a sluggish nucleation of a single crystalline Li particle, with no preferential growth directions. Remarkably, we find that retarded surface transport of Li plays a decisive role in the subsequent deposition morphology. We then explore the validity of these findings in practical cells using a series of carbonate-poisoned ether-based electrolytes. Finally, we show that Li whiskers can yield, buckle, kink or stop growing under certain elastic constraints.

Nucleation and Growth of Iron Whiskers during Gaseous Reduction of Hematite Iron Ore Fines

A high-temperature confocal scanning laser microscope and an online reduction–water quenching experiment system were used to systematically study the generation of iron whiskers during the reduction of hematite ore particles with CO/CO2 gas. The "blooming" phenomenon of the surface during the reduction of iron ore particles was found in this experiment. The orientation of the grain on the longitudinal section of an iron whisker was measured to be uniform by applying the electron back-scattered diffraction technique, which proved that the iron whiskers are most likely to exist in single crystal form. According to the in-situ online experimental video, the average diffusion flux of iron atoms when the layered iron completely covers the surface of the ore particle is about 0.0072 mol/(m2·s). While the iron atom diffusion flux at the root of the iron whisker during the pre-growth process is much larger than the flux when the layered iron is produced, which are defined to be 0.081 mol/(m2·s), 0.045 mol/(m2·s), 0.013 mol/(m2· s), and 0.0046 mol/(m2·s), respectively during the four stages of the growth of an iron whisker. The quantitative relationship between the chemical driving force and the whisker growth is established as Δ G θ + R T ln p CO 2 p CO + 2 n 0.056 r ρ E s T = 0 .

Chemo-thermo-mechanically Coupled Crystal Plasticity Simulation of Stress Evolution in Thermally Strained β-Sn Films

Whisker formation in tin films is a mode of stress relaxation, but the exact conditions causing them are yet to be established. In this work, a three-dimensional full-field chemo-thermo-mechanically coupled crystal plasticity simulation of thermally strained tin films was performed to evaluate the stress evolution and connect it to the redistribution of vacancies. Spatial heterogeneity in the hydrostatic stress along the grain boundary network (that served as the primary conduit for mass transport) was observed, which became more homogeneous towards the film surface. Normal and shear tractions on the columnar grain boundaries were evaluated as they might be crucial to breaking of the oxide layer (formed on the film surface) especially when inclined grain boundaries are present. With such an advanced multi-physics framework, several crystallographic and geometrical factors influencing whisker formation can be analyzed thereby leading to a better understanding of the factors modulating the nucleation and growth of such whiskers.

Studying the Effect of Grain Size on Whisker Nucleation and Growth Kinetics Using Thermal Strain

To understand how and why Sn whiskers nucleate, measurements were performed in which the thermal expansion mismatch was used to induce strain in the samples. The current work focuses on the effect of grain size by studying samples with a thickness of 8 μm to compare with previous studies on 2.5 μm samples. The stress evolution during cycles of heating is determined by monitoring the resulting curvature in the sample. At the same time, the number of whisker nuclei are measured with optical microscopy which enables the relationship between film stress and nucleation rate to be quantified, where the stress relaxation kinetics depend on the grain size. The rate of stress relaxation is greater in the thick samples than the thin samples for the same heating cycle and the final stress after relaxation is smaller for the thicker samples. The whisker volume evolution was determined by using short sequences of heating and then measuring the size of different features after each interval in the scanning electron microscope. This enabled quantification of the relationship between film stress and whisker growth rate. The growth rate depends on the stress above the final stress, similar to our previous results for the thinner samples. The whisker nucleation kinetics in the 8 μm samples can also be explained with the same model used for the 2.5 μm samples. However, the stress-dependent activation energy is shifted to lower values.

A full-field crystal plasticity study on how texture and grain structure influences hydrostatic stress in thermally strained β-Sn films

The present study uses full-field thermo-mechanically coupled crystal plasticity to simulate the stress distribution in thermally strained β-Sn films to better understand the conditions causing the generally rare nucleation of whiskers from such films. Following the notion that stress-driven diffusion along the grain boundary network is leading to and supporting whisker formation, the goal is to identify critical factors of crystallographic and/or geometric nature that influence the hydrostatic stress in such films prior to the onset of actual whisker formation. Approximating the film as a periodic structure of around a hundred columnar grains on an isotropic substrate, the simulations reveal a strong spatial variation of hydrostatic stress over distances comparable to the grain size but without apparent long-range gradients, suggesting whisker nucleation to be a phenomenon that depends on specifics within a relatively small neighborhood of grains. Furthermore, neither the grain size distribution nor the presence of oblique surface grains alters the width of the stress distribution by much. In contrast, the variation of hydrostatic stress notably depends on the crystallographic texture of the film, where β-Sn films for which the film normal is aligned with ⟨0 0 1] show a narrower distribution than films with ⟨1 0 0] parallel to the normal. The former film texture, therefore, is predicted to be less prone to whisker formation than the latter.

Metal whisker growth induced by localized, high-intensity DC electric fields

ABSTRACTIn this work, a very high, locally applied electric field was used to induce whisker nucleation on an Sn film. The field was generated by using a conductive AFM tip and applying a voltage bias between the sample and the conductive cantilever. The tip-sample separation distance was thus controllable, and any dielectric breakdown could be avoided. At locations where the AFM tip was positioned for an extended period, minuscule whiskers were observed, whose growth direction matched vertical orientation of the field.

Quantifying the Effect of Stress on Sn Whisker Nucleation Kinetics

Although Sn whiskers have been studied extensively, there is still a need to understand the driving forces behind whisker nucleation and growth. Many studies point to the role of stress, but confirming this requires a quantitative comparison between controlled stress and the resulting whisker evolution. Recent experimental studies applied stress to a Sn layer via thermal cycling and simultaneously monitored the evolution of the temperature, stress and number of nuclei. In this work, we analyze these nucleation kinetics in terms of classical nucleation theory to relate the observed behavior to underlying mechanisms including a stress dependent activation energy and a temperature and stress-dependent whisker growth rate. Non-linear least squares fitting of the data taken at different temperatures and strain rates to the model shows that the results can be understood in terms of stress decreasing the barrier for whisker nucleation.

In situ tensile testing of tin (Sn) whiskers in a focused ion beam (FIB)/scanning electron microscope (SEM)

Tin and tin-alloyed electroplated films are known to be susceptible to whisker growth under a range of conditions, many of which result in the generation of compressive stresses in the film. Compressive stress is considered to be one of the primary causes for whisker nucleation and growth. While extensive investigations have been performed on whisker growth, there have been few studies on the mechanical properties of tin whiskers themselves. We report on the tensile behavior of tin whiskers that were obtained by indentation and furnace aging of electroplated tin films on copper disks. Tensile tests of the whiskers were conducted in situ in a dual beam focused ion beam (FIB)-scanning electron microscope (SEM) system using a micro electro-mechanical systems (MEMS) based tensile testing stage. The strength of the whiskers was found to decrease with an increase in gage length and aged whiskers were found to be weaker than their indented counterparts. The observed gage length effect can be attributed to the probability of finding more defects as the whisker length increases. The effect of processing on the observed strength variation was investigated by analyzing the oxygen content in the whiskers via energy dispersive spectroscopy and the microstructure through transmission electron microscopy (TEM). The deformation mechanisms of whiskers were also inferred using post-mortem TEM. It was observed that the whiskers grown by indentation were dislocation free both before and after deformation. In contrast, whiskers grown by aging showed notable dislocation content (arranged in low energy configurations) even before deformation.

Investigation of Tin (Sn) Film Using an Aerosol Jet Additive Manufacturing Deposition Process

The quality of a Sn film deposited by the aerosol process is compared against the quality of Sn films deposited with traditional electroplating. Using the aerosol additive deposition technique, a Sn film was deposited on a brass substrate and exposed to room (25°C) temperature environments for 30 days, followed by a laser photosintering process. The film characteristics and content, formation of intermetallic compounds, residual stress distribution, grain texture, and the tendency of the film to grow Sn whiskers were analyzed. The preliminary results show a successful deposition of Sn film with an aerosol jet process and tensile residual stresses, whereas it was compressive in nature for electroplated Sn film. X-ray diffraction results also show the absence of intermetallic compound (IMC) formation in the aerosol jet-deposited film, while electroplated Sn film has a significant presence of IMC. The aerosol jet-deposited Sn film has the potential to resist nucleation of Sn whiskers under the operating conditions used in this study.