Formation Of Whiskers Research Articles

Coupled reforming of methane to syngas (2H2-CO) over Mg-Al oxide supported Ni catalyst

We report bi-reforming and coupled reforming of methane with carbon dioxide, steam and/or oxygen to produce syngas over Ni supported on Mg-Al mixed oxide. The catalyst has been characterised in terms of specific surface area, TPR, XRD, TGA-DTG, SEM and TPO-MS analysis. Ni/Mg-Al mixed oxide exhibited Ni particle size range (11–30nm) with a mean of 20.7nm. Syngas H2/CO=2.0, suitable for methanol/Fischer-Tropsch fuel synthesis, has been achieved for both reactions (T=1048K, P=1atm). The impact of process parameters including temperature, feeding concentration and GHSV on conversion and H2/CO ratio has been demonstrated. The Ni catalyst suffered temporal activity decline in bi-reforming that can be linked to formation of carbon whiskers encapsulated Ni particles resulting in a loss of active sites. Coupled reforming delivered higher CH4 conversion and enhanced stability, but lower CO2 conversion than bi-reforming under similar conditions. The enhanced stability in coupled reforming can be attributed to lower carbon deposition on Ni particles due to combustion of carbon by oxygen.

Effects of in-situ formed TiB whiskers on microstructure and mechanical properties of spark plasma sintered Ti–B4C and Ti–TiB2 composites

Monolithic titanium, Ti–1 wt% B4C, Ti–2.5 wt% TiB2 were spark plasma sintered at 1050 oC for 5 min under 50 MPa. The effect of B4C and TiB2 additions on densification process, microstructural development and mechanical properties of titanium was investigated. The results revealed that relative density of undoped, B4C- and TiB2-doped Ti samples reached ~98-99%. X-ray diffraction patterns, thermodynamic assessments, and microstructural investigations verified the in-situ formation of TiB whiskers in both composite samples as well as appearance of TiC spheres in Ti–B4C composite. However, trace unreacted TiB2 and B4C additives were remained in the composites as a result of incomplete chemical reactions due to short-time SPS process. Compared to undoped Ti sample, grain growth was hindered when the sample was doped by B4C or TiB2. Elongation, ultimate tensile strength and Vickers hardness of B4C- or TiB2-doped samples were higher than those of monolithic titanium but bending strength of ceramic-doped samples significantly lower, compared to undoped titanium. These outcomes were discussed in detail and related to presence/formation of several ceramic phases with different morphologies in Ti matrix.

Improving mullite-silicon carbide refractory in coke dry quenching using aluminum nitride whiskers formed in situ

The formation conditions and their influence on the properties of mullite-SiC refractory were studied by adding metallic Al powder to the raw materials to form AlN whiskers in a coke dry quenching furnace. The results show that the AlN whiskers were formed in situ and interacted to form networks in the matrix when the metallic Al powder was added into the mullite-SiC refractory after heat treatment above 850°C for 8h under nitrogen flow. The whiskers grew lengthwise as the firing temperature increased and the cold modulus of rupture, cold crushing strength, and hot modulus of rupture increased. The amount and particle size of the metallic Al powder also affected the properties of the refractory. The optimum amount of Al powder addition was determined to be 6wt%, which provided the highest strength and excellent toughness after heat treatment at 1000°C for 8h under nitrogen. Reducing the particle size of the metallic Al powder could promote formation of the AlN whiskers and improve the strength of the refractory.

Tin Whiskers Formation and Growth on Immersion Sn Surface Finish under External Stresses by Bending

Deposited tin (Sn) layers used as lead-free solderable finish on the semiconductor devices are known to form whiskers. These whiskers are a single crystal of tin that spontaneously grows from the surface of tin within weeks to years. Thus, they can cause shorts and the failure of a whole electronic circuit. In this research, the effect of external stresses applied by bending on the tin whiskers formation and growth was investigated on immersion Sn surface finish. The plating time was 10 minutes to produce 1.4 μm coating thickness on the copper substrate and exposed for 1, 4, and 8 weeks under the environment of 30°C/60% RH for bent and non-bent samples. It was found that the non-bent Sn surface had uniform ts distributed on the entire surface. Unlike the non-bent Sn surface, the surface of bent samples had non-uniform tin whiskers distribution. The tin whiskers formed more and grew longer at the lower stress region of the bent surface as compared to the higher stress region, based on the micrograph observed using the field emission scanning electron microscopy (FESEM).

Influence of carbonaceous materials on the interactions among (Al2O3-C)/Fe system with temperature and soaking time

It has practical significance for improving the service life of Al2O3-C refractories and reducing its influence on steel quality to reveal the reaction mechanism among (Al2O3-C)/Fe system under high temperature. The influence of carbonaceous materials on the interactions among (Al2O3-C)/Fe system with temperature and soaking time were studied using thermalgravity method in this paper in order to get a better understanding on the reaction mechanism among (Al2O3-C)/Fe system. The weight loss of (Al2O3-C)/Fe samples and the formation of Al2O3 crystal whiskers along with the change of aluminium and carbon content in iron indicates the mechanism of the effects of carbonaceous materials on the reactions among (Al2O3-C)/Fe system under high temperature. The result showed that the aluminium pickup of iron was not only due to the dissolution of Al2O3 in molten iron but mostly due to the carbothermic reduction of Al2O3 and carbonaceous materials in the presence of molten iron.

Synthesis and growth mechanisms of ZrC whiskers fabricated by a VLS process

Abstract The mechanisms of nano-sized ZrC whisker formation by a vapor–liquid–solid process are investigated, which produces a very high purity, single crystal whisker. Rectangular ZrC whiskers with a cross-sectional diameter of 100–200 nm and lengths up to tens of microns are formed under the catalytic effect of nickel. The ZrC whiskers are characterized using scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. ZrC whiskers can be used as a potential reinforcing and strengthening phase for ceramic composites.

Mechanical stability of monolithic catalysts: Improving washcoat adhesion by FeCrAl alloy substrate treatment

Alumina-washcoated FeCrAl alloy sheets are prepared to investigate the effects of metallic substrate pretreatments on the mechanical stability of monolithic catalysts. It is shown that chemical and thermal pretreatments produce different effects on the metallic surface morphology and phase, and different mechanisms are involved in their ways of improving the washcoat adhesion. Combination of chemical and thermal pretreatments leads to the formation of α-alumina whiskers on corroded and rough metallic surface, thus giving the most satisfactory washcoat adhesion. It is also pointed out that not only mechanical shock resistance but also thermal shock resistance is crucial to metallic monolithic catalysts.

Behavioral Consequences of a Bifacial Map in the Mouse Somatosensory Cortex.

The whisker system is an important sensory organ with extensive neural representations in the brain of the mouse. Patterned neural modules (barrelettes) in the ipsilateral principal sensory nucleus of the trigeminal nerve (PrV) correspond to the whiskers. Axons of the PrV barrelette neurons cross the midline and confer the whisker-related patterning to the contralateral ventroposteromedial nucleus of the thalamus, and subsequently to the cortex. In this way, specific neural modules called barreloids and barrels in the contralateral thalamus and cortex represent each whisker. Partial midline crossing of the PrV axons, in a conditional Robo3 mutant (Robo3R3-5cKO) mouse line, leads to the formation of bilateral whisker maps in the ventroposteromedial, as well as the barrel cortex. We used voltage-sensitive dye optical imaging and somatosensory and motor behavioral tests to characterize the consequences of bifacial maps in the thalamocortical system. Voltage-sensitive dye optical imaging verified functional, bilateral whisker representation in the barrel cortex and activation of distinct cortical loci following ipsilateral and contralateral stimulation of the specific whiskers. The mutant animals were comparable with the control animals in sensorimotor tests. However, they showed noticeable deficits in all of the whisker-dependent or -related tests, including Y-maze exploration, horizontal surface approach, bridge crossing, gap crossing, texture discrimination, floating in water, and whisking laterality. Our results indicate that bifacial maps along the thalamocortical system do not offer a functional advantage. Instead, they lead to impairments, possibly due to the smaller size of the whisker-related modules and interference between the ipsilateral and contralateral whisker representations in the same thalamus and cortex.SIGNIFICANCE STATEMENT The whisker sensory system plays a quintessentially important role in exploratory behavior of mice and other nocturnal rodents. Here, we studied a novel mutant mouse line, in which the projections from the brainstem to the thalamus are disrupted. This led to formation of bilateral whisker maps in both the thalamus and the cortex. The two whisker maps crowd in a space normally devoted to the contralateral map alone and in a nonoverlapping fashion. Stimulation of the whiskers on either side activates the corresponding region of the map. Mice with bilateral whisker maps perform well in general sensorimotor tasks but show poor performance in specific tests that require whisker-dependent tactile discrimination. These observations indicate that contralateral, instead of bilateral, representation of the sensory space plays a critical role in acuity and fine discrimination during somesthesis.

ZrB2‐SiCw ceramic composites synthesized by in situ reaction and spark plasma sintering

AbstractSiC whisker reinforced ZrB2‐SiCw composites were prepared by sol‐gel method and spark plasma sintering (SPS) with designed composition of 10‐30 vol% SiCw. The ZrB2‐SiCw composite powder was synthesized by sol‐gel method using zirconium diboride, tetraethoxysilane (TEOS), and activated carbon as starting materials, then densified by SPS at 1700°C under a pressure of 40 MPa for 5 minutes. The phase compositions and microstructures of ZrB2‐SiCw composites were investigated by X‐ray diffraction and scanning electron microscope, respectively. The relative density of ZrB2‐SiCw composites increased from 83% to 98.5% with increasing SiCw contents from 10 vol% to 30vol% after sintering at 1700°C without additional sintering additives. Meanwhile, the fracture toughness and flexural strength of as‐obtained composites exhibited a similar trend as relative density. ZrB2‐30vol% SiCw composite showed the highest fracture toughness of 6.1 MPa·m1/2 and flexural strength of 350 MPa, which were attributed to the formation of SiC whisker.

Synthesis and characterization of AlN whiskers by nitridation of Al4O4C

A novel route has been developed to synthesise AlN whiskers by heat-treating Al4O4C via Si atom doping under a nitrogen atmosphere. The resulting AlN whiskers have dimensions of greater than 200 nm in diameter and several hundred microns in length, and the whiskers, with their zig-zag shaped, noodle-like morphology, were formed by adjusting the heat-treatment conditions. TEM observation indicates that these whiskers are mono-crystalline and grow along the [001] direction. Possible reaction mechanisms responsible for the formation of these AlN whiskers were discussed. In addition, N2 can accelerate the decomposition of Al4O4C, which provides meaningful information for the application of Al4O4C under a nitrogen atmosphere at high temperatures.

Nonlinear Etch Rate of Au-Assisted Chemical Etching of Silicon.

We demonstrated time-dependent mass transport mechanisms of Au-assisted chemical etching of Si substrates. Variations in the etch rate and surface topology were correlated with catalyst features and etching duration. Nonlinear etching characteristics were associated with the formation of pinholes and whiskers. Variable rates of mass transport as a function of whisker density accounted for the nonlinear etch rates of Si. Nanopinholes on Au catalysts facilitated the vertical mass transport of reactants and byproducts, which dramatically changed the etch rate, surface topology, and porosity of Si. The suggested transport models describe the transient mass transport and the corresponding chemical reactions.

38 - Enasidenib (AG-221), A Selective Oral Inhibitor of Mutant Isocitrate Dehydrogenase 2 (IDH2) Enzyme, In Patients with Myelodysplastic Syndromes (MDS)

Different types of Sn coatings are widely used in the electronics industry. However, they are susceptible to spontaneous whisker formation which can cause reliability issues. In this paper, whisker growth from submicron Sn thin films was studied at room temperature for 150 days. For this purpose, 99.99% pure Sn was vacuum evaporated onto 1.5 mm thick Cu and ceramic substrates, with an average thickness of 400 nm. It was found that the submicron thick Sn layers on a Cu substrate can generate intense whiskering directly after layer deposition. The microstructure, composition and grain orientation of the whiskers and the areas underneath were studied with a Scanning Ion Microscope (SIM) and a Transmission Electron Microscope (TEM). It was proven that the whisker growth was induced by Cu6Sn5 intermetallic layer growth between the Cu substrate and the Sn thin film which resulted in large stress within the film. We found the first evidence in a tin-copper layer system that the high stress due to the intermetallic formation itself can initiate the interfacial flow mechanism (without a direct mechanical load) between the Sn layer and the intermetallic layer. This phenomenon can explain the intense whisker growth from submicron Sn thin films.

Stress evolution and whisker growth during thermal cycling of Sn films: A comparison of analytical modeling and experiments

Sn whiskers are a re-emerging reliability issue for Pb-free manufacturing. Although stress is considered as the most likely driving force for whisker formation, there is little quantitative understanding of how stress and whisker growth are correlated. To address this issue, we have developed an analytical model which accounts for strain generation (via thermal expansion mismatch) and multiple relaxation processes (rate-dependent plasticity and diffusion-controlled whisker growth). This model is used to analyze the results of thermal cycling experiments in which the evolution of stress and average volume of whiskers/hillocks were tracked as the features grew on the Sn surface. Predictions of the model are found to be in a good agreement with the time-dependent whisker growth and stress relaxation behaviors that have been observed in the experimental measurements.

Spontaneous Sn whisker formation on Ti2SnC

Spontaneous Sn whisker growth on Sn-containing substrates has resisted interpretation for several decades. In this paper, Ti2SnC-Sn and Ti2SnC samples were prepared and then cultivated in the same conditions. Many Sn whiskers had appeared on the ball milled Ti2SnC-Sn sample after being stored in air for 40 days, while no whisker grew on the Ti2SnC-Sn samples without ball milling although the cultivation time had been extended to more than half a year. Moreover, even though it was ball milled, the Sn-free Ti2SnC sample did not grow any whisker. The whiskers formed on this system share typical features with the ones grown on Sn platings and solders, while they do not follow any of the existing growth models. A catalysis-based whisker formation model, in which the cleavage planes of Ti2SnC grains act as heterogeneous nucleation sites, is proposed, which well interprets Sn whisker formation in the ball milled Ti2SnC-Sn system.

Whisker growth from vacuum evaporated submicron Sn thin films

Different types of Sn coatings are widely used in the electronics industry. However, they are susceptible to spontaneous whisker formation which can cause reliability issues. In this paper, whisker growth from submicron Sn thin films was studied at room temperature for 150days. For this purpose, 99.99% pure Sn was vacuum evaporated onto 1.5mm thick Cu and ceramic substrates, with an average thickness of 400nm. It was found that the submicron thick Sn layers on a Cu substrate can generate intense whiskering directly after layer deposition. The microstructure, composition and grain orientation of the whiskers and the areas underneath were studied with a Scanning Ion Microscope (SIM) and a Transmission Electron Microscope (TEM). It was proven that the whisker growth was induced by Cu6Sn5 intermetallic layer growth between the Cu substrate and the Sn thin film which resulted in large stress within the film. We found the first evidence in a tin-copper layer system that the high stress due to the intermetallic formation itself can initiate the interfacial flow mechanism (without a direct mechanical load) between the Sn layer and the intermetallic layer. This phenomenon can explain the intense whisker growth from submicron Sn thin films.

Microstructural characterisation and mechanical properties of spark plasma-sintered TiB2-reinforced titanium matrix composite

ABSTRACTTitanium and titanium matrix composites, reinforced with TiB2 particles, have been synthesised by the spark plasma sintering method at 1050°C under 50 MPa pressure, using mixtures of 2.4 wt.-% TiB2 and 97.6 wt.-% Ti powders. The changes in microstructural features and mechanical properties were investigated. XRD results and SEM observations confirm the formation of TiB whiskers as a result of the reaction between Ti and TiB2. However, some unreacted TiB2 particles have remained in the composite owing to the incomplete chemical reaction between matrix and additives. The measured mechanical properties demonstrate that the increase in hardness and tensile strength with TiB2 addition is mainly attributed to the generation of TiB whiskers, increase in relative density and decrease in grain size, while the reduction in bending strength is possibly due to the plastic restraint imposed on the matrix by the TiB whiskers and unreacted TiB2 particles.

Theoretical study on influence of CaO and MgO on the reduction of FeO by CO

Coating of CaO or MgO on the particle surface can prevent the sticking among iron ore particles effectively during fluidization process. However, CaO and MgO promote the formation of iron whiskers at high temperature, leading to the catastrophic defluidization. The density functional theory (DFT) calculations were implemented to investigate the influence of CaO and MgO on reduction of FeO/Fe2O2 by CO. Our results show that the CO molecule tends to bind to FeO/Fe2O2 on CaO(100) and MgO(100) surfaces through newly formed C-Fe and CO bonds. The CaO(100) surface will accelerate the reduction reactions which occur on it, in particular, in the initial stage of reactions, however, will slow down the reactions in the posterior stage. For the MgO(100) surface, the reduction reactions which occur on it will be promoted. The positive roles displayed by CaO and MgO in promoting the reduction of FeO by CO accelerate the precipitation of fresh iron and therefore, leading to the formation of iron whiskers.

Morphology of AlN whiskers grown by reacting N2 gas and Al vapor

We have investigated the morphology of AlN whiskers on a polycrystalline AlN substrate by using Fe-Al alloy melts under the different synthesis conditions. Formation density of the AlN whiskers increases and the diameter of the whisker decreases with increasing Al content of the Fe-Al alloy melt. Most of the AlN whiskers were zigzag shape with the hexagonal cross section. The longitudinal direction was the [0001] direction. The facet with the zigzag shape was the {11¯01} or {11¯01¯} pyramidal plane and the period of the zigzag facet was almost constant. The average diameter of the whiskers and the period of the zigzag facet decrease with increasing Al content. At the initial stage of the whisker formation, the island of AlN formed on the AlN substrate and the pyramidal facet grows via step-flow growth. From the observation, we discussed the possible mechanism for the formation of the zigzag-shape AlN whiskers.

Investigation of Whisker Growth from Alkaline Non-cyanide \tZinc Electrodeposits

Electroplated zinc finishes have been widely used in the packaging of electronic products for many years as a result of their excellent corrosion resistance and relatively low cost. However, the spontaneous formation of whiskers on zinc electroplated components, which are capable of resulting in electrical shorting or other damaging effects, can be highly problematic for the reliability of long-life electrical and electronic equipment. This work investigated the mechanism for whisker growth from zinc electrodeposited mild steel substrates. The incubation time for whisker growth from the surface of nodules on the surface of the electrodeposit was considerably reduced compared with that from the planar deposit surface. Recrystallisation of the as-deposited columnar structure was observed at the whisker root. This result is consistent with some recent whisker growth models based on recrystallisation. There was no evidence of iron-zinc (Fe-Zn) intermetallic formation at the iron/zinc (Fe/Zn) interface or within the zinc coating beneath the whiskers.

Microstructure evolution and its effect on thermo-mechanical properties of low-carbon Al2O3-C refractories

Low-carbon Al2O3-C refractory samples were prepared from raw materials of tabular alumina, reactive alumina powder, flake graphite and carbon black. Silicon powder and metallic Al powder were used as additives and phenol resin was used as binder. The effects of nitrogen partial pressure and firing temperature on microstructural evolution and the thermo-mechanical properties of low-carbon Al2O3-C refractories were investigated. The thermodynamic calculations for Al–Si–O–C–N systems suggested that SiC, AlN and β-Sialon could be stable under different nitrogen partial pressures. The experimental results showed that short columnar AlN and SiC whiskers were formed at 1200°C. The aspect ratio of AlN was higher and SiC production become larger with nitrogen partial pressure increasing. Two-dimensional flaky β-Sialon could be generated as the nitrogen partial pressure increased to higher than 0.11MPa at 1400°C. Due to the formation of SiC whiskers and β-Sialon, the cold modulus of rupture and the cold crushing strength were improved from 12.8MPa to 19.3MPa and from 57.3MPa to 74.7MPa, respectively. The load displacement could be increased by 62% to 0.59mm. At the same time, the maximum values of flexural modulus and hot modulus of rupture reached 4.2GPa and 13.2MPa, respectively. After three thermal shock cycles, the CCS of refractories containing the SiC whiskers and β-Sialon only decreased by 6.2MPa. The in situ formation of SiC whiskers and β-Sialon could greatly enhance the thermo-mechanical properties of low-carbon Al2O3-C refractories.