Synthesis of Ti2SnC from Ti/Sn/TiC powder mixtures by pressureless sintering technique

Ti/Sn/TiC powder mixtures were first employed to synthesize Ti₂SnC powder by pressureless sintering in the temperature range of 950–1250 °C at vacuum atmosphere. Ti₂SnC began to form at 950 °C, its content increased with increasing temperature. High purity of Ti₂SnC was obtained by sintering the mixtures with deficient Sn and TiC at 1200 °C for 15 min. A reaction mechanism was proposed to explain the formation of Ti₂SnC. The Ti₂SnC powder was characterized by scan electron microscopy (SEM) and X-ray diffraction (XRD). Using the above mixtures and process, the Ti₂SnC ceramic powder can be obtained on a larger scale.     

Fabrication and Wear Performance of (Cu–Sn) Solution/TiC<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> Bonded Diamond Composites

The Cu(Sn)–TiC_x bonded diamond composites were prepared by in situ reaction sintering of Cu, Ti₂SnC and diamond powders. Effect of Ti₂SnC content on the phase composition, microstructure and grinding properties were studied. The result shows that Ti₂SnC was decomposed to TiC_x and Sn. And then, Sn atom dissolved into the crystal lattice of Cu and formed Cu(Sn) solution. The rich C formed at the interface between diamond and the matrix. Excess Ti₂SnC inhibited the formation of Cu solid solution and reacted with Cu to form Cu₃Sn. Additionally, its matrix was mainly composed of TiC_x with better wear resistance, which may improve obviously the grinding performance of the composites. The grinding ratio value of copper–diamond composite was only 132. The grinding ratio value of the composite contained higher Ti₂SnC content in the raw materials was 636.     

Preparation of Ti2SnC by solid–liquid reaction synthesis and simultaneous densification method

Polycrystalline Ti₂SnC was prepared by a novel solid–liquid reaction synthesis and simultaneous densification method using elemental Ti, Sn and graphite as starting materials. The method is a one-step process and it provides the advantages of simultaneous synthesis and densification. The microstructure evolution during the high temperature processing was described and the reaction route was described. The reaction path for the formation of Ti₂SnC can be described in the following steps. Sn melted at 230 °C, which provided a favorable liquid circumstance for the reactions between Ti and Sn to form Ti–Sn intermetallic compounds. At high temperatures, TiC_x formed and reacted with Ti–Sn intermetallics at ∼1100 °C to yield ternary carbide Ti₂SnC. The material prepared by this method was dense and showed a layered anisotropic microstructure. The electrical conductivity of polycrystalline Ti₂SnC was metallic and anisotropic due to the anisotropic microstructure. The Vickers hardness was 3.7 GPa and the flexural strength was 313 MPa. Under compression, Ti₂SnC was damage tolerant at room temperature and deformed plastically at above 1000 °C. Electronic Publication     

Anisotropic mullitization in CuO-doped oxide mixture activated by high-energy ball milling

Mullite phase formation and grain growth, in a CuO-doped Al₂O₃ and silica mixture, has been investigated. The oxide mixture was activated with both stainless steel and tungsten carbide milling media. The milled powders demonstrated a much lower mullite formation temperature when compared to the conventional solid-state reaction process. Anisotropic grain growth was observed in the powders milled with stainless steel media, while well-shaped mullite whiskers were produced in the WC milled samples. The lowered mullitization temperature together with the anisotropic grain growth behavior was attributed to the refined microstructure of the powders as a result of the high-energy ball milling process.     

Splitting behavior and structural transformation process of K<Subscript>2</Subscript>Ti<Subscript>6</Subscript>O<Subscript>13</Subscript> whiskers under hydrothermal conditions

The splitting behavior and structural transformation process of K₂Ti₆O₁₃ whiskers in various hydrothermal solutions were investigated by the X-ray diffraction technique, scanning electron microscopy, transmission electron microscopy, and atomic force microscopy. TiO₂ (B) particle aggregates and rutile twinned crystals were produced respectively in diluted and concentrated HCl solutions via “dissolution-precipitation” mechanism, while no changes were observed in deionized water. In contrary to the chemical inertia of K₂Ti₆O₁₃ whiskers in KOH solution, trititanate nanowires were synthesized by splitting the bulk K₂Ti₆O₁₃ whiskers in NaOH solution. The driving force for the formation of nanowires originated from the intrinsic strain induced by the phase transition from K₂Ti₆O₁₃ with a tunnel structure to layered trititanate. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Effects of reflow atmosphere and flux on Sn whisker growth of Sn–Ag–Cu solders

We evaluated the Sn whisker growth behavior of Sn–Ag–Cu solder fillets on lead frames of quad flat packages (QFPs) upon OSP printed circuit boards that were exposed to 85 °C/85% relative humidity (RH) exposure. Three different concentrations of halogen flux for activated Sn-3.0wt%Ag–0.5wt%Cu were used to solder in air and in an inert N₂ reflow atmosphere. The lead frames of the QFPs consisted of Sn plated Cu and Fe-42wt%Ni (alloy 42). Sn whiskers were observed on the surface of the QFP solder fillet joints that were reflowed with halogen containing flux in an air atmosphere. A substantial amount of Sn oxides were formed in those solder fillets while whisker growth and the amount of Sn oxides increased with the halogen content. Sn oxide formation apparently enhanced whisker formation. The combination of air reflow atmosphere and high halogen flux was the worst combination for solder fillet oxidation resulting in Sn whisker formation regardless of the electrode’s lead frame composition of Cu or alloy 42. In contrast, an inert N₂ reflow atmosphere obviously prevented Sn whisker formation on Sn–Ag–Cu solder fillets under all conditions used in this work.     

Microstructure and oxidation behaviour of TiAl(Nb)/Ti<Subscript>2</Subscript>AlC composites fabricated by mechanical alloying and hot pressing

TiAl-based intermetallic matrix composites with dispersed Ti₂AlC particles and different amounts of Nb were successfully synthesized by mechanical alloying and hot pressing. The phase evolution of Ti–48 at%. Al elemental powder mixture milled for different times with hexane as a process control agent was investigated. It was found that after milling for 25 h, a Ti(Al) solid solution was formed; also with increase in the milling time to 50 h, an amorphous phase was detected. Formation of a supersaturated Ti(Al) solid solution after 75 h milling was achieved by crystallization of amorphous phase. Addition of Nb to system also exhibited a supersaturated Ti(Al,Nb) solid solution after milling for 75 h, implying that the Al and Nb elements were dissolved in the Ti lattice in a non-equilibrium state. Annealing of 75 h milled powders resulted in the formation of equilibrium TiAl intermetallic with Ti₂AlC phases that showed the carbon that originates from hexane, participated in the reaction to form Ti₂AlC during heating. Consolidation of milled powder with different amounts of Nb was performed by hot pressing at 1000°C for 1 h. Only the presence of γ-TiAl and Ti₂AlC was detected and no secondary phases were observed on the base of Nb. Displacement of γ-TiAl peaks with Nb addition implied that the Nb element was dissolved into TiAl matrix in the form of solid solution, causing the lattice tetragonality of TiAl to increase slightly. The values for density and porosity of samples indicated that condition of hot pressing process with temperature and pressure was adequate to consolidate almost fully densified samples. The isothermal oxidation test was carried out at 1000°C in air to assess the effect of Nb addition on the oxidation behaviour of TiAl/Ti₂AlC composites. The oxidation resistance of composites was improved with the increase in the Nb content due to the suppression of TiO₂ growth, the formation and stabilization of nitride in the oxide scale and better scale spallation resistance.     

Confining effect of oxide film on tin whisker growth

Spontaneous tin whisker growth has been mysterious and catastrophic for more than half century. The difficulty in the research on this topic consists of the randomness of the whisker growth, the slow growth rate and many other tricky factors. Herein, with Ti₂SnC-Sn as a new platform, fast tin whisker growth is realized to facilitate the research. The whisker morphology is found to be modulated by oxide film. A striated whisker morphology forms as growing in air, whereas a faceted morphology forms in vacuum. Furthermore, the evolution to the faceted morphology is attributed to the reconstruction of the whisker surface driven by surface energy reduction. The findings might open a new avenue to uncover the myths of this long-standing issue, and thus develop a long-awaited lead-free tin whisker mitigation strategy.     

Development of two-dimensional titanium tin carbide (Ti2SnC) plates based on the electronic structure investigation

Titanium tin carbide (Ti₂SnC) is a novel layered ternary compound. The ab initio calculations on the electronic structure and bonding properties indicated that Ti₂SnC exhibit anisotropy of chemical bonding and properties. The electrical conductivity parallel to the basal plane is metallic and is much higher than that in c-axis. Thus Ti₂SnC material in two-dimensional quasi-infinite form with the sheet surface parallel to the basal plane will show superior properties and have diverse device applications. Based on the theoretical predicted anisotropic electronic structure and properties, two-dimensional Ti₂SnC plates were synthesized through a solid–liquid reaction process utilizing elemental Ti, Sn and C as starting materials. X-ray diffraction and scanning electron microscopy demonstrated that the morphology of the as-prepared plates were two-dimensional sheets. And the sheet surface was parallel to the (001) plane of Ti₂SnC. Received: 4 April 2000 / Reviewed and accepted: 1 July 2000     

Effects of humidity on tin whisker growth — Investigated on Ni and Ag underplated layer construction

The effect of humidity on nickel and silver underlayered tin platings has been observed in relation to whisker formation. Using nickel or silver underlayers between the copper substrate and the tin coating is known to be a useful technique for mitigating the occurrence of whiskers. The underlayer blocks the formation of Cu₆Sn₅ intermetallics which is one of the root causes of whisker growth. Samples with a bronze (94Cu/6Sn) substrate covered with a 1-2 μm underlayer (silver or nickel) and 5-7 μm of tin on the top were tested. The samples were stored in high humidity and at various temperature conditions; 40 °C/95% RH (Relative Humidity), 105 °C/100% RH and 50 °C/25% RH for over 4200 h to demonstrate the importance of humidity in whisker growth. Results have shown differences in whisker growth on the samples depending on the nature of the environmental conditions. The differences originate from the various stress-causing mechanisms. Growth differences have also been found on the two types of underlayer materials.     

Inorganic whiskers reinforced bismaleimide composites

The whisker-reinforced polymer composites have good friction and wear properties and widely used in many fields. Potassium titanate (K₂Ti₆O₁₃) whiskers have good properties, lower prices and show good foreground in whisker-reinforced polymer matrix composites. The surface properties of the whisker are vital for the performance of the reinforced composites. In this paper, the friction and wear properties of potassium titanate whiskers reinforced bismaleimide copmposites and the surfaces of whiskers treated by coupling agents were studied. Two coupling agents, a silane compound (KH550) and a titanate (NDZ311) were selected to treat the surface of K₂Ti₆O₁₃ whiskers, respectively. Three whisker-reinforced BMI composites, K₂Ti₆O₁₃/BMI, K₂Ti₆O₁₃ (KH550)/BMI and K₂Ti₆O₁₃ (NDZ311)/BMI, were prepared and their tribological behaviors were investigated. Results show that the wear-resistance of the matrix improved by the incorporation of whiskers into the matrix, while the improvement efficiency is depended on the nature of the surface of whiskers and whisker content. The composite containing KH550 treated whiskers has the best wear-resistance, and that containing untreated whiskers has the poorest wear-resistance among the three composites. Experiment results were explained from the point of the interfacial adhesion between the matrix and whiskers as well as the surface morphologies of worn surface and wear particles of the matrix and composites.     

Whisker growth on SnAgCu–xPr solders in electronic packaging

The addition of rare earth Pr into Sn3.8Ag0.7Cu solder results in the formation of PrSn₃ phase, which can induce the whiskers growth. After several hours’ exposure at room temperature in air, different morphologies of whiskers appear in the regions of PrSn₃ intermetallic compounds. The Pr content and storage time are the main parameters for affecting the whiskers growth at ambient temperature. The oxidation mechanism of PrSn₃ phase was used to explain the whiskers growth, the compressive stress is proposed as the driving force for whisker growth.     

Electromigration-induced Bi-rich whisker growth in Cu/Sn–58Bi/Cu solder joints

Effect of current stressing on whisker growth in Cu/Sn–58Bi/Cu solder joints was investigated with current densities of 5 × 10³ and 10⁴ A/cm² in oven at different temperatures. Two types of whiskers, columnar-type and filament-type, were observed on the solder film propagating along the surface of the Cu substrate and at the cathode interface, respectively, accompanied with many hillocks formation. Typically, these whiskers were 5–15 μm in length and 0.06–2 μm in diameter. EDX revealed that these whiskers and hillocks were mixtures of Sn and Bi rather than single crystal. It should be noted that the sprouted whiskers would not grow any more even if the current-stressing time increased again when the solder joint was stressed under lower current density. Nevertheless, when the current density was up to 10⁴ A/cm², the whiskers would melt along with the increasing current-stressing time. Results indicated that the compressive stress generated by precipitation of Cu₆Sn₅ intermetallics provides a driving force for whisker growth on the solder film, and the Joule heating accumulation should be responsible for whisker growth at the cathode interface.     

Synthesis,phase formation study and magnetic properties of CoFe<Subscript>2</Subscript>O<Subscript>4</Subscript> nanopowder prepared by mechanical milling

In this work we report the phase formation and magnetic properties of CoFe₂O₄ nanopowder prepared by mechanical alloying technique using metallic cobalt and hematite powder (1:1 molar ratio) as the initial raw material in ambient air atmosphere. The formation of single phase cobalt ferrite of (Co _0.18 ²⁺ Fe _0.82 ³⁺ )[Co _0.82 ²⁺ Fe _1.18 ³⁺ ]O₄ stoichiometry was confirmed for the samples milled above 15 h without any heat-treatment by XRD and Mössbauer techniques. The average crystallite size of the sample milled for 30 h was ～13 nm. The highest room temperature value of the magnetization measured at 1.5 T was 51 e.m.u/g for the sample milled for 25 h which was much lower than the corresponding value of the bulk cobalt ferrite (80.8 e.m.u/g at 300 K) due to the size effect.     

Reaction mechanism for synthesis of Ti2SnC

The reaction mechanism for the formation of Ti₂SnC has been investigated. A mixture of Ti₅Sn₃ and TiC powders was adopted to synthesize Ti₂SnC. It confirms that Ti₂SnC forms from the reaction of Ti₅Sn₃ and TiC. Scanning electron microscopy, differential scanning calorimetry and X-ray diffraction were used to analyze the reaction mechanism.     

Influence of Primary Milling on Structural and Electrical Properties of Bi<Subscript>4</Subscript>Ti<Subscript>3</Subscript>O<Subscript>12</Subscript> Ceramics Obtained by Sintering Process

In this article, the influence of primary mechanical milling of precursors on the microstructure and dielectric properties of Bi₄Ti₃O₁₂ ceramics was studied. Precursor material (mixture of Bi₂O₃ and TiO₂ powders) was ground by a high-energy attritorial mill for (1, 3, 5, and 10) h. Bi₄Ti₃O₁₂ ceramics were obtained by a solid-state reaction process, synthesized at an intermediate temperature (800 °C) and finally sintered at high temperature (1140 °C). Structure studies were performed by X-ray diffraction (XRD) and scanning electron microscopy techniques. XRD patterns were analyzed by the Rietveld method using the DBWS 9807a program. The thermal properties of the studied materials were measured using differential thermal analysis and thermal gravimetric techniques. These studies indicate that one-, three-, and five-hour primary high-energy ball milling followed by sintering is a promising technique for pure Bi₄Ti₃O₁₂ ferroic ceramics preparation. The investigation of Bi₄Ti₃O₁₂ shows that ceramics obtained from a precursor and milled for 5 h have the best dielectric properties.     

On the mechanism of mechanochemical synthesis of Ti2SC from Ti/FeS2/C mixture

In this study, the synthesis of Ti₂SC MAX phase by high energy ball milling, and the effects of heat treatment on ball milled powder was investigated. To this aim, a mixture of Ti, FeS₂ (as sulfur source), and C according to Ti₂SC stoichiometry, were ball milled by a planetary ball mill for different milling periods up to 10 h. The structural evolution, and the morphology of the products was studied by x-ray diffraction (XRD), and scanning electron microscopy (SEM) equipped with energy-dispersive spectroscopy (EDS), respectively. The results showed that after 10 h of ball milling, the raw materials reacted together and resulted in the formation of Ti₂SiC and TiC phases. The ball milled powder was then compacted and heat treated at 1000 and 1200 °C. Heat treatment caused the progressing of synthesis reactions, and led to increasing the purity of Ti₂SC phase. The heat-treated powder was leached in 1 M HCl for 2 h to remove iron from the product. The XRD results confirmed successful iron removal by leaching. SEM micrographs of the final product revealed the specific lamellar structure of MAX phases. Elemental mapping confirmed the homogeneous distribution of Ti, S and C elements.     

Preparation of K<Subscript>2</Subscript>Ti<Subscript>6</Subscript>O<Subscript>13</Subscript>/TiO<Subscript>2</Subscript> bio-ceramic on titanium substrate by micro-arc oxidation

K₂Ti₆O₁₃/TiO₂ bio-ceramic coatings are prepared successfully by micro-arc oxidation on titanium substrate in pure KOH electrolyte solution. The coating is prepared at various applied current density (150–500 mA/cm²) and in KOH electrolyte with different concentrations (0.5–1.2 mol/L). The composition and surface morphologies of coatings are strongly dependent on the applied current density and the electrolyte concentration. On the condition of lower current density and electrolyte concentration, K₂Ti₆O₁₃ phase almost cannot be formed. The phase is mainly composed of rutile and K₂Ti₆O₁₃ with increasing current density and electrolyte concentration. The surface morphologies are composed of whiskers and porous structures. The ability of K₂Ti₆O₁₃/TiO₂ bio-ceramic films inducing apatite deposition is evaluated by soaking it in biological model fluids. The results show the K₂Ti₆O₁₃/TiO₂ bio-ceramic coatings possess excellent capability of inducing bone-like apatite to deposit.     

Synthesis of TiC Whiskers through Carbothermal Reduction of TiO2

TiC whiskers were produced through carbothermal reduction of TiO₂ in the presence of potassium (K₂CO₃) and nickel (NiCl₂). The effect of potassium, nickel, and heating rate on the formation of whiskers was studied. Potassium was found to be an essential constituent for whisker formation. Nickel acts as a catalyst for TiC whisker formation only in the presence of potassium. The yield of whiskers was maximum at 1000–1200°C. At higher temperatures, formation of particulates of TiC was the dominant process. An increase in K₂CO₃ concentration during fast heating and decrease in K₂CO₃ concentration during slow heating was found to be beneficial in increasing the formation of TiC whiskers. A vapor–liquid–solid growth mechanism of whisker formation was explained.     

Compressive stress-strain response of directionally aligned SiC<Subscript>w</Subscript>/Al composite

A SiC_w/6061Al composite was fabricated through a squeeze-casting route and hot extruded to obtain a composite with directionally aligned whiskers. Based on observed changes in whisker orientation and length before and after deformation, compressive deformation behaviour of the directionally aligned SiC_w/Al composite was investigated. It is found that when the compressive temperature is much lower than the solidus of the matrix alloy, the compressive flow stress of the directionally aligned composite is increased with compressive strain first and then decreased. When the compressive temperature equals the solidus of the matrix, however, the compressive flow stress of the directionally aligned composite is increased monotonously with compression strain. During compression, whisker rotation and breakage occurred, and the higher the compressive temperature, the easier the whisker rotation and hence the smaller the degree of whisker breakage. When the compressive strain was quite high, the degree of whisker breakage was serious even at the temperature as high as the solidus of the matrix. Analyzing changes in whisker orientation and breakage before and after compression indicates that the decreased compressive flow stress with compressive strain is the result of the decreased load carrying ability of whiskers caused by whisker rotation and breakage. Compared with whisker rotation, whisker breakage has a bigger contribution to the decreased compressive flow stress. No strain softening in the composite compressed at 580°C can be thought to be a result of the very low strengthening effect of whiskers at such a high temperature. From the point of view of whisker breakage, to get higher properties of SiC_w/Al composite parts made by means of plastic forming, too high plastic strain should not be suffered by SiC_w/Al composites during the plastic forming.