Single Crystal Whiskers Research Articles

Infrared Transmission Spectra of TiS3: Fundamental Absorption Edge, Phonons, and Excitons

Transmission spectra of TiS3 single crystal whiskers in the infrared and optical bands have been studied at various light polarizations in the temperature range from room to liquid helium. The absolute absorption coefficients have been determined. Previously unknown phonon modes have been observed in the far infrared region. The study of the temperature dependence of the band gap width has shown that it increases by 50 meV under cooling from 300 to 5 K. The comparison of spectra measured at different polarization directions indicates that the optical gap is anisotropic. A 1.28-eV absorption peak has been detected below ~150 K and has been attributed to the excitation of excitons.

Mechanistic insights into single crystal cadmium whisker growth on Ti2Cd intermetallic

Metal whisker growth poses a significant reliability threat to electronic devices. Previous investigations have predominantly focused on metal platings and alloy solder samples, yet the intricate factors involved have yielded inconsistent comprehension of the phenomenon. To achieve a comprehensive understanding, exploring intermetallic compounds (IMCs) may offer a novel standpoint. Herein, we present the first report on the growth of single crystal cadmium (Cd) whiskers on Ti2Cd IMC, accompanied by the characterization of their composition, crystal structure, and morphological features. Furthermore, the influence of mechanical damage and temperature on the whisker growth is examined. Ball milling induces the decomposition of Ti2Cd, liberating active Cd atoms with elevated chemical potentials that consequently feed the whisker growth. These findings augment the existing knowledge on the topic of the metal whisker growth and provide fresh perspectives and references for further investigations in this field.

Growth of REBa2Cu3O x single-crystal whiskers utilizing the concept of high-entropy alloys

Single-crystal whiskers of REBa2Cu3O x (RE-123, RE: rare Earth element) cuprate superconductors were successfully grown using Te-doped precursors utilizing the concept of high-entropy-alloys at the RE site. The obtained whiskers were 0.5–1.0 mm in length with approximately 10 μm thickness. The width of the flat surface, which corresponds to the ab-plane, was 10–50 μm. The RE site of the grown whiskers could be easily substituted by large ionic radius RE elements (Gd, Dy and Ho). Substitution with small ionic radius RE elements (Tm, Yb and Lu) resulted in the opposite trend. The superconducting transition temperature and superconducting anisotropy of the grown RE-123 whiskers were 89–93 K and 6–10, respectively. The mixed entropy at the RE site in the RE-123 whiskers did not affect the superconducting transition temperature and superconducting anisotropy.

Microstructure and evolution of hafnium carbide whiskers via polymer-derived ceramics: A novel formation mechanism

Polymer-derived ultra-high-temperature ceramic (UHTC) nanocomposites have attracted growing attention due to the increasing demands for advanced thermal structure components in aerospace. Herein, hafnium carbide (HfC) whiskers are successfully fabricated in carbon fiber preforms via the polymer-derived ceramic (PDC) method. A novel carbon nanotube (CNT) template growth mechanism combined with the PDC method is proposed in this work, which is different from the conventional vapor–liquid–solid (VLS) mechanism that is commonly used for polymer-derived nanostructured ceramics. The CNTs are synthesized and proved to be the templates for fabricating the HfC whiskers, which are generated by the released low-molecular-weight gas such as CO, CO₂, and CH₄ during the pyrolysis of a Hf-containing precursor. The formed products are composed of inner single crystal HfC whiskers that are measured to be several tens of micrometers in length and 100–200 nm in diameter and outer HfC/HfO₂ particles. Our work not only proposes a new strategy to prepare the HfC whiskers, but also puts forward a new thinking of the efficient utilization of a UHTC polymer precursor.

Features of the photoconductivity of the quasi-one-dimensional compound TiS3

Problem formulating. The features of linear and nonlinear conductivity, magnetic conductivity, thermopower, and the Hall effect observed on TiS3 whiskers indicate phase transitions to a state of CDW in this compound, but there is no direct evidence yet. The study of photoconductivity would allow us to learn more about the nature of these phase transitions. Goal. Study of the effect of radiation in the range from near-IR to near-UV on the conductivity of single-crystal TiS3 whiskers at 4.2 <T<300 K and search for features of photoconductivity in the temperature range of the suggested phase transitions. Result. Features of the photoconductivity of TiS3 at temperatures of 50 K and 15 K, at which the −dlnσ/d(1/T) maxima were previously observed, are found. A decrease in photoconductivity at these temperatures indicates a change in the electronic spectrum, which may be associated with the formation of CDW. Practical meaning. In the case of developing TiS3-based photosensors, it is important to study the photoconductivity in a wide temperature range.

Defect structure of TiS3 single crystals with different resistivity

A defect structure of single-crystal whiskers TiS3 with different resistivity has been studied by the high resolution scanning transmission electron microscopy. The whiskers crystallize in one monoclinic lattice, but in two variants, A and B. The high-resistivity whiskers crystallize according to the A-variant, while the crystal structure of the low-resistivity whiskers is a mixture of the A- and B-variants. All planar defects in the crystals are described in terms of twinning with the twin boundaries running parallel to the (001) plane and located in the middle of the atomic layers. Sulfur vacancies are found both in high- and low-resistivity whiskers, but the density of vacancies is noticeably higher in the low-resistivity ones. It is shown that the magnitude and temperature dependence of the resistivity are determined by the density of vacancies and the number of B-domains in the TiS3 crystals, whereas the effect of twinning on the resistivity is negligible.

Investigation of semiconducting materials for magnetic field sensors in strong magnetic fields under cryogenic temperatures

Influence of strong magnetic fields B ˃ 3 T under cryogenic temperatures (1.5 ÷ 4.2) K on the signals of Hall sensors based on single-crystal whiskers (InSb, InAs) and nano-sized heterostructures (InSb/i-GaAs, InAs/i-GaAs) have been investigated. There are distinct Shubnikov – de Haas oscillations for whisker-based sensors, whereas heterostructures-based sensors demonstrate the linear field dependence of the output signal. This difference explained by the higher concentration of structure defects in the heterostructures. The derived results confirm the operability of Hall sensors based on indicated heterostructures for the magnetic fields diagnostics in the temperature and field conditions of modern particle accelerators and fusion reactors.

Investigation of the Microstructure and Mechanical Properties of Copper-Graphite Composites Reinforced with Single-Crystal α-Al₂O₃ Fibres by Hot Isostatic Pressing.

Single-crystal α-Al2O3 fibres can be utilized as a novel reinforcement in high-temperature composites owing to their high elastic modulus, chemical and thermal stability. Unlike non-oxide fibres and polycrystalline alumina fibres, high-temperature oxidation and polycrystalline particles boundary growth will not occur for single-crystal α-Al2O3 fibres. In this work, single-crystal α-Al2O3 whiskers and Al2O3 particles synergistic reinforced copper-graphite composites were fabricated by mechanical alloying and hot isostatic pressing techniques. The phase compositions, microstructures, and fracture morphologies of the composites were investigated using X-ray diffraction, a scanning electron microscope equipped with an X-ray energy-dispersive spectrometer (EDS), an electron probe microscopic analysis equipped with wavelength-dispersive spectrometer, and a transmission electron microscope equipped with EDS. The mechanical properties have been measured by a micro-hardness tester and electronic universal testing machine. The results show that the reinforcements were unevenly distributed in the matrix with the increase of their content and there were some micro-cracks located at the interface between the reinforcement and the matrix. With the increase of the Al2O3 whisker content, the compressive strength of the composites first increased and then decreased, while the hardness decreased. The fracture and strengthening mechanisms of the composite materials were explored on the basis of the structure and composition of the composites through the formation and function of the interface. The main strengthening mechanism in the composites was fine grain strengthening and solid solution strengthening. The fracture type of the composites was brittle fracture.

The local strength of individual alumina particles

We implement the C-shaped sample test method and micro-cantilever beam testing to measure the local strength of microscopic, low-aspect-ratio ceramic particles, namely high-purity vapor grown α-alumina Sumicorundum® particles 15–30 µm in diameter, known to be attractive reinforcing particles for aluminum. Individual particles are shaped by focused ion beam micromachining so as to probe in tension a portion of the particle surface that is left unaffected by ion-milling. Mechanical testing of C-shaped specimens is done ex-situ using a nanoindentation apparatus, and in the SEM using an in-situ nanomechanical testing system for micro-cantilever beams. The strength is evaluated for each individual specimen using bespoke finite element simulation. Results show that, provided the particle surface is free of readily observable defects such as pores, twins or grain boundaries and their associated grooves, the particles can achieve local strength values that approach those of high-perfection single-crystal alumina whiskers, on the order of 10 GPa, outperforming high-strength nanocrystalline alumina fibers and nano-thick alumina platelets used in bio-inspired composites. It is also shown that by far the most harmful defects are grain boundaries, leading to the general conclusion that alumina particles must be single-crystalline or alternatively nanocrystalline to fully develop their potential as a strong reinforcing phase in composite materials.

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.

Engineer in Situ Growth of α-Al2O3 Whiskers by Axial Screw Dislocations

Screw dislocation growth mechanism has recently drawn much attention in the micro/nanostructures due to its unique existence conditions and impact on the morphology. Here we reported the observation of a screw dislocation growth of single crystal α-Al2O3 whiskers via a facile and novel in situ synthesis method. By programming synthesis temperatures, we observed an interesting circular to polygonal thread morphology transition on the surface of the as-synthesized α-Al2O3 whiskers. Clear observations of the terraces and the axial dislocations under SEM and TEM characterizations revealed the presence of screw dislocation growth in the as-grown α-Al2O3 whiskers. This mechanism was further validated by analyzing both the supersaturation of the system and the thermodynamics of chemical reaction in the system. This strategy provides a detailed understanding of the screw dislocation growth in the Al2O3 whiskers as well as a new strategy for engineering Al2O3 whiskers.

Sr1.98Eu0.02SiO4 luminescence whisker based on vapor-phase deposition: Facile synthesis, uniform morphology and enhanced luminescence properties

A high performance strontium silicate phosphor has been successfully synthesized though a facile vapor-phase deposition method. The product consists of single crystal whiskers which are smooth and uniform, and with a sectional equivalent diameter of around 5μm; the aspect ratio is over 50 and no agglomeration can be observed. X-ray diffraction result confirmed that the crystal structure of the whisker was α’-Sr2SiO4. The exact chemical composition was Sr1.98Eu0.02SiO4 which was analyzed by energy dispersive spectrometer and inductively coupled plasma-mass spectrometer. The whisker shows broad green emission with peak at 523nm ranging from 470 to 600nm (excited at 370nm). Compared with traditional Sr2SiO4:Eu phosphor, durability (at 85% humidity and 85°C) and thermal stability of the whisker are obviously improved. Moreover, growth mechanism of the Sr1.98Eu0.02SiO4 whiskers is Vapor–Liquid–Solid. On a macro-scale, the product is still powder which makes it suitable for the current packaging process of WLEDs.

Effect of Environment on Sn Whisker Growth during Welding of Electronic Wires

In Pb-free Al-Sn welding of electrolytic parts, single-crystal Sn whiskers easily form and can cause problems such as short circuits. Here we report that the growth of Sn whiskers in the weld zone of Al electrolytic condenser leads was suppressed in a vacuum environment. We examined the effect of the environment and weld metal microstructure in order to understand how to control and prevent whisker growth. In vacuum, the weld zone did not form whiskers after more than 100 h, whereas in air, whiskers grew within several hours. This suggests that whiskers require oxygen to form. The growth can be explained by the energy balance between the potential energy of the weld metal and the surface energy of the whisker. Our results will contribute to developing techniques for suppressing the formation of Sn whiskers during the percussion welding of Al electrolytic capacitor leads.

Room-temperature ferromagnetism with high magnetic moment in Cu-doped AlN single crystal whiskers**Project supported by the National Basic Research Program of China (Grant No. 2013CB932901), the National Natural Science Foundation of China (Grant Nos. 51372267, 51210105026, and 51172270), the Funds from the Chinese Academy of Sciences, the International Centre for Diffraction Data, USA (2013 Ludo Frevel Crystallography Scholarship Award), and the

Ferromagnetism is investigated in high-quality Cu-doped AlN single crystal whiskers. The whiskers exhibit room-temperature ferromagnetism with a magnetic moment close to the results from first-principles calculations. High crystallinity and low Cu concentrations are found to be indispensable for high magnetic moments. The difference between the experimental and theoretical moment values is explored in terms of the influence of nitrogen vacancies. The calculated results demonstrate that nitrogen vacancies can reduce the magnetic moments of Cu atom.

Preparation and characterization of continuous alumina based fiber reinforced with orientated mullite whisker

Single crystal mullite whiskers were directly added into the precursor sols to synthesis the orientated whisker-reinforced continuous alumina based fibrous matrix composite. The solid content and spinnable viscosity range were enhanced but the zeta potential was reduced by the addition of the mullite whiskers into the sol. The Al oligomers content increased while monomer and Al13 decreased with the increase of mullite whisker. After sintering at 1250°C, the obtained compact nano grains fibers were reinforced with accordant axial direction whiskers which were orientated by the axial force derived from the spinning process. The phase composition and compact structure were not affected by the presence of the mullite whiskers, while the continuous spinning length became low. Although tensile strength of single composite fiber was decreased at a high mullite whisker content due to the low densification and cracks of composite fiber, whisker bridging and pullout during fiber fracture may have increased the strength at a mullite whisker content of 1wt.% to 642.6MPa.

Magnetotransport and power-law I–V curves of the layered quasi one-dimensional compound TiS3

The current–voltage characteristics, Hall effect and magnetoresistance of TiS3 single crystal whiskers have been studied experimentally. Below 60K the I–V curves are non-linear along all crystallographic directions and exhibit a power law behavior. The Hall voltage shows a pronounced maximum at around 60K. The results indicate a change of the conductivity mechanism at low temperatures. Possible explanations of the observed effects are discussed.

Synthesis of BSCCO Single Crystal Whiskers for Terahertz EM Wave Oscillators

We prepared Bi2Sr2CaCu2O8+δ (BSCCO, Bi2212) and Bi2-xPbx Sr2CaCu2Oy (BPSCCO) single crystal whiskers from Bi2+xSr2-xCa2Cu2Te0.5Oy and Bi2-xPbxSr2 Ca2Cu2Te0.5Oy precursors using a tellurium-doped method, respectively. X-ray diffraction patterns revealed that the resulting whiskers showed a single crystalline Bi2212 phase. Although the temperature dependence of the resistance in the ab-plane showed a superconducting transition at T = 84 K and 107 K, a transition along the c-axis was observed only at T = 84 K. This indicates the intergrowth of the Bi2Sr2Ca2Cu3O10+δ (Bi2223) phase. The Pb- and Te-dual doping method suppresses the crystal growth of the BPSCCO whisker, and the superconductivity of the BPSCCO whisker is easily destroyed by annealing at 450°C in air. A mesa-type BSCCO intrinsic Josephson junction (IJJ) oscillator was fabricated with a single crystalline whisker. The current-voltage ( I-V) characteristics of the fabricated oscillator show a multibranch structure. The critical current is 14 mA at 4.2 K for a mesa structure ~ 300 μm in length and ~20 μm in width, which is higher than that previously reported for a whisker-based IJJ structure. Furthermore, a negative differential resistance in the I-V characteristics was observed below 40 K. A BSCCO whisker might be a potential candidate material for constructing an IJJ terahertz electromagnetic wave oscillator.

Pd nanoparticles on SnO2(Sb) whiskers: Aggregation and reactivity in CO detection

Single crystal antimony-doped SnO2 whiskers have been synthesized by in situ doping process in horizontal flow reactor. The produced whiskers were modified with 0.1, 0.2, 0.5, 1 or 2wt.% Pd. The processes of Pd particles growth and aggregation are described on the base of AFM and STEM data. Depending on the content of introduced Pd precursor, the various mechanisms (Volmer–Weber or Stranski–Krastanov) of Pd nanoparticles growth realize. The dependence of sensor signal to CO on Pd concentration has non-monotonous character determined by the size of Pd nanoparticles and their aggregation degree. The best sensor signal toward CO was observed for whiskers decorated with 0.1wt.% Pd. This concentration corresponds to the presence of individual 3–5nm Pd nanoparticles on the surface of the whiskers.

An Approach to Use a Raw Single Crystal Whisker as Intrinsic Josephson Junctions Stack

We have developed a technique to observe the intrinsic Josephson effect in Bi2Sr2CaCu2O8+δ (Bi-2212) single crystal whiskers. In this technique, a raw Bi-2212 single crystal whisker used as intrinsic Josephson junctions (IJJs) along the c-axis. The technique is simple, quick, and less-cost processing. First, a whisker made to stand on ac-plane and then two electrodes were made in ab-planes on either side. The standing whisker with this configuration worked as IJJs. The standing whisker (1.6 mm×40 μm×3 μm) showed the transition temperature of about 84 K. The critical current was about 15 mA at 8 K (critical current density ∼23 A/cm2). We observed voltage gap of about 500 mV in current–voltage (I–V) characteristics. This corresponds to a few hundred of IJJs out of ∼2,000 IJJs in the whisker thickness. Observations reflect that the technique can be further improved with the single crystal quality, shape of single crystal whisker, and annealing conditions.

Thermodynamic growth model of single crystal Bi-2212 whisker

In this study the growth mechanism of superconducting Bi-2212 whiskers is investigated from a thermodynamic point of view. The evolution of the free energy of the growth region in the main matrix is evaluated by estimating that of its surface energy. It is shown that the local free energy change for partial melting is the same as that of the whole structure of the sample. It is pointed out that the formation of whiskers requires a higher energy than that of plate like grains.