Pee Yew Lee

Nanocrystalline WO3-based H2S sensors

Nanocrystalline (NC) materials, exhibiting a large surface area, may be applied to gas sensors for which an excellent surface effect is required. In this study, tungsten oxide is synthesized by the gas-evaporation method and a mean particle size of about 10 nm is obtained. A step-heating sintering process is used to obtain the porous network-like structure of NC WO3. The results indicate that nanocrystalline WO3 is better than thin-film WO3 in sensing the H2S gas. The amount of dopants influences the sensitivity and the optimum operating temperature. An increasing weight percentage of Pd dopant will at first increase the sensitivity and lower the optimum operating temperature in 100 ppm H2S/air. But if excess dopant is added, e.g., if NC WO3 is 13.5 wt.% Pd-doped, it will oxidize at a high temperature and its characteristics of activating the detected gases on the surface of NC WO3 will disappear. The sensor thus loses the ability to sense at low temperature. Sensitivities of 9.9 and 9.7 can be achieved in 7.7 wt.% Pt-doped NC WO3 at 220 °C and 7.2 wt.% Pd-doped NC WO3 at 170 °C under 100 ppm H2S/air, respectively. The response times of 7.2 wt.% Pd-doped NC WO3 at 170 °C and 7.7 wt.% Pt-doped NC WO3 at 220 °C are less than 0.11 s.

The formation and thermal stability of amorphous Ni-Nb alloy powder synthesized by mechanical alloying**

NixNb1−x alloys were synthesized from pure crystalline Ni and Nb powder by mechanical alloying under a controlled atmosphere. The mechanically alloyed powders were amorphous in the range 0.20

Phase formation behavior and diffusion barrier property of reactively sputtered tantalum-based thin films used in semiconductor metallization

Abstract Tantalum (Ta) and nitrogen-contained tantalum (Ta–N) thin films are sputter deposited at different argon/nitrogen flow ratios onto (001) silicon-based substrates with and without a titanium adhesion layer. The impact of varying the nitrogen flow rate and the underlying titanium on the phase formation process is also investigated using X-ray diffractometry, resistivity measurement and scanning electron microscopy. In contrast to previous works on bare silicon and thermally oxidized silicon wafers, our results indicate that a thin titanium adhesion layer inhibits the formation of high-resistivity (200 μΩ cm) tetragonal Ta over a wide range of realistic deposition conditions. The titanium layer leads to the deposition of a low-resistivity (29 μΩ cm) body-centered cubic α-Ta arising from its epitaxial orientation on the underlying titanium. The thresholds of nitrogen flow rates for depositing nitrogen-saturated α-Ta, amorphous Ta2N (a-Ta2N) and stoichiometric NaCl-type TaN on silicon are 0.25, 1.0 and 2.0 sccm, respectively. However, the underlying titanium can increase the thresholds for forming nitrogen-saturated α-Ta, a-Ta2N and stoichiometric TaN to 1.0, 1.5 and 2.5 sccm, respectively. Consequently, the electrical properties and microstructures for Ta and Ta–N thin films on Ti are significantly changed. Moreover, the barrier properties of 40-nm-thick stoichiometric a-Ta2N (Ta67N33) and nitrogen over-saturated a-Ta2N thin films are evaluated. According to X-ray diffraction analyses and sheet resistance measurements, all of the a-Ta2N barrier layers degrade in a similar manner, triggered mainly by an entire crystallization of the amorphous barrier layers. This is followed by a phase transformation process, sequentially forming Cu3Si and TaSi2. Cross-sectional transmission electron microscopy reveals that copper can penetrate through the crystallized films either along grain boundaries or thermal-induced crevices to react with silicon, subsequently forming Cu3Si precipitates. As adequately doping nitrogen into stoichiometric a-Ta2N can dramatically increase the crystallization temperature by approximately 150°C, the effectiveness of the nitrogen over-doped a-Ta2N barrier layers can be greatly improved, subsequently elevating the degrading temperature by at least 100°C.

Formation of amorphous Ni‐Zr alloys by mechanical alloying of mixtures of the intermetallic compounds Ni11Zr9 and NiZr2

Amorphous Ni40Zr60 and Ni50Zr50 alloy powders were synthesized by mechanical alloying of mixtures of the intermetallic compounds Ni11Zr9 and NiZr2. Milling these compounds together in the proper proportions resulted in material transfer and amorphization of alloys with compositions Ni40Zr60 and Ni50Zr50. After crystallization in a differential scanning calorimeter, the same products of crystallization were observed as for crystallization of liquid quenched amorphous alloys of the same compositions. The driving force for the amorphization of Ni11Zr9+NiZr2 mixtures is believed to be either the steep rise in free energy of the line compounds as material transfer moves their compositions off stoichiometry, or the creation of a critical defect concentration in the intermetallic compounds.

Formation of amorphous Ni―Zr alloy powder by mechanical alloying of intermetallic powder mixtures and mixtures of nickel or zirconium with intermetallics

Mechanical alloying was used to synthesize NixZr1−x alloys from mixtures of intermetallic compound powders, and also from mixtures of intermetallic compound powders and pure elemental powders. The mechanically alloyed powders were amorphous in the range 0.24 ⩽ x ⩽ 0.85. This range is larger than amorphous alloys produced by the melt-spinning technique and mechanical alloying of elemental crystalline powders. Two-phase mixtures of the amorphous phase and the corresponding crystalline terminal solid solution were formed in the range 0.10 ⩽ x ⩽ 0.22, and x=0.90. It is found that the morphological development during mechanical alloying of these powders is different from mechanical alloying using only pure ductile crystalline elemental powders. The thermal stability has been investigated. The enthalpy and activation energy of crystallization for Ni-Zr amorphous powders prepared by mechanical alloying are lower than those for melt-spun samples of the same composition. The crystallization temperature of the mechanically alloyed Ni-Zr amorphous powders is higher than that of meltspun samples in the composition range Ni20Zr80 to Ni33Zr67 and Ni40Zr60 to Ni60Zr40. The presence of tiny crystallites as nucleation centres and high oxygen levels in the mechanically alloyed amorphous alloys might be responsible for the differences in crystallization behaviour. A new crystalline metastable phase was observed during crystallization studies of Ni24Zr76 amorphous powder.

Formation of NiAl–Al2O3 intermetallic-matrix composite powders by mechanical alloying technique

This study investigated the feasibility of preparing intermetallic-matrix composite powders (NiAl/Al2O3) by mechanical alloying of Ni, Al and Al2O3 powder mixtures with various compositions of (NiAl)x(Al2O3)100–x. The as-milled powders were examined by X-ray diffraction, scanning electron microscopy, and differential thermal analysis. The formation of NiAl phase was noticed after 5 h of milling. Intermetallic-matrix composite powders (NiAl/Al2O3) were prepared successfully at the end of milling for (NiAl)x(Al2O3)100–x (x=79, 66, and 49), but no alumina phase was detected for (NiAl)95(Al2O3)5. It is suspected that the additions of alumina hampered the cold welding and fracturing process. The thermal analysis of (NiAl)x(Al2O3)100–x powders after 1 h of milling revealed that the transition temperature of NiAl phase increased with increasing amount of Al2O3 additions.

Consolidation of amorphous Ni–Zr–Ti–Si powders by vacuum hot-pressing method

Abstract In the current study, we investigated the feasibility of fabricating amorphous Ni 57 Zr 20 Ti 20 Si 3 powders by mechanical alloying, and consolidating them into bulk metallic glasses by a vacuum hot pressing technique. The as-milled and hot-pressed samples were examined by X-ray diffraction, scanning electron microscope, and differential thermal analysis. According to the results, amorphous Ni 57 Zr 20 Ti 20 Si 3 powders were prepared after milling elemental powder mixtures for 5 h. The amorphous powders were found to exhibit a wide supercooled liquid region of 88 K before crystallization. Bulk metallic glasses were prepared successfully by consolidating the as-milled Ni 57 Zr 20 Ti 20 Si 3 amorphous powders using vacuum hot pressing in the supercooled liquid region. Vickers microhardnesses of the as-pressed samples are in the range of 788–830 kg/mm 2 .

Amorphization by mechanical alloying: The role of mixtures of intermetallics

Abstract This paper reports selected results of a continuing study of the systematics and mechanisms of amorphization by mechanical alloying of mixtures of intermetallic compound powders. Mechanical alloying was carried out on mixtures of “line” intermetallic compounds ( i.e . those having a narrow range of homogeneity) in the following binary alloy systems: Cr-Ti, Mn-Ti, Cu-Ti, Fe-Ti, Co-Ti, Ni-Ti, Cu-Zr, Ni-Zr and Mn-Si. Amorphization by mechanical alloying was observed to be complete, to the resolution of X-ray diffraction, for intermetallic compound mixtures ( e.g . NiZr 2 + Ni 11 Zr 9 → a-Ni 40 Zr 60 ) in all the above systems except Cr-Ti, which exhibited partial amorphization, and Mn-Si, which showed no amorphization. The amorphization tendencies could not be predicted from calculated enthalpies of mixing for the amorphous alloys, all of which were negative, or from calculated differences in free energy between the amorphous phase and the mixture of equilibrium intermetallic compounds. Preliminary transmission electron microscopy studies of the evolution of the amorphous phase for the system NiZr 2 + Ni 11 Zr 9 → a-Ni 40 Zr 60 revealed an amorphous halo-like region surrounding the transforming intermetallic particles in an amorphous matrix. Energy dispersive X-ray analysis measurements indicate that the halo-like region has a composition which falls between those of the intermetallic particle and the amorphous matrix.

Synthesis of MoSi2 powder by mechanical alloying

Abstract Systematic investigations were carried out on the synthesis of MoSi 2 powders by using a combination of mechanical alloying (MA) and vacuum heat treatment techniques. MA was conducted by two different high-energy ball mills under various milling conditions. The mechanically alloyed powders were characterized by X-ray diffraction and scanning electron microscopy as well as differential scanning calorimetry. The results showed that the preparation of MoSi 2 phase is not successful for all the mechanically alloyed powders after being milled with a less energetic planetary ball mill. However, the low-temperature α -MoSi 2 phase and the high-temperature β -MoSi 2 phase were formed when the same powder mixtures were mechanically alloyed with a more energetic shaker ball mill after a long milling time. Microstructural investigation indicates that cold welding behavior of Mo and Si powders during MA treatment has a pronounced effect on the formation of MoSi 2 phases. Vacuum heat treatment can also promote the formation of MoSi 2 phases in mechanically alloyed powders which are processed with shaker ball mill.

An optimal quasisuperlattice design to further improve thermal stability of tantalum nitride diffusion barriers

X-ray diffraction and transmission electron microscopy, along with electrical and film stress measurements, were used to evaluate the effectiveness of 40-nm-thick amorphous Ta2N and microcrystalline TaN diffusion barriers, both single and multilayered, against Cu penetration. Failure of the single-layered Ta2N diffusion barrier upon annealing is initialized by crystallization/grain growth, mainly helped by frozen-in compressive stress (3–4 GPa) to transform itself into a columnar structure with a comparable grain size to the thickness of the barrier. However, when subjected to annealing, the Ta2N/TaN alternately layered diffusion barrier with an optimum bilayer thickness (10 nm) remains almost stress-free (0–0.7 GPa) and transforms itself into an equiaxed structure with grain sizes of only ⩽3 nm. Such quasisuperlattice films can present lengthening and complex grained structures to effectively block Cu diffusion, thus acting as much more effective barriers than Ta2N (and TaN) single-layered films.