Zr Pd Research Articles

Effect of Nature of the Precursor on Crystallinity and Microstructure of MOCVD-Grown ZrO2 Thin Films

ABSTRACTIn the present work, we report the deposition of zirconia thin films on Si(100) at various substrate temperatures by low-pressure metalorganic chemical vapor deposition (MOCVD). Three different zirconium complexes, viz., tetrakis(2,4-pentadionato)zirconium(IV), [Zr(pd)4], tetrakis(2,2,6,6-tetramethyl-3,5-heptadionato)zirconium(IV), [Zr(thd)4], and tetrakis(t-butyl-3-oxo-butanoato)zirconium(IV), [Zr(tbob)4] are used as precursors. The relationship between the molecular structures of the precursors and their thermal properties, as examined by TG/DTA is presented. The films deposited using these precursors have distinctly different morphology, though all of them are of the cubic phase. The films grown from Zr(thd)4 are well crystallized, showing faceted growth at 575°C, whereas the films grown from Zr(pd)4 and Zr(tbob)4 are not well crystallized, and display cracks. These differences in the observed microstructure may be attributed to the different chemical decomposition pathways of the precursors during the film growth, which influence the nucleation and the growth processes. This is also evidenced by the different kinetics of growth from these three precursors under otherwise identical CVD conditions. The details of thin film deposition, and film microstructure analysis by XRD and SEM is presented. The dielectric behavior of the films deposited from different precursors, as studied by C-V measurements, are compared.

Formation and Stability of Metastable Pd(Zr) Solid Solution Developed during Ball Milling and/or Heat Treatment of Pd<sub>3</sub>Zr

Formation and Stability of Metastable Pd(Zr) Solid Solution Developed during Ball Milling and/or Heat Treatment of Pd<sub>3</sub>Zr

Formation and Stability of Metastable Pd(Zr) Solid Solution Developed during Ball Milling and/or Heat Treatment of Pd<sub>3</sub>Zr

Formation and Stability of Metastable Pd(Zr) Solid Solution Developed during Ball Milling and/or Heat Treatment of Pd<sub>3</sub>Zr

Nanoicosahedral quasicrystalline phase in Zr–Pd and Zr–Pt binary alloys

It is found that a nanoicosahedral phase in the diameter less than 20 nm is formed as a primary crystalline phase in the melt-spun Zr70Pd30 and Zr80Pt20 binary amorphous alloys. The nanoicosahedral phase is also formed in the as-quenched state in the Zr80Pt20 binary alloy by controlling the quenching rate. The slight redistribution of approximately 3 at % is observed during the quasicrystallization in the Zr70Pd30 alloy. In contrast, no significant compositional change between the nanoicosahedral and residual amorphous phases is observed in the Zr80Pt20 alloy. It is suggested that the precipitation of nanoicosahedral phase in the Zr70Pd30 alloy takes place by a diffusion-controlled growth mode accompanying an increase in nucleation rate. The activation energy for grain growth is calculated to be 270 kJ mol−1, which implies the growth of icosahedral phase without a long-range atomic redistribution. The icosahedral medium-range order in the diameter range less than ∼2 nm is observed in the high-resolution electron micrographs of the melt-spun Zr70Pd30 and Zr80Pt20 amorphous alloys. It is realized that the icosahedral quasicrystalline phase can grow easily with assimilating the icosahedral medium-range order due to a slight redistribution of constitutional elements during quasicrystallization. The formation of the nanoicosahedral phase in the Zr70Pd30 and Zr80Pt20 binary alloys appears to be attributed to the existence of the icosahedral medium-range order in the amorphous and/or liquid states.

Investigation of short-range order in nanocrystal-forming Zr60Cu20Pd10Al10 metallic glass and the mechanism of nanocrystal formation

Rapidly solidified ribbons from the nanocrystal-forming Zr60Cu20Pd10Al10 alloy prepared at various melting liquid temperatures were used to study the influence of the liquid state upon quenched-in nuclei. With lowering quenching liquid temperatures, the small-angle x-ray scattering shows increased related periodic composition fluctuations and the radial distribution function analysis from x-ray diffraction method reveals that the coordination number of Pd around Zr increases. These results provide evidence for the stronger attractive interaction in Zr–Pd, which exhibits large negative mixing enthalpy, leads to the formation of (Zr, Pd)-rich domains of short-range order in the liquid. They remain in the amorphous phase as quenched-in nuclei and therefore contribute to nanocrystalline formation.

Nanoquasicrystalline phase formation in binary Zr–Pd and Zr–Pt alloys

This paper reports nanoquasicrystalline phase formation in Zr100−xPdx (x=30 and 35) and Zr80Pt20 binary alloys and the kinetics of the nanoquasicrystallization process. While the icosahedral phase (i-phase) forms as a metastable phase in the transient stage during the crystallization of Zr–Pd amorphous alloy, it forms directly from the liquid during melt-spinning of Zr–Pt alloy. The isothermal kinetics studies show that i-phase forms from the Zr70Pd30 amorphous alloy by the primary crystallization process with the Avrami exponent in the range of 1.5–2.5. Three-dimensional atom probe analysis results suggest that the i-phase is slightly enriched with Zr with respect to the matrix and its composition is close to Zr75Pd25. The tendency of quasicrystallization of Zr-based alloys appears to have correlation with the enthalpy of mixing of the system.

Effect of metal composition on hydrogen selectivity in steam reforming of methanol over catalysts prepared from amorphous alloys

Methanol steam reforming was carried out over catalysts prepared from amorphous Cu 50Zr 50, (Cu 50Zr 50) 100− x Au x and (Cu 50Zr 50) 100− x Pd x alloys in a fixed bed reactor operated at atmospheric pressure. The selectivity for carbon dioxide, which is a measure of steam reforming, was dependent on the composition of the amorphous alloy. When the reaction was performed over Cu 50Zr 50 amorphous alloy, the selectivity for carbon dioxide was essentially unity, whereas methanol conversion was very low. On the other hand, methanol conversion increased with the palladium content over the Cu–Zr–Pd amorphous alloy, but the selectivity for carbon dioxide was lower than that over Cu–Zr alloy. When the reaction was carried out over catalysts prepared from amorphous Cu–Zr–Au alloy, the methanol conversion increased with the gold content, whereas the carbon dioxide selectivity was almost unity over the range of gold content. The surface area measurement revealed that the dispersion of copper, which was effective for the steam reforming reaction, increased with the content of gold in the amorphous alloy. On the other hand, the dispersion of palladium, which was effective for dehydrogenation of methanol, increased with the palladium content. These results indicate that gold in the Cu–Zr alloy plays an important role in increasing the dispersion of copper.

Nanoquasicrystallization of binary Zr–Pd metallic glasses

The formation of an icosahedral phase during crystallization of Zr–Pd binary amorphous alloys is reported. The icosahedral phase forms in the nanocrystalline state with a grain size of about 10 nm as a metastable transient phase during the transformation from an amorphous to a crystalline state in Zr70Pd30 and Zr65Pd35 alloys. The amorphous phase coexists with the nanoquasicrystalline phase before complete crystallization to Zr2Pd occurs.

Influence of the liquid states on the crystallization process of nanocrystal-forming Zr–Cu–Pd–Al metallic glasses

Rapidly solidified ribbons of nanocrystal-forming Zr–Cu–Pd–Al metallic glasses were prepared at various liquid temperatures (TL). Differential scanning calorimetry (DSC) traces show clearly the influence of the liquid states on the thermal properties and crystallization process. Namely, with increasing TL, the exothermal peaks of the DSC traces shift to higher temperatures, the super-cooled-liquid region ΔTx increases, and the decomposition of the metastable compound Zr2(Cu, Pd) becomes more difficult. These results suggest that the liquid state strongly controls the crystallization process of the nanocrystal-forming metallic glasses. This behavior may originate from the variation of the quenched-in nuclei, which highly depends on the short-range-order domains in liquid with different TL. We suggest that the stronger attractive interaction in Zr–Pd, which exhibits large negative mixing enthalpy, leads to the short-range order domains.

An X-ray photoelectron spectroscopy study of the stability of ZrO 2 films on Pd(110)

Thermal stability of ZrO 2 overlayers on a Pd(110) substrate is studied using X-ray photoelectron spectroscopy (XPS). The stoichiometric ZrO 2 films, produced in situ by oxidation of physical vapour deposited Zr metal, are stable in 1×10 −6 Torr of oxygen up to 1000 K. Under vacuum conditions, the oxide films are found to decompose at temperatures above 840 K, resulting in the formation of Zr–Pd alloy. The decomposition of ZrO 2 is found to take place at the oxide-Pd interface and the rate limiting step is the release of oxygen through the boundaries between small oxide crystallites.

High-strength bulk nanocrystalline alloys containing compound and amorphous phases

Partial crystallization of cast bulk amorphous alloys in ZrAlCuPd and ZrAlCuPdFesystems was found to cause bulk nanocrystalline alloys with high tensile strength ( σ f ). The nanostructure alloys consist of nanoscale bet- Zr 2(Cu, Pd) surrounded by the remaining amorphous phase. The crystallization of a ternary Zr 60Al 10Cu 30 amorphous alloy occurs by the simultaneous precipitation of Zr 2Cu and Zr 2Al phases with large particle sizes of 400 to 500 nm and hence the addition of Pd is essential for formation of the nanostructure. The Pd has much larger negative heats of mixing against Zr and the resulting ZrPd atomic pair seems to act as preferential nucleation sites leading to the precipitation of Zr 2(Cu, Pd). The nanostructure alloys in the cast cylinder of 4 to 5 mm in diameter keep good ductility in the volume fraction (V f) range of the compound below 30 to 40 %.

Local structure of (U,Zr)Pd 3 by EXAFS

Measurements of the extended X-ray absorption fine structure (EXAFS) were used to determine the local configuration around uranium and zirconium atoms in (U,Zr)Pd 3 solid solutions. Interatomic distances and lattice constants from EXAFS and X-ray diffraction, respectively, are discussed in relation to the effect of addition of ZrPd 3 to UPd 3. The decrease in U–Pd distance with ZrPd 3 addition was appreciably smaller than the average M–Pd (M=U, Zr) distance of the solutions. The result suggests that the 5-f localized electronic structure of UPd 3 resists compression by the addition of ZrPd 3.

The synthesis of substituted bis[(diarylphosphinomethyl)cyclopentadienyl]zirconocene dichloride complexes for the preparation of heterodimetallic complexes containing early/late transition metal combinations

Fulvenes ( 1a–e) derived from condensation of cyclopentadiene with acetone or a variety of aldehydes were treated with LiPAr 2 (Ar = phenyl, p-tolyl) to yield the respective substituted (diarylphosphinomethyl)cyclopentadienides ( 2, 3). Subsequent reaction with ZrCl 4(THF) 2 gave the respective bis[(diarylphosphinomethy])cyclopentadienyl]zirconium dichlorides ( Ar = phenyl ( 4), p-tolyl ( 5)). The complex rac-[C 5H 4-CH(CH 3)-PPh 2] 2ZrCl 2 ( rac- 4b) was characterized by X-ray diffraction. The reaction of complexes 4a and 5a [(Cp-CMe 2-PAr 2) 2ZrCl 2] with PdCl 2(NCPh) 2 or PtCl 2(NCPh) 2 leads to the formation of the trans-(metallocene-chelate-phosphane)metal complexes 6–9 (e.g. trans-Cl 2Pd(Ph 2P-CMe 2-Cp) 2ZrCl 2]. Chloride abstraction from the reaction product of [Cp-CH(CMe 3)PPh 2] 2ZrCl 2 with PdCl 2(NCPh) 2 eventually gave the cationic complex [ meso,trans-(Cp-CH(CMe 3)PPh 2) 2(Cl)Zr( μ-Cl)Pd(Cl)] + ( 10) that was also characterized by X-ray diffraction. It features a dimetallabicyclic framework with two Cp-CHR-PPh 2 ligands and a chloride bridging between the early and the late transition metal center.

Combustion of Methane over Palladium/Zirconia Derived from a Glassy Pd–Zr Alloy: Effect of Pd Particle Size on Catalytic Behavior

A Pd25Zr75glassy metal alloy has been activated by controlled oxidation in air, resulting in highly active catalysts for the catalytic combustion of methane. The fully oxidized alloy was reduced in hydrogen at different temperatures prior to catalytic investigations resulting in a catalyst containing metallic palladium and zirconia. The reduced materials showed marked differences in BET surface area and specific surface area of palladium, both decreasing with increasing reduction temperature. The loss of surface area was accompanied by an increase of the palladium crystallite size as evidenced by XRD line broadening measurements. Catalysts reduced at low temperatures were faster reoxidized than catalysts reduced at high temperatures. Changes in the chemical behavior of palladium oxide were indicated by different decomposition behavior and reducibility with hydrogen and methane. Kinetic measurements revealed profound differences in catalytic activity for catalysts in dependence of the reduction temperature. Strong correlations between the catalytic activity and the crystallite size as well as the reducibility of palladium oxide with methane were found for catalysts reduced at different temperatures. The correlation between the reduction with methane and the catalytic performance is explained by a redox mechanism involving palladium oxide. The influence of the particle size on the catalytic activity is attributed to a strong interaction of the Pd-containing phases (Pd, PdO) with zirconia. This (support) effect is suggested to become prominent with decreasing Pd-particle size.

Preparation of amorphous alloys in Zr–Mo–Pd system

All boundary systems of the Zr–Mo–Pd system show at least one eutectic. This indicates the existence of low melting temperature (~1000°C) eutectic alloys in this ternary system, and is therefore directly related to the possibility of glass formation. Some amorphous and microcrystalline ternary samples were prepared via rapid solidification (~106 K s−1 ) in a splat cooling apparatus with a radio frequency levitation coil and a high velocity two piston arrangement driven by solenoids. To obtain the starting material, homogeneous Zr–Mo–Pd samples of differing composition were prepared via arc melting techniques. Some amorphous alloys have also been obtained in the Zr–Pd–(Si,B) systems.MST/1662

Crystal structure of the Zr 3Pd 4 phase

A new binary ZrPd phase with Zr 3Pd 4 stoichiometry was found. The structure of the phase was investigated by transmission electron microscopy and powder neutron diffraction. The structure belongs to the Pu 3Pd 4 structure type, which has a rhombohedral R 3 space group with 42 atoms in a hexagonal cell. Modifications of the ZrPd phase diagram which accommodate the new phase are suggested.

Oxidation of carbon monoxide over palladium on zirconia prepared from amorphous PdZr alloy: II. The nature of the active surface

The oxidation of dilute mixtures of CO with dilute oxygen to C0 2 procedes efficiently at moderate temperatures over a catalyst prepared from in-situ activation of amorphous PdZr 2 alloys. The process of activation and the nature of the activated surface are investigated by XPS, UPS, and low pressure reaction experiments. It turned out to be crucial to perform the activation “ in situ” in a special reaction chamber. The simultaneous presence of oxidizing and reducing components in the gas leads to the formation of particles of a solid solution of oxygen in Pd (Pd + O) in intimate contact to an ion conducting phase of zirconia. The interface between the two types of particles is large and influences the chemical stability of the Pd + O. It is preformed in the rapid solidification process which leads to a disc-shaped morphology of the micrograins in the alloy. The arrangement is capable of storing chemically active oxygen. The superior catalytic performance of this system relative to a conventional impregnated Pd/ZrO 2 catalyst is traced back to the intimate contact of the oxygen storage phase, zirconia, with the active phase, Pd + O, which is stabilized against reduction to the pure metal by a constant supply of oxygen ions through the bulk of the supporting matrix and the surface of the particle.

Palladium-zirconia diffusion bonds: Mechanical properties and interface reactions

High strength and toughness diffusion bonds have been fabricated using palladium foils between TZP zirconia blocks at temperatures above 1000°C in vacuum. Bonds fabricated below 1000°C in vacuum and under all conditions in air showed negligible strength. Strong, vacuum made bonds lost almost all their strength on annealing in air above 1000°C, anneals in vacuum also resulted in a decrease in bond strength but with a much less marked effect. PdZrO 2 interfaces have been characterised by cross-sectional TEM and a thin reaction zone identified. Microanalysis identified the presence of Pd, Zr and O in a ratio of approximately 30:52:18 in the reaction interlayer. This composition has a Pd:Zr ratio close to that of a steep eutectic in the PdZr binary system and evidence for the presence of a liquid phase at the PdZrO 2 interface during bonding is presented. The strength and toughness of the bonds are shown to be strongly dependent on the perfection of the bonded interface with the presence of a small fraction of voids causing a significant reduction in bond strength and toughness. Simple slip-line field methods are used to illustrate the influence of interface voids on the plastic constraint of bonded thin ductile layers.

Reactions of Cp 2ZrH 2 with Ni(dppe)(edt) and [Pd(PPh 3)(edt)] 2. Synthesis and structure of (Cp 2Zr) 2Pd(edt) 2, a novel, ethanedithiolate stabilized palladium(O) complex

Both Ni(dppe)(edt) ( 1) (dppe = Ph 2PCH 2CH 2PPh 2; edt = ethanedithiolate) and [Pd(PPh 3)(edt)] 2 ( 2) react with zirconocene dihydride without any evidence of CS bond cleavage. Reduction of the late transition metals to the zero oxidation state occurs together with the evolution of hydrogen. For palladium the major product of the reaction is the trinuclear complex Cp 2Zr(edt)Pd(edt)ZrCp 2 · C 6H 6 ( 3) for which the crystal structure was determined. The structure was refined to R = 0.045 ( R W = 0.032) for 2640 reflections with I > 3.0σ( I). The stereochemistry around palladium is distorted tetrahedral, and the ZrPd separations are 2.866(1) Å, suggesting bonding interactions between the metals. In reactions of Cp 2ZrH 2 with 1, spectroscopic evidence suggested that Cp 2Zr(edt)Ni(dppe) was first formed, possibly with the trinuclear nickel analogue of 3, but all attempts to recrystallize these complexes failed, and the major products isolated stemmed from ligand redistribution reactions.

Influence of Zr ions on the properties of Pd supported on graphite and diamond

FTIR, TEM and XPS methods have been applied to study the formation, dispersion, electronic and catalytic properties of the (Pd+Zr)/C system obtained by consecutive anchoring of (π-C 3H 5) 4Zr and C 3H 5PdC 5H 5 on the surface of oxidized graphite and diamond. The interaction of (π-C 3H 5) 4Zr with the surface of oxidized diamond proceeds with participation of carboxyl groups of the support, and the supported Pd complex then seems to be coordinated with anchored Zr ions. According to the XPS data, Zr in the reduced (Pd+Zr)/C catalysts is found in oxidized states and Pd in metallic states. The size of Pd particles in (Pd+Zr)/C catalysts reduced in mild conditions ranges from 10 to 20 Å. Pd sintering over graphite proceeds more rapidly compared to diamond. The introduction of Zr ions to Pd/C results in the following effects: appearance of a new absorption band at 1600 cm −1 in the spectrum of adsorbed CO, increase in the specific activity in methanol synthesis by 1–2 orders of magnitude but a drastic decrease in the specific activity in ethane hydrogenolysis, while high temperature oxidation of the samples completely eliminates the inhibitory action of Zr ions. These peculiarities are similar to those previously observed for (Pd+Zr)/SiO 2 as well as for Pd/ZrO 2, and are likely to be due to mixed PdZr n+ active sites formed in the systems.