Amorphous Chromium Research Articles

Condensation and Crystallization of Amorphous/Lamellar Chromium Sesquioxide

Amorphous chromium oxide nanocondensates were fabricated by energetic pulsed laser ablation on a Cr target in vacuum for a fine particle size and a pronounced quenching effect. Analytical electron microscopy indicated the amorphous phase thus quenched has corrugated lamellar layers with bimodal interspacings of 0.259−0.266 and 0.355−0.371 nm, which are close to that of the specific lattice planes of the stable α-type structure, that is, (1̅104) and (112̅0), having the Cr-filled octahedral sites assembled as 0 and 1 periodic bond chains, respectively. Such amorphous nanocondensates were observed in situ to become more polymerized by forming (011̅2)-like layers and then fully crystallized as α-Cr2O3 for further (011̅2)-specific coalescence when irradiated by electron beam. The partially crystallized lamellae showed Raman shifts similar to that of the ambient α-Cr2O3, yet at higher frequencies due to a residual compressive stress up to ∼4 GPa. This implies a rather tight 6-coordination of Cr3+ in the corundum-like structure for the rapidly quenched amorphous phase.

Carboxylate-containing chelating agent interactions with amorphous chromium hydroxide: Adsorption and dissolution

Anthropogenic chelating agents and biological chelating agents produced by indigenous organisms may dissolve Cr III (hydr)oxides in soils and sediments. The resulting dissolved Cr III-chelating agent complexes are more readily transported through porous media, thereby spreading contamination. With this work, we examine chelating agent-assisted dissolution of amorphous chromium hydroxide (ACH) by the (amino)carboxylate chelating agents iminodiacetic acid (IDA), nitrilotriacetic acid (NTA), tricarballylic acid (TCA), citric acid (CIT), ethylenediaminetetraacetic acid (EDTA), trans-1,2-cyclohexanediaminetetraacetic acid (CDTA), and trimethylenediaminetetraacetic acid (TMDTA). The extent of chelating agent adsorption onto ACH increased quickly over the first few hours, and then increased more gradually until a constant extent was attained. The extent of chelating agent adsorption versus pH followed “ligand-like” behavior. All chelating agents with the exception of TCA and IDA effectively dissolved significant amounts of ACH within 10 days from pH 4.0 to 9.4. IDA dissolved ACH below pH 6.5 and above pH 7.5. Rates of ACH dissolution normalized to the extent of chelating agent adsorption were pH dependent. IDA, NTA, CIT, and CDTA exhibited an increase in normalized dissolution rate with decreasing pH. EDTA and TMDTA exhibited a maximum in normalized dissolution rate near pH 8.5. Use of acetic acid as a pH buffer in experiments decreased the extent of chelating agent adsorption for IDA, NTA, and CIT but increased normalized rates of chelating agent-assisted dissolution for all chelating agents except EDTA. The results from this study provide the necessary information to calculate the extents and time scales of ACH dissolution in the presence of (amino)carboxylate chelating agents.

Electrodeposition and tribological behavior of amorphous chromium-alumina composite coatings

The amorphous chromium-alumina (a-Cr-Al 2O 3) composite coatings with thickness of 50 μm were electrodeposited from Cr(III) electrolytes containing Al 2O 3 particles and Al 3+. The effects of Al 3+ on the dispersibility and Zeta potential of Al 2O 3 particles in electrotyles were investigated. It was found that the Al 3+ promotes the Al 2O 3 particles to uniformly disperse in the Cr coatings. Tribological behavior of a-Cr-Al 2O 3 composite coatings was studied as compared with amorphous chromium (a-Cr) coatings electrodeposited from single Cr(III) electrolytes. The results demonstrate that the introduction of Al 2O 3 particles significantly improves the tribological performance of a-Cr coatings.

Tribological Behaviors of Amorphous Cr Coatings Electrodeposited from Cr(III) Baths under Ionic Liquid Lubrication

The amorphous chromium (a-Cr) coatings with thickness of 50 μm were electrodeposited from Cr(III) instead of Cr(VI) bath. The electrochemical and tribological behaviors of the a-Cr coatings in the ionic liquid were investigated. Possessing high hardness, steady passive film, and few grain boundary structures, the a-Cr coatings exhibit much better corrosion and wear resistance compared with Fe, Cu, and nanocrystalline Cr coatings under the ionic liquid lubrication. © 2007 The Electrochemical Society. [DOI: 10.1149/1.2746129] All rights reserved.

Corrosive behavior of chromium carbide-based films formed on steel using a filtered cathodic vacuum arc system

The formation of chromium carbide-based hard-coatings on steels using a 90°-bend filtered cathodic vacuum arc (FCVA) has extensive industrial applications; such coatings are free of macroparticles and exhibit excellent characteristics. In this investigation, a working pressure of C 2H 2/Ar was adopted to synthesize amorphous chromium carbide film (a-C:Cr) and crystalline chromium carbide film (cryst-Cr 3C 2) from a Cr target (99.95%) at 500 °C under a substrate voltage of −50 V. The corrosion behavior of a-C:Cr coated on steel (a-C:Cr/steel) and cryst-Cr 3C 2 coated on steel (cryst-Cr 3C 2/steel) were compared in terms of open-circuit potentials (OCP) and polarization resistance ( R p) in an aerated 3.5 wt% NaCl aqueous solution, as determined by electrochemical impedance spectroscopy (EIS). The XRD results indicated that the transformation of a-C:Cr to cryst-Cr 3C 2 is distinct as the working pressure declines from 1.2 × 10 −2 to 2.9 × 10 −3 Torr. The OCP of a-C:Cr/steel and cryst-Cr 3C 2/steel resemble each other and both assembly are nobler than uncoated steel. The R p of the coatings exceeds that of the uncoated steel. The SEM observation and the EIS results demonstrate that the cryst-Cr 3C 2/steel more effectively isolates the defects than dose a-C:Cr/steel.

Mechanochemical Synthesis of Rutile‐Type CrMO4 (M: V, Sb) and Their Solid Solutions.

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Mechanochemical synthesis of rutile-type Cr MO 4 ( M=V, Sb) and their solid solutions

Grinding a mixture of hydrous amorphous chromium oxide (Cr 2O 3· nH 2O), vanadium oxide (V 2O 5) and antimony oxide (Sb 2O 5) was conducted by using a planetary ball mill, to investigate their mechanochemical reactions to form chromium vanadium oxide (CrVO 4) and chromium antimony oxide (CrSbO 4). The synthesis reactions proceed with an increase in grinding periods of time. The ground samples consist of agglomerates with particle size of about ten nanometers. The synthesized CrVO 4 sample exhibits a rutile-type tetragonal crystal structure, which is a high pressure phase. Additionally, solid solutions, CrV 1− x Sb x O 4 ( x = 0 ∼ 1 , Δ x = 0.2 5 ), have been synthesized mechanochemically from the mixtures of Cr 2O 3· nH 2O, V 2O 5 and Sb 2O 5.

KrF pulsed laser deposition of chromium oxide thin films from Cr8O21 targets

Chromium oxides, CrxOy, are of great interest due to the wide variety of their technological applications. Among them, CrO2 has been extensively investigated in recent years because it is an attractive compound to be used in spintronic heterostructures. However, its synthesis at low temperatures has been a difficult task due to the metastable nature of this oxide. This is indeed essential to ensure interface quality and the ability to coat thermal-sensitive materials such as those envisaged in spintronic devices. Pulsed Laser Deposition (PLD) is a technique that has the potential to meet the requirements stated above. In this work, we describe our efforts to grow chromium oxide thin films by PLD from Cr8O21 targets, using a KrF excimer laser. The as-deposited films were investigated by X-ray diffraction and Rutherford backscattering spectrometry. Structural and chemical composition studies showed that the films consist of a mixture of amorphous chromium oxides exhibiting different stoichiometries depending on the processing parameters, where nanocrystals of mainly Cr2O3 are dispersed. The analyses do not exclude the possibility of co-deposition of Cr2O3 and a low fraction of CrO2. PACS: 81.15.Fg, 75.50.Dd

Preparation of Chromium Hydroxide Sub-micro- and Nanoparticles by Microwave Dielectric Heating

This paper reports the preparation of spherical amorphous chromium hydroxide particles by the forced hydrolysis method combined with microwave dielectric heating. The sols were obtained by aging both chrome alum (KCr(SO4)2·12H2O) solutions and solutions of Cr(NO3)3 - K2SO4 with different initial [Cr3+]/[SO42-] ratios, at the boiling temperature. In the first case the experiments yielded sols composed of submicroparticles. The aging times were shorter (two order of magnitude less) than those reported in the literature by using conventional heating. In the second case the mean size of particles decreased drastically from the submicrometric range for [Cr3+]/[SO42-] ratios of 0.5,0.8 and 1.0 to the nanometric one for ratios 1.6 and 2.0. This finding is the first successful demonstration of applicability of the forced hydrolysis method to obtain nanoparticles of this material. A mechanism of nanoparticles formation is proposed.

Observation of amorphous chromium in modified C4 flip chip solder joints after thermal stress testing

Flip chip reliability was evaluated using thermal stress tests at 150 °C. Electrical failures of flip chip devices were found to occur at the solder/under-bump-metallization interface by forming a porous amorphous chromium layer. The formation of the porous amorphous layer responsible for electrical failures resulted from the outdiffusion of copper atoms from a copper–chromium co-deposit, used as one of the under-bump-metallization layers. A strong interaction of Cu with the Sn component of the solder is the driving force of the Cu outdiffusion.

Evidence for the formation of chromium (III) diphenoquinone complexes during oxidation of guaiacol and 2,6-dimethoxyphenol with chromic acid

The reaction of guaiacol and 2,6-dimethoxyphenol with metal oxidants has been used as a model in order to better understand the chromium mediated reaction and photostabilization of lignin on wood surfaces. Even with a simple lignin model such as guaiacol the reaction is complex and some of the reaction products are not easily characterized. The reaction of aqueous ferric chloride with guaiacol in a 1:2 molar ratio yields an organic product with the isomeric diphenoquinones 5-(4-hydroxy-3-methoxy-phenyl)-3,3′-dimethoxy-bicyclohexylidene-2,5,2′,5′,-tetraene-4,4′-dione (A) and 3-(4-hydroxy-3-methoxy-phenyl)-5,3′-dimethoxy-bicyclohexylidene-2,5,2′,5′,-tetraene-4,4′-dione (B) as major components. By comparison the reaction of chromic acid with guaiacol at 1:2 and 1:5 molar ratios affords amorphous chromium (III) complexes in good yield. Reductive degradation of these complexes with sodium borohydride in tetrahydrofuran yields the coupled guaiacol trimer 3,5′,3″-trimethoxy-[1,1′;3′,1″]terphenyl-4,4′,4″-triol as the major component. The formation of this trimer is consistent with the presence of bound diphenoquinones (A) and (B) in the original chromium (III) complex. Oxidation of 2,6-dimethoxyphenol is less complicated and clarifies the nature of chromium (III) complexes formed during these reactions. Oxidation of 2,6-dimethoxyphenol with aqueous ferric chloride yields the symmetrical diphenoquinone coerulignone. Oxidation of 2,6-dimethoxyphenol with chromic acid unambiguously yields an amorphous chromium (III) coerulignone complex which was characterized by solid state 13C MAS NMR and IR spectroscopy as well as by reductive degradation. It is postulated that chromic acid oxidizes lignin phenols in wood affording related chromium (III) quinone complexes that confer weather stability to the treated wood surface.

Properties and preparation of amorphous chromium carbide electroplates

The electrochemical and corrosion behavior of chromium electroplates formed in sulfuric acid solutions of Cr(III) in the presence of oxalates was studied by measuring steady-state polarization curves in 0.5 M H 2SO 4 solution. These electroplates demonstrate no active dissolution region and their open-circuit potentials are located in the passivity region, i.e. shifted substantially in the positive direction as compared with those of metallurgical chromium and electroplates from standard chrome-plating baths. The results of X-ray photoelectron spectroscopy (XPS) studies evidence that the passive metal surface layer several nanometers thick consists of chromium oxides with incorporated carbides formed during electroplating. It is supposed that the peculiarities of the corrosion and electrochemical behavior of the deposits under study can be attributed to the presence in them of chromium carbides, which operate as cathodic agents. At the same time, the formation of these carbide compounds during the cathodic deposition of chromium electroplates from sulfuric acid Cr(III) solutions in the presence of sodium oxalate is a result of electrocatalytic activity of metal chromium. The latter assumption is confirmed by XPS analysis of surface layers formed during the exposure of chromium to sulfuric acid solutions containing organic substances.

Mechanochemical synthesis of LaCrO 3 by grinding constituent oxides

Hydrous amorphous chromium oxide (Cr 2O 3· nH 2O) and crystalline one (Cr 2O 3) powders prepared by heating Cr(OH) 3· nH 2O at different temperatures were separately ground with La 2O 3 powder using a planetary ball mill at room temperature to investigate mechanochemical synthesis of LaCrO 3. Hydrous amorphous chromium oxide prepared reacts with La 2O 3 to form LaCrO 3 and the reaction proceeds with an increase in grinding time. No reaction takes place in the case of crystalline Cr 2O 3. The ground sample consists of agglomerates and large particles with size up to around 100 μm and shows low specific surface area of about 5 m 2/g. The mechanochemical method can be also used to synthesize other chromium rare earth complex oxides by grinding the mixture of hydrous chromium oxide and rare earth (Pr, Nd and Sm) oxides.

Studies of the fractal microstructure of nanocrystalline and amorphous chromium obtained by electrodeposition

Nanocrystalline materials are of high technological and scientific interest because their internal microstructure can affect many macroscopic properties like hardness, corrosion resistance and magnetic properties. In this paper we discuss the properties of electrodeposited Cr. The nanocrystalline (n-Cr) and amorphous (am-Cr) samples were prepared by an electrochemical method. The deposited n-Cr has crystallites with grain diameters of about 30 nm. The microstructure of the obtained materials has been studied by density measurements, X-ray diffraction and small angle neutron scattering (SANS). X-ray diffraction shows completely different atomic arrangements in n-Cr and am-Cr, but the SANS results are very similar to each other. The microstructure of both n-Cr and am-Cr can be described as a surface fractal with fractal dimension of 2.24(3) and 2.22(3) for n-Cr and am-Cr, respectively.

Polycrystalline and Amorphous Chromium Oxide MOCVD

AbstractThis paper will report the methods to deposit polycrystalline or amorphous chromium oxide (Cr2O3) thin films, which have potential applications in optoelectronics such as components for flat panel display devices. An atmospheric cold wall chemical vapor deposition (CVD) system was used to thermally decompose sublimed chromium acetylacetonate(Cr(C5H7O2)3) precursor, which gave Cr2O3 films. Oxygen concentration in gas phase is the key factor determining chromia film structure. In oxygen containing CVD system, the deposited films were polycrystalline Cr2O3, with (006), and (110) orientations. In inert, O2 free environment, the grown films were amorphous chromium oxide, as identified by XPS and XRD results. Based on depth profiling AES results, even in an oxygen free ambient, the elemental Cr/O ratio in films still remained stoichiometry(Cr:O = 2:3). Oxygen atoms might originate from acetylacetonyl (C5H7O2) ligands in the Cr(C5H7O2)3precursor.

Surface Layers on Chromium in Sulfuric Acid Solutions of Organic Substances: An X-ray Photoelectron Spectroscopy Study

Surface layers formed at a contact of chromium with sulfuric acid solutions containing some organic substances (sodium oxalate, dimethylformamide, methanol) are analyzed by the X-ray photoelectron spectroscopy method. The layers are shown to contain products of deep destruction of organic molecules, i.e. carbon species and those containing functional groups. The data obtained point to the electrocatalytic activity of chromium in cathodic processes. The results of this study may provide a deeper insight into the mechanism of carbon incorporation (in the form of carbide compounds as well) into amorphous chromium deposits formed in sulfuric acid baths containing some organic substances.

Investigation of the anodic behavior of x-ray-diffractometrically amorphous electrolytic chromium deposits on copper

The anodic behavior of x-ray-diffractometrically amorphous chromium deposits in 1 N sulfuric acid is investigated with the use of potentiostatic and potentiodynamic techniques. The morphology of the surface of coatings was studied with digital microscopy. The substantial effect of a substrate (copper) on the anodic behavior of a porous coating is noticed. Pores and cracks may appear in the coating due to the surface somatoids formed during electroplating. The possible mechanisms of the passivation of coatings are considered.

Studies of Corrosion Resistance of Amorphous Chromium Deposits in Acidic Solution

SUMMARYAmorphous chromium deposit has been prepared by electrodeposition technology. The amorphous structure of the deposit was examined by XRD and TEM. The deposit was spontaneously passive showing a significantly lower corrosion rate in comparison with conventional chromium deposit in l mol/l HCl and l mol/l H2SO4 solutions. X-ray photoelectron spectroscopy analysis of passive film shows that the passive film formed is similar to that on a conventional chromium deposit and consists mainly of chromium oxyhydroxide.

The structure of thin Cr film prepared by the vacuum evaporation method

In order to clarify the structures of thin Cr films by their real space images, Cr films of various thicknesses, prepared using vacuum deposition, have been observed using a high resolution electron microscope. The amorphous chromium films were composed of microcrystallites of the order of 3 nm. The crystallite structure was δ-Cr. The well-known bcc chromium particles were observed in thick and large crystal particles. The stability of small particles is discussed.

The Structure of Amorphous CrS3 Containing [CrIII2((S−I)2)3]x Chains: An X-Ray Diffraction Modeling Study

The amorphous chromium sulfide, CrS3, has no crystalline counterpart. A model is constructed to account for the interference function, Qi(Q), and the real space total correlation function, T(r), obtained from X-ray diffraction measurements. The structural unit (S−I)2/2CrIII((S−I)2))2CrIII(S−I)2/2 gives a good fit to the Q-space data over the range 2–20 Å−1 and explains features in T(r) to 3.6 Å. The unprecedented coordination geometry around chromium explains the unusual magnetic moment of the compound, corresponding to one unpaired electron per CrIII. The individual structural units are linked by the terminal disulfide groups to give [–Cr(S2)2Cr(S2)–]n chains. The flexibility of the single disulfide linkages within the chains and the difficulty of rearranging these chains to form an ordered structure after the rapid formation of CrS3 from solution explains the amorphous nature of CrS3.