Jagadish C. Ray

Chemical synthesis and structural characterization of nanocrystalline powders of pure zirconia and yttria stabilized zirconia (YSZ)

Abstract Nanocrystals of pure zirconia and yttria stabilized zirconia (YSZ) are obtained by a simple chemical synthesis route using sucrose, polyvinyl alcohol (PVA) and metal nitrates. The reaction mixture on pyrolysis and calcination gives nanocrystals. These are characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD). The size of the nanocrystallites for pure zirconia is in the range of about 7.0–45.0 nm and for yttria stabilized zirconia, is in the range of about 5.0–24.0 nm at 200°C and above, according to the preparative condition. At 200°C, pure zirconia forms cubic phase and this cubic phase is stable up to 600°C and then slowly transformed into monoclinic form. For yttria stabilized zirconia, the crystals are tetragonal in the temperature range from 200 to 1200°C.

Formation of Cr3+ stabilized ZrO2 nanocrystals in a single cubic metastable phase by a novel chemical route with a sucrose-polyvinyl alcohol polymer matrix

Abstract Cr 3+ -stabilized ZrO 2 nanocrystals in a single metastable cubic (c) phase are prepared by a novel chemical method using a polymer matrix-based precursor with sucrose and polyvinyl alcohol. A pyrolysis of precursor over a hot plate at 250°C followed by calcination at temperatures between 400°C and 1000°C yields ZrO 2 nanocrystals in form of a powder. X-ray diffraction determines formation of a single c-ZrO 2 phase at 2–15 mol% Cr 3+ at 900°C or lower temperatures. Crystallites are in a near-spherical shape, 5–10-nm diameter, in electron micrograph as per the calcination temperature. A fast decomposition/combustion of Cr 3+ modified precursor over a narrow 250–500°C range during calcination controls formation of c-ZrO 2 nanocrystals in the confined size. It occurs with a vigorous exothermic signal with 70–80% loss in initial precursor mass. Average position of the exothermic peak shifts from 460°C to 442°C to 385°C at 1, 5 and 15 mol% Cr 3+ , respectively, in the precursor.

A novel chemical route for the synthesis of nanocrystalline α-Al2O3 powder

Nanocrystalline α-Al2O3 powders have been synthesized by pyrolysis of a precursor material prepared by evaporation of an aqueous solution of sucrose with polyvinyl alcohol and stoichiometric amount of the desired metal nitrate. The single-phase α-Al2O3 powder was obtained after heat treatment at 1150°C for 2 h. X-ray diffractometry (XRD), infrared spectroscopy, differential thermal and thermogravimetric analysis (TG/DTA), and transmission electron microscopy (TEM) were used to characterize the precursors and the heat-treated final powders. The average particle size as measured from X-ray line broadening and TEM studies was around ∼25 nm. The particles have sizes of the same order to that of crystallite which indicates poor agglomeration of crystallites.

Chemical synthesis of nanocrystals of tantalum ion-doped tetragonal zirconia

Abstract A polymer matrix-based precursor solution method is explored for the preparation of tantalum ion-doped tetragonal (t) zirconia polycrystals. The matrix is composed of sucrose and polyvinyl alcohol (PVA), which are the dispersing agents for the metal cations and finally act as a template for the nanoparticle through the formation of a mesoporous structure. The t-phase is formed in powders of zirconia with 5, 10, 15 and 20 at.% tantalum ion, and the phase is stable up to the calcining temperature of 1100 °C. The particle size distribution is narrow with the size range 20–40 nm for the sample containing 5 at.% tantalum ions calcined at 700 °C. The stabilization of the metastable tetragonal form is probably due to the strong TaO bonds, which hinder the reorientation of atoms during phase transformation.

Chemical synthesis of nanocrystalline zirconia by a novel polymer matrix-based precursor solution method using triethanolamine

Nanocrystalline cubic ZrO2–Y2O3 powders were synthesized by a novel polymer matrix-based method, using triethanolamine. Solid solutions of ZrO2 containing various mole percent of Y2O3 with cubic structure were produced at low temperature (approximately bed-temperature 250°C). The produced powder was characterized using differential thermal analysis (DTA), differential thermogravimetry (DTG), X-ray diffraction (XRD) and transmission electron microscopy (TEM). The average crystalline size of the powder calcined at 650°C ranges from 7.0 to 13.0 nm and the particle size ranges from 20.0 to 40.0 nm.

Chemical synthesis of nanocrystalline tin-doped cubic ZrO2 powders

Abstract The nanocrystalline powders of the system SnO 2 –ZrO 2 has been prepared using zirconium oxalate and tin tartarate, which are also synthesised from their organic and inorganic precursors. The aqueous solutions of oxalate and tartarate are mixed with proper proportions and with polyvinyl alcohol to form the polymer precursor solution. This is evaporated, pyrolysed and calcined to nanocrystalline powders. The phase of the powders is cubic at the temperature of calcination of 700 °C with the crystallite size ranges from 15 to 25 nm, whereas the particles size ranges from 30 to 50 nm for the sample containing 5 mol% SnO 2 . The shapes of the particles are oval and spherical. The alloying can be done up to 20 mol% with SnO 2 . The material is promising for chemical sensors.

A novel polymer matrix method for synthesizing ZrO2 nanocrystals at moderate temperature

Bulk zirconia (ZrO2) exists in three monoclinic (m), tetragonal (t), and cubic (c) polymorphs. m-ZrO2 transforms to t-phase at ∼1170 ◦C and then to c-phase at 2370 ◦C [1]. The high temperature phases, which have potential applications as oxygen sensors [2], solid fuel cells [3], and several ceramic components [2, 3], cannot be retained at room temperature because the transformation is reversible. Efforts have been made to partially or fully stabilize them by addition of small amounts of oxides, e.g., MgO, CaO, Y2O3, etc. [4–6], or Mg3N2, Si3N4 and AIN [4]. The additive refines microstructure of sample at a nanometer scale through a proper method. This includes vapor phase reactions [2], hydrothermal process [5], alkoxide or gel processes [7], mechanical attrition [8], and combustion methods [9]. In this letter, we report a novel polymer matrix method and its validity in synthesizing stabilized ZrO2 or other nanoceramics in a controlled crystallite size to a few nanometers at moderate temperature. The polymer encapsulates metal cations in small micelles in an aqueous solution and permits a controlled nucleation and growth of them as ceramics on its decomposition and combustion. This occurs at moderate temperature. It is demonstrated with example of Cr3+ stabilized c-ZrO2 nanocrystals. Otherwise, Cr3+ is hardly soluble in bulk ZrO2 and hardly stabilize the c or t-phase alone [10, 11]. A pure hydrous zirconia, obtained by hydrolyzing an aqueous ZrOCl2 · 8H2O solution (∼3 M) by NH4OH, is dissolved in nitric acid to have a clear solution. As a source of Cr3+, (NH4)2Cr2O7 is added in predetermined Cr3+/Zr4+ ratio through an aqueous solution. Addition of sucrose and PVA (in 25 : 2 molar ratio) by 50 to 70% mass in a batch of 20 g of the total sample forms a polymer matrix solution in a transparent light blue color. On evaporating over a water bath, it turns into a dried precursor mass in a dark black characteristic color. On pyrolyzing at ∼250 ◦C on a hot plate, it results in a fluffy powder in a light brown color to deep blue color depending on the Cr3+ content. After pulverizing, it is calcined at selected temperatures between 400 and 1000 ◦C to obtain the Cr3+ stabilized ZrO2 nanocrystals. A series of samples with Cr3+ contents up to 15 mol% thus were obtained. Phase analysis of samples calcined at various temperatures is carried out with X-ray diffraction with the help of an X-ray powder diffractometer using λ = 0.15418 nm wavelength of Cu Kα radiation. Average crystallite size is calculated from broadening in characteristic peaks with the Debye Scherrer formula. Size and morphology of stabilized ZrO2 particles are studied with electron microscopy. The sucrose and PVA polymer matrix behaves as an efficient dispersoid to disperse metal cations in it at a microscopic scale. Here, sucrose plays a multifunctional role. At first, it forms a complex with metal cations by coordinating through hydroxyl groups and those get encapsulated in micelles of it in the solution. The micelle circumvents selective precipitation of encapsulated cations inside it during evaporation of precursor solution. Sucrose, being in excess to the cations, behaves as a strong chelating agent and ensures an atomistical distribution of the cations through the polymer network structure. It forms a branched polymer with PVA [9]. It has been observed that sucrose in presence of PVA in a very small 10% amount gives rise to a desirably crushable, fluffy metal oxide powder. Moreover, the polymer matrix serves as an efficient internal fuel to decompose and burn out the precursor spontaneously into a refined metal oxide powder at 250 to 500 ◦C. A typical TEM micrograph (a) of a stabilized ZrO2 powder with 5 mol% Cr3+, obtained after a complete decomposition and combustion of the precursor at 600 ◦C for 2 h, is shown in Fig. 1. It consists of mostly clusters of very tiny crystallites. The size of the clusters varies from 10 to 30 nm. A magnified picture of these clusters is shown in micrograph (b) in Fig. 1. The particles in the cluster are so intimately compacting together that an analysis of individual particles is difficult. Moreover, a few single particles are present in these micrographs. These are in spherical shape in 4 to 6 nm diameter (d). Morphology of spherical particles in clusters can be visualized by micrograph of a selected cluster (d ∼ 20 nm) in Fig. 1c. A spherical particle of radius r involves a surface area = 4πr2 ÷ 3πrρ ≡ 3/rρ. It gives a value of ∼ 197 m2/g for a c-ZrO2 particle in r = d/2 = 2.5 nm with density ρ = 6.10 g/cm3 [12]. It is ∼4 times larger that in the cluster, d ∼ 20 nm, in Fig. 1c. In approximation of a close packing of particles in the cluster and neglecting the interparticle space, it has as many particles as 64. At moderate temperature, as 600 ◦C used above, insufficient to cause a significant grain growth, ZrO2