Sintering kinetic studies in nonstoichiometric strontium titanate ceramics

The effect of nonstoichiometry on the densification of SrTiO₃ ceramics with Sr/Ti ratios from 0.997 to 1.02 was systematically addressed. The kinetics of densification was studied by dilatometric analysis. X-ray diffraction, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used for crystallographic and microstructure characterization. Ti excess enhanced matter transport during sintering whereas Sr excess decreased it. The shrinkage rate and average grain size increased with the decrease of Sr/Ti ratio. Close values of the activation energy for the initial densification and the near constant onset temperature for densification suggest that identical transport mechanisms control the densification of all the compositions. Small excesses of TiO₂ and SrO were mostly incorporated into the perovskite lattice inducing alterations in the defect chemistry of the material and the mass transport during sintering is controlled by Sr vacancies. Very small stoichiometric variations have a strong influence on the sintering kinetics and resulting microstructure of ST ceramics.     

Sintering of SrTiO3 in a reducing atmosphere in the presence of the perovskite “SrLi1/4Nb3/4O3”

The sintering of SrTiO₃ in a reducing atmosphere in the presence of perovskite Sr(Li_1/4Nb_3/4)O₃ as a dopant and lithium salts (LiNO₃, Li₂CO₃ and LiF) as sintering agents has been studied. Three compositions, Ti/Sr=1.0125, 1 or 0.975, were investigated. The latter two were shown to exhibit inhomogeneous microstructures, especially when sintering in the presence of LiNO₃ or Li₂CO₃. The dielectric properties of those materials were interpreted in terms of anionic defects and intergrowths. Different processing parameters were investigated such as the mixing grinding method and the grinding time; the influence of the heating rate and temperature sintering was also studied. The changes induced by each parameter on the dielectric properties and the microstructure were discussed. From these results it appears that the inhomogeneous character of the microstructure could be due to the formation of a liquid phase involving lithium. This study shows the possibility of synthesizing SrTiO₃-based type III ceramic capacitors following a suitable process with multilayer technology.     

Sintering of titanate based dielectrics doped with lithium fluoride and calcium borosilicate glass

The effect of LiF and CBS glass additives on the microstructure and dielectric property of BaTiO₃ (BT) ceramics was studied. The phase of BaLiF₃ was observed from the interaction between BT and LiF when the samples were sintered at 900 °C for 1 h. Crystal grains with Ba and F in atomic ratios of 1:3 were observed from the SEM and TEM analysis of the microstructures. Abnormal grain growth (AGG) was observed to occur in the BT ceramics sintered at 950 °C. Further improvement to the composition was achieved by employing a two-stage process. This included CBS glass shell coating of the BT particles in the first stage, and the addition of LiF in the second stage. The composition thus formed had very good permittivity (1725) and an extremely low dielectric loss value (0.008).     

Switching time of planar ferroelectric capacitors using strontium titanate and barium strontium titanate films

A microstripe cavity was used to make microwave (f∼10 GHz) measurements of the time of variation of the permittivity of SrTiO₃ and (Ba,Sr)TiO₃ films exposed to unipolar video-frequency voltage pulses. It was shown that the permittivity of SrTiO₃ films varies over times less than 30 ns. For (Ba,Sr)TiO₃ films two characteristic times of variation of the permittivity were identified: a fast variation over times less than 30 ns and a slower variation, on the order of 20 μs. Pis’ma Zh. Tekh. Fiz. 24, 19–25 (October 12, 1998)     

Sandwich-type composite multilayer films of strontium titanate and barium strontium titanate and their controllable dielectric properties

It is a challenge to reduce the dielectric loss and increase the tunability of pure barium strontium titanate(BST)films for microwave tunable application because these two properties change simultaneously.Herein,a novel composite of strontium titanate(ST)and potassium-doped BST(KBST)has been designed as ST/KBST/ST sandwich-type film with various ST and KBST layers.X-ray diffraction patterns show that the film exhibits cubic perovskite polycrystalline structure composed of BST and ST phase,mainly grow along(110)crystal plane with average grain size of less than 20 nm and decreasing BST phase/ST phase ratio with increasing film thickness.Scanning electron microscope shows that no interfacial layer can be observed,indicating that ST and KBST are fully compounded.Low dielectric loss and high tunability at-10-10V and stable and excellent dielectric properties at 1 GHz are achieved,meeting the needs of microwave tunable application at high frequency.The surface structures are also studied by other analysis methods,and ST/MgBST/ST sandwich-type film is compared.     

Transitions of barium strontium titanate ferroelectric ceramics for different strontium content

Ceramic based barium strontium titanate (BST) solid solutions with the formula Ba_1 − xSr_xTiO₃ are very important candidates for a wide range of device applications. Several doped (Mn and Mg) and undoped samples were prepared by standard solid-state reaction. Special emphasis was put on compositions with x = 0.35 and 0.60, with high potential for applications. The samples were sintered at temperatures in the 1200 ÷ 1260 °C range. Structural X-ray diffraction analysis preformed confirms the perovskite structure. The dielectric parameters were investigated in a wide temperature range between − 150 and 150 °C. The temperature was cyclically changed in both directions, up and down, at a rate of less than 2 °C/min. Both permittivity and dielectric loss were measured at low frequencies, 1 kHz. The peak values of the permittivity are increasing from 2000 to 4000 with the sintering temperature increase. Moreover the dielectric parameters were measured at room temperature in microwave domain (1 ÷ 2 GHz). The Curie temperature of BST samples with x = 0.35 and x = 0.6 is in agreement with the Curie point dependence on Sr content, as we have previously reported.     

Solubility of lanthanum in strontium titanate in oxygen-rich atmospheres

Perovskite (ABO₃)-type lanthanum substituted strontium titanate ceramics (lanthanum content x), which had been sintered in pure oxygen at 1400 °C, were investigated from x = 0 up to x = 0.6 by light optical and scanning electron microscopic means in conjunction with X-ray analysis, X-ray diffraction, and pyconometry in order to determine the mechanism being responsible for the compensation of the electronic excess charge resulting from the "donor" lanthanum. A pure strontium vacancy compensation mechanism was observed for lanthanum contents up to x = 0.3. Above x = 0.4 titanium vacancies occur additionally but their concentration remains negligible compared to the predominating strontium vacancies. No indication of a solubility limit of lanthanum at x = 0.4, as stated in former works was observed. At x = 0.5 and 0.6 the lattice structure was found to be slightly distorted, tetragonally and orthorhombically, respectively. The lattice parameter obeys Vegard’s law up to the end member La_2/3□_1/3TiO₃ (□: vacant site). These results were completely confirmed by pycnometry data. This revised version was published online in November 2006 with corrections to the Cover Date.     

Sintering of a nanodiamond in the presence of cobalt

A composite with hardness over 55 GPa is obtained by cobalt infiltration into a nanodiamond at a pressure of 8 GPa. Graphitization of a nanodiamond in pores is found to precede cobalt infiltration into them. In the presence of cobalt, graphite-like carbon is inversely transformed into diamond with a slight time delay and the nanodiamond recrystallizes, a polycrystal diamond frame being formed. The minimum cobalt concentration in compacts required for nanodiamond sintering using the infiltration technique is 6 vol %.     

The structure of a migrating low-angle tilt grain boundary in strontium titanate

Weak beam transmission electron microscopy and stereomicroscopy have been used to characterize the three-dimensional structure of a severely deformed low-angle tilt grain boundary in a strontium titanate ceramic. Various interactions between crystal lattice dislocations and grain boundary dislocations in this boundary have been analysed. The deformed low-angle tilt grain boundary is a result of partial glide. The boundary is composed of dislocations with Burgers vectorsa [¯100] and the deformation of the boundary is interpreted as having occurred by the interactions of the boundary with crystal lattice dislocations during grain boundary migration. Observed dislocation network in the grain boundary are a result of the reactionb ₃=b₁+b₂=a [¯100]+a [00¯1]=a [¯10¯1], and the resultant dislocations are sessile. Also, many crystal lattice dislocations are pinned by the grain boundary and produce a complex dislocation structure for the boundary.M. Fujimoto is a Visiting Scientist at the Massachusetts Institute of Technology on leave from Taiyo Yuden Co., Ltd., Tokyo, Japan.     

Synthesis of barium strontium titanate nanorods in reverse microemulsion

Perovskite Ba_0.7Sr_0.3TiO₃ nanorods were synthesized in Triton X-100 reverse microemulsion at room temperature and characterized by XRD, ICP-AES, ED and TEM. The Ba_0.7Sr_0.3TiO₃ nanorods with a diameter of 60-100 nm and a length of 450-1200 nm showed a single-crystalline tetragonal structure. The size of nanorods was effectively tuned by changing w value (molar ratio of water to surfactant), aging time and reactant concentration.     

Sintering mechanisms in strontium doped lanthanum chromite

The sintering behavior of (La_0.7Sr_0.3)_xCrO₃ (0.95 x 1.05) is investigated to compare liquid phase sintering phenomena occuring in stoichiometric and non-stoichiometric compositions. Shrinkage analysis revealed marked contrast between the densification characteristics of the A-site enriched (x > 1.00) and A-site depleted (x < 1.00) materials. A-site depleted samples typically exhibited a single liquid phase sintering event at 1250 °C attributed to the melting of an exsoluted SrCrO₄ phase. A-site enriched samples indicated two rapid shrinkage events due to the melting of SrCrO₄, and a Sr_2.67(CrO₄)₂ phase with a melting temperature of 1450 °C. Sr_2.67(CrO₄)₂ was shown to evolve from a decomposition reaction between SrCrO₄ and La₂CrO₆, detected together in A-site enriched samples from 800–1000 °C. Maximum densities (93% theoretical density) were achieved for (La_0.7Sr_0.3)_xCrO₃ x = 1.00 after sintering at 1700 °C for two hours.     

Preparation of barium strontium titanate using a modified polymeric precursor method

Barium strontium titanate with Ba/Sr ratio of 50/50 (BST5) was prepared using a modified polymeric precursor method. Unlike other methods, it proved to be cost effective since neither moisture sensitive nor costly starting materials were used for the preparation. The optimum preparation condition was established, and it was found that an annealing temperature of 600 °C with a dwell time of 20 h followed by a heat treatment at 800 °C with dwelling time of 4 h is enough to obtain well crystallized, phase pure fine powder of BST5. The oxide powder thus obtained was single phased, crystalline with cubic structure. The average lattice constant of the polycrystalline oxide was examined using XRD and it was determined to be 0.3950 nm (±0.0003 nm). The average crystallite size of the ceramic was found to be about 25–30 nm. The reaction pathway, temperature of decomposition of the precursor and the temperature of formation of the oxide was found using the data from the EGA-TGA-MS coupled instrument. TGA data showed about 75% of total weight loss over the entire heating process. The SEM studies showed that the grains were in micrometer size range, having high degree of agglomeration and irregular morphology. This process is interesting as it yields phase pure BST5 ceramic at a temperature of 800 °C devoid of secondary phases like (Ba,Sr)₂TiO₄.     

Lattice constant of strontium titanate at low temperatures

A double-crystal X-ray diffraction technique has been applied to the lattice constant measurement of SrTiO₃ in the temperature range 17 to 300 K. A relative accuracy of 10^?5 is attained. No anomaly on the lattice constant vs. temperature curve is detected except at the cubic-to-tetragonal transition at 106 K. The splitting of the h00 and 00l Bragg peaks are completely resolved below 85 K.     

Anomalous crystallization behavior of strontium titanate aluminosilicate glass-ceramics

The crystallization of glasses with a nominal composition consisting of 65 wt% SrTiO₃, 29 wt% SiO₂, and 6 wt% Al₂O₃, were investigated. Two glass batches of this composition, prepared from different chemicals and similar processing conditions, resulted in glass-ceramics with very different crystalline phases. One glass batch resulted in glass-ceramics with perovskite SrTiO₃ as the primary crystalline phase, while the second glass batch resulted in glass-ceramics with fresnoite Sr₂TiSi₂O₈ as the primary crystalline phase. The different crystallization sequences of glass-ceramics of the two glass batches were traced to differences between the first phases to crystalline from each glass. The crystallization behavior of these glass-ceramics was further complicated by the presence of several unidentified phases.     

On the defect structure of strontium titanate with excess SrO

The defect structure of strontium titanate with Sr to Ti ratios of 1.002, 1.005 and 1.008 was determined by measuring the equilbrium electrical conductivity at 950 and 1000° C as a function of oxygen partial pressure in the range 10^o to 10^–22 atm. The measured conductivity data were similar to those obtained in the sample with Sr/Ti=1.000. These observations indicate that the incorporation of excess SrO in strontium titanate with ideal composition does not generate any charged defects. A model involving the formation of a shear-structure is considered in order to accommodate the excess SrO in the system.     

Dielectric properties of lead zirconate titanate thin films seeded with barium strontium titanate nanoparticles

A low temperature synthetic method recently proposed by the authors was applied to the fabrication of lead zirconate titanate (PZT) thin films containing crystalline seeds of barium strontium titanate (BST) nanoparticles. PZT precursor and the BST particles were prepared with complex alkoxide methods. Precursor solution suspending the BST particles was spin-coated on Pt/Ti/SiO₂/Si substrate to film thickness of 500–800 nm at particle concentrations of 0–25.1 mol%, and annealed at various temperatures. Seeding of BST particles prevented the formation of pyrochlore phases, which appeared at temperatures above 400 °C in unseeded PZT films, and induced crystallization of PZT into perovskite structures at 420 °C, which was more than 100 °C below the crystallization temperature of the unseeded PZT films. Measurement of dielectric properties at 1 kHz showed that the 25.1 mol% BST-seeded PZT films annealed at 450 °C had a dielectric constant as high as 300 with a dissipation factor of 0.05. Leakage current density of the film was less than 1×10⁻⁶ A/cm² at applied electric field from 0 to 64 kV/cm.     

Atomic layer deposition of ruthenium films on strontium titanate

Atomic layer deposition of ruthenium on SrTiO3 layers was investigated using (C2H5C5H4). (NC4H4)Ru and air as precursors. For comparison, the growth was studied also on ZrO2 films and SiO2/Si surfaces. Deposition temperature was 325 degrees C. Using rather short but intense air pulses, smooth and uniform Ru films were deposited on SrTiO3. The films were crystallized at early stages of the growth. The nucleation density and rate on SrTiO3 were notably lower compared to that on ZrO2 and SiO2, but the physical qualities including the film conductivity were considerably enhanced after reaching Ru film thickness around 10 nm.