M. Sayer

Preparation of Pb(Zr,Ti)O3 thin films by sol gel processing: Electrical, optical, and electro‐optic properties

Crack‐free transparent ferroelectric polycrystalline Pb(Zr,Ti)O3 thin films were prepared by spin‐coating solutions of complex alkoxides. The preparation of stock solution, firing, and annealing of films was described. The coating of the intermediate layer of Al2O3 increased the adhesion between Pb(Zr,Ti)O3 thin films and glass substrates. The crystalline phases of films with varying Zr/Ti ratios were investigated. The dielectric constants were about 260. The remanent polarization and coercive field were 6.6 μC/cm2 and 26.7 kV/cm, respectively. The refractive index of the perovskite Pb(Zr,Ti)O3 films was 2.6 at 6328 A, and the absorption edge was at 3400 A. The quadratic and linear electro‐optic effects were measured with respect to the Zr/Ti ratio from 40/60 to 60/40 for films grown on glass substrates. The quadratic and linear electro‐optic coefficients were about 1×10−18 m2 /V2 and 2.4×10−11 m/V at 6328 A, respectively.

Characterization of thick lead zirconate titanate films fabricated using a new sol gel based process

Lead zirconate titanate (PZT) films 60 μm in thickness have been fabricated using a new sol gel based process. PZT powders are dispersed in a sol gel matrix to form a 0–3 ceramic/ceramic composite. The dielectric properties of these films have been studied as a function of powder concentration, frequency, and temperature. The characteristic Curie point is observed at 420 °C. The ferroelectric behavior measured in terms of the remanant polarization (Pr=35 μC/cm2) and coercive field (Ec=20 kV/cm) was an improvement over values quoted for thin PZT films but lower than that of bulk ceramic. The piezoelectric properties d33 (325 pC/N) and d31 (−80 pC/N) were comparable with those of the bulk ceramic.

rf planar magnetron sputtering and characterization of ferroelectric Pb(Zr,Ti)O3 films

Ferroelectric films of lead zirconate titanate (PZT) have been fabricated by rf planar magnetron sputtering. Films having a resistivity >106 Ω cm and a dielectric constant >800 at 300 K have been achieved using 100% oxygen as a sputtering medium, 10% excess PbO added to the target, and appropriate post‐deposition annealing. The sputtering rate is in the range 0.2–1.0 μm/h and varies with sputtering pressure and substrate temperature. Post‐deposition annealing affects both crystallinity and grain size, but incipient crystallization formed only in as‐grown films deposited at substrate temperatures >400 °C is essential for this process to be effective. A clear ferroelectric transition is observed at 350 °C, while the activation energy for dc conductivity of around 0.8 eV is consistent with bulk properties of PZT. The spontaneous polarization and coercive field measured from ferroelectric hysteresis loops is 20.75 μC/cm2 and 10 kV/cm, respectively. The optical transmittance of the films is determined by a Pb–...

Characterization of Pb(Zr,Ti)O3 thin films deposited from multielement metal targets

Lead zirconate titanate [Pb(Zr,Ti)O3 or PZT] thin films have been grown by sputtering a multi‐element metal target in oxygen using dc planar magnetron sputtering. Growth parameters and annealing conditions have been optimized. The kinetics of reactive sputtering and the implications of sputtering parameters on film composition have been studied. The studies reveal the requirement for operation at low substrate temperatures (200 °C), high sputtering pressures (4–5 Pa), and a large substrate‐to‐target distance (10 cm) for obtaining good control over composition. The structural and electrical properties of films were found to depend on the compositional ratio of Zr/Ti, similar to that observed in bulk PZT ceramics. Films having a resistivity of 1010 Ω cm and a dielectric constant e’∼820 at room temperature (300 K) have been achieved. Ferroelectric hysteresis loop measurements indicated a remanent polarization of 30.0 μC/cm2 and coercive field of 25 kV/cm for the rhombohedral phase composition (Zr/Ti=58/42). ...

Resorbable bioceramics based on stabilized calcium phosphates. Part II: evaluation of biological response

Synthetic materials capable of being remodelled in vivo by the same processes responsible for natural bone turnover have long been sought for use as an artificial bone substitute. These materials must ideally combine osteoinductive capacity with the ability to withstand random dissolution at normal physiological pH, while being resorbed by natural cell-mediated processes. Resorbable calcium phosphate based coatings and bulk ceramics have been developed which promote the uniform deposition of new mineralized bone matrix thus enabling rapid integration with the surrounding host bone tissue in vivo. Furthermore, a critical result of this study is the determination that the silicon-stabilized calcium phosphate ceramics are essentially insoluble in biological media but are resorbed when acted upon by osteoclasts. In vitro biological testing and preliminary in vivo testing show that the important features of this new biomaterial are a characteristic calcium phosphate phase composition and a unique microporous morphology.

DC conduvtivity of V2O5TeO2 glasses

The dc conductivity of semiconducting vanadium tellurite glasses of compositions in the range 50 to 80 mol% V2O5 has been measured in the temperature region 77 to 400 K. Measurements have been made on annealed samples at different annealing temperatures. Annealing the samples at temperature of about 250°C causes the appearance of a complex crystalline phase resulting in an increase of conductivity. Results are reported for amorphous samples of different compositions. The conductivity of tellurite glasses is slightly higher than the corresponding composition of phosphate glasses, but the general trend of the increase of conductivity and decrease of high temperature activation energy with increasing V2O5 content is similar in the two systems. The data have been analysed in the light of existing models of polaronic hopping conduction. A definite conclusion about the mechanics of conduction (adiabatic or nonadiabatic) is difficult in the absence of a precise knowledge of the characteristic phonon frequency v0. Adiabatic hopping is indicated for v0∼1011 Hz, however this value leads to unreasonably low value for the Debye temperature θD, and higher values for v0∼1013 hz satifiies the conditions for nonadiabatic hopping which appears to be the likely mechanism of conduction in V2O5TeO2 glasses. The low temperature data (< 100 K) can be fitted to Motts variable range hopping, which when combined with ac conductivity data gives reasonable values of α, but a high value for the disorder energy.

Polaronic Hopping Conduction in Vanadium Phosphate Glasses

Conductivity and dielectric properties of semiconducting glass of composition 80% V2O5:20% P2O5 have been measured as a function of frequency from dc to 3.6 GHz over a temperature range from 77°K to 420°K. A fit of the dc results to a model of polaronic hopping conduction leads to an activation energy due to disorder ΔW=0.09 eV at 77°K and an optical phonon energy of 0.053 eV compared to a calculated Miller‐Abrahams energy of approximately 0.12 eV. The real and imaginary parts of the ac conductivity are shown to increase with frequency according to power law σ=Aωs, where s=0.85 to 0.95, and it is found that the dielectric behavior can be described by a Debye‐type relaxation behavior only if a broad distribution of relaxation times exists. The results are shown to be consistent with a model of ac conduction normally applied to impurity‐doped broad‐band semiconductors, and differences between the temperature dependence of the dc and ac conductivity are attributed to a distribution of site energies in the gl...

Resorbable bioceramics based on stabilized calcium phosphates. Part I: rational design, sample preparation and material characterization

It has long been the goal of biomaterials research in the field of orthopedics to develop synthetic structures exhibiting comprehensive bioactivity. In particular, an ideal bone-biomaterial would support the activity of osteoblasts in the development of new bone, while simultaneously being resorbed by osteoclasts as part of the lifelong orderly process of bone remodelling. Such resorbable calcium phosphate-based thin films and bulk ceramics have now been created by the high-temperature processing of a fine precipitate, formed from a colloidal sol and stabilized using an additive such as silicon. The materials have two characteristic features: a phase composition which is a mixture of calcium hydroxyapatite and a silicon stabilized tricalcium phosphate, and a microporous morphology based on inter-connected particles (0.2-1 microm in diameter). X-ray diffraction, infrared spectroscopy, nuclear magnetic resonance spectroscopy, and light scattering experiments indicate that the characteristic phase composition arises during sintering through substitution reactions where silicon enters the calcium phosphate lattice under conditions of high chemical reactivity. The crystallographic features are linked through the glaserite form of the apatite structure.

Crystallization of sputtered lead zirconate titanate films by rapid thermal processing

A rapid thermal annealing (RTA) technique has been employed to process lead zirconate titanate (PZT) films prepared by reactive magnetron sputtering. The films were fabricated by dc sputtering a multielement metal target in an oxygen ambient at a substrate temperature of 200 °C. A subsequent postdeposition RTA at 600 °C for 5 s crystallizes the films into a perovskite‐type structure through various intermediate phases. Due to the short postdeposition processing times inherent in the RTA method, the initial nature of the as‐grown films has a critical influence on the crystallization kinetics. The reaction sequence in the formation of perovskite PZT from the films deposited at low substrate temperatures by the sputtering technique has been evaluated, and various key factors influencing the crystallization of PZT have been identified. As‐grown films are constituted of polycrystalline orthorhombic lead oxide in an amorphous matrix of titania and zirconia. During annealing lead oxide transforms into a cubic ph...

Structure and composition of silicon-stabilized tricalcium phosphate

Silicon stabilized tricalcium phosphate [Si-TCP] is formed within the calcium hydroxyapatite (HA)-tricalcium phosphate (TCP) system when a stoichiometric precipitate of hydroxyapatite is fired at 1,000 degrees in the presence of SiO(2). This paper proposes a composition range and crystallographic structure for Si-TCP. Reitveld XRD powder diffraction, transmission electron microscopy, infrared and proton nuclear magnetic resonance measurements show that crystalline Si-TCP is associated with the displacement of OH from an initial hydroxyapatite structure. The resulting calcium phosphate is modified by the incorporation of silicon into its structure with excess silica contributing to an amorphous component. Si-TCP has a monoclinic structure with a space group P2(1)/a akin to alpha-TCP with estimated lattice constants of a=12.863+/-0.004 A, b=9.119 +/-0.003 A, c=15.232+/-0.004 A, beta=126.3+/-0.1 degrees. It is proposed that Si(4+) substitutes for P(5+)in the TCP lattice with the average chemical composition of Si-TCP set primarily by the mechanisms available for charge compensation. While the formation of OH vacancies in HA initiates the transformation to Si-TCP, two mechanisms of charge compensation in the Si-TCP structure are plausible. If O(2-) vacancies provide charge compensation, the composition of Si-TCP is Ca(3)(P(0.9)Si(0.1)O(3.95))(2) derived for the addition of 0.33 mol SiO(2):mol HA. If excess Ca(2+) compensates, the composition is Ca(3.08)(P(0.92)Si(0.08)O(4))(2) derived for the addition of 0.25 mol SiO(2):mol HA. The reaction occurs most effectively when SiO(2) is added as a colloidal suspension rather than by the in-situ thermal decomposition of a silicon metallorganic compound. The material is a bioceramic of major biological interest because of its osteoconductivity and unique influence on skeletal tissue repair and remodeling.