Thomas Vils Pedersen

A Comparative Study of Phosphate Sorption in Lowland Soils under Oxic and Anoxic Conditions

Phosphate (P(i)) release due to Fe(III) oxide dissolution is well documented for soils undergoing reduction. The P(i) sorption properties of soils in anoxic conditions are, however, still under consideration. In this investigation, P(i) sorption to strictly anoxic soils was compared with oxic conditions to assess the potential of lowland soils to function as traps for P(i) when flooded with drainage water. Batch sorption experiments were performed on seven minerogenic soils. Sorption to the anoxic soils was conducted after anoxic incubation, resulting in reduction of 36 to 93% of the dithionite-extractable Fe(III) (Fe(BD)). Langmuir fitted P(i) sorption isotherms showed a P(i) release of up to 1.1 mmol kg(-1) in six soils when P(i) concentrations in the matrix (P(sol)) were lower than 10 microM. Phosphate desorption was attributed to dissolution of amorphous iron oxides, and higher pH under anoxic conditions. The point of zero net sorption (EPC(0)) increased 2- to 10-fold on reduction. Five soils showed higher P(i) sorption capacities in the anoxic than in the oxic state at higher P(sol) concentrations. Solubility calculations indicated that precipitation of vivianite or similar Fe(II) phosphates may have caused the higher sorption capacities. Use of maximum sorption capacity (S(max)) is therefore misleading as a measure of P(i) sorption at low P(sol) concentrations. The results demonstrate that none of the strongly anoxic soils, irrespective of the initial Fe(III) oxide content, the P saturation, and the degree of Fe(III) oxide reduction, could retain P(i) at natural P(sol) concentrations in agricultural drainage water.

Fabrication of high-frequency pMUT arrays on silicon substrates

A novel technique based on silicon micromachining for fabrication of linear arrays of high-frequency piezoelectric micromachined ultrasound transducers (pMUT) is presented. Piezoelectric elements are formed by deposition of lead zirconia titanate into etched features of a silicon substrate such that the depth of these features determine the element thickness and hence the resonance frequency. The process leaves a near planar surface which is ideal for further wafer level processing such as top electrode and interconnect formation. A fabricated element is characterized by pulse echo response.

Investigation of Top/Bottom Electrode and Diffusion Barrier Layer for PZT Thick Film MEMS Sensors

In this work screen printed piezoelectric Ferroperm PZ26 lead zirconate titanate (PZT) thick film is used for two MEMS devices. A test structure is used to investigate several aspects regarding bottom and top electrodes. 450 nm ZrO2 thin film is found to be an insufficient diffusion barrier layer for thick film PZT sintered at 850degC. E-beam evaporated Al and Pt is patterned on PZT with a lift-off process with a line width down to 3 mum. The roughness of the PZT is found to have a strong influence on the conductance of the top electrode.

Screen printed thick film based pMUT arrays

This article reports on the fabrication and characterization of lambda-pitched piezoelectric micromachined ultrasound transducer (pMUT) arrays fabricated using a unique process combining conventional silicon technology and low cost screen printing of thick film PZT. The pMUTs are designed as 8 element membrane based devices dimensioned to operate in bending mode in the 1-10 MHz regime. The devices are characterized using impedance measurements as well as acoustic pulse echo measurements. The characterization showed resonance frequencies for the bending mode around 6-8 MHz.

Weak-star properties of homomorphisms from weighted convolution algebras on the half-line

Let L 1 ( ω ) be the weighted convolution algebra L 1 ω (ℝ + ) on ℝ + with weight ω . Grabiner recently proved that, for a nonzero, continuous homomorphism Φ: L 1 ( ω 1 )→ L 1 ( ω 2 ), the unique continuous extension to a homomorphism between the corresponding weighted measure algebras on ℝ + is also continuous with respect to the weak-star topologies on these algebras. In this paper we investigate whether similar results hold for homomorphisms from L 1 ( ω ) into other commutative Banach algebras. In particular, we prove that for the disc algebra every nonzero homomorphism extends uniquely to a continuous homomorphism which is also continuous with respect to the weak-star topologies. Similarly, for a large class of Beurling algebras A + v on (including the algebra of absolutely convergent Taylor series on ) we prove that every nonzero homomorphism Φ: L 1 ( ω )→ A + v extends uniquely to a continuous homomorphism which is also continuous with respect to the weak-star topologies.

Some properties of the Pisier algebra

The Pisier algebra [Pscr ] consists of those continuous functions f on the unit circle [ ] for which formula here is continuous on [ ] almost surely, that is, for almost every choice of a sequence of signs (±1, ±1, …). In this paper, we prove that spectral synthesis holds in [Pscr ]. Moreover, we show that certain closed ideals in [Pscr ] with infinite hull have bounded approximate identities and give an example of a closed ideal in [Pscr ] which does not have a bounded approximate identity.

Properties of derivations on some convolution algebras

For all convolution algebras L1[0, 1); Lloc1 and A(ω) = ∩nL1(ωn), the derivations are of the form Dμf = Xf * μ for suitable measures μ, where (Xf)(t) = tf(t). We describe the (weakly) compact as well as the (weakly) Montel derivations on these algebras in terms of properties of the measure μ. Moreover, for all these algebras we show that the extension of Dμ to a natural dual space is weak-star continuous.

New technique for fabrication of high frequency piezoelectric Micromachined Ultrasound Transducers

A novel technique for fabrication of linear arrays of high frequency piezoelectric Micromachined Ultrasound Transducers (pMUT) on silicon substrates is presented. Piezoelectric elements are formed by deposition of PZT ((PbZrxTi1-x)O3) into etched features of the silicon substrate such that the depth of these features determine the element thickness and hence the resonance frequency. The process leaves a near planar surface which is ideal for further wafer level processing such as top electrode and interconnect formation. A fabricated single element is characterized by pulse echo response.