The application of quartz crystal microbalance (QCM) as a device to measure the rheology of colloidal suspensions has been studied. Using a commercial dip-probe QCM, the yield stress of magnesium hydroxide suspensions has been correlated to the resonance properties of a 5 MHz AT-cut quartz sensor. A stable resonance baseline was first established in air before submerging the sensor into the colloidal suspension. The response of the sensor resistance was shown to correlate to changes in the suspension yield stress, while the frequency response was found to result from more complex contact mechanics and suspension viscoelasticity contributions. Since the QCM is a relatively simple technique with no mechanically moving parts, this approach offers the potential for rapid in situ rheology assessment.

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The resonance properties, frequency and half-band-half-width, of a quartz crystal microbalance (QCM) immersed in concentrated suspensions of 16.2 vol% TiO2 are shown to be a function of pH. The overall QCM response is dependent on the complex interactions between the QCM sensor and overlying particle suspension. Atomic force microscopy confirms pH dependent interaction forces between the QCM sensor (gold-coated) and a TiO2 particle: a strong attraction is measured between pH 4-4.5, and the interaction becomes increasingly repulsive at all pH > 6.5. Yield stress measurements of the concentrated TiO2 suspensions also confirm the changing particle-particle interaction strength as the pH is adjusted from acidic to basic conditions. For the chosen system, the total potential energy of interaction (VT) between the sensor-suspension (Au-TiO2) is comparatively stronger than the particle-particle (TiO2-TiO2) interaction; hence the QCM responds to changes in VT sensor-suspension, as verified by the calculated interaction energy between two dissimilar surfaces (Hogg-Healy-Fuerstenau (HHF) theory), and not the suspension yield stress. Slight deviation between the measured QCM responses and the theoretical sphere-plate interaction strength is shown over a narrow pH range and likely corresponds to strengthening particle-particle interactions. Although the suspensions exhibit significant yield strengths, the QCM response can be suitably described by the sensorsuspension contact mechanics of inertial loading. Combined with our previous study 1 , the current study confirms that the suspension yield strength can only be measured when VT sensor-suspension is attractive and comparatively weaker than VT particle-particle.

In view of suitable long‐term waste forms for depleted U, metal uranates(VI) are solubility limiting U(VI) phases under high pH conditions.

Large stores of unstable waste uranic materials such as fluorides or nitrates exist internationally due to legacy civil nuclear enrichment activities. Conversion of these uranic materials to layered metal uranates prior to disposal is possible via aqueous quench -precipitation type reactions. Previous studies 1 have shown facile in-situ formation of geologically persistent and labile uranate colloids 2 under simulated nuclear waste repository conditions, though the effects of local solution metal-uranium ratios on uranate stoichiometry have yet to be covered. This affects our understanding of how key radionuclides present in repository porewaters such as strontium or caesium may be sequestered in these uranate structures. In this work, we demonstrate a synthesis reaction for calcium monouranate particles via rapid anhydrous curing of a sol-gel. We present some results showing aqueous nucleation of uranate nanoparticles and their phase transformations during thermal curing as well as the effects of solution phase calcium loading on uranate phase purity in the cured particles.