The driving force behind tool-stone selection in the African Middle Stone Age.

In the Stone Age, the collection of specific rocks was the first step in tool making. Very little is known about the choices made during tool-stone acquisition. Were choices governed by the knowledge of, and need for, specific properties of stones? Or were the collected raw materials a mere by-product of the way people moved through the landscape? We investigate these questions in the Middle Stone Age (MSA) of southern Africa, analyzing the mechanical properties of tool-stones used at the site Diepkloof Rock Shelter. To understand knapping quality, we measure flaking predictability and introduce a physical model that allows calculating the relative force necessary to produce flakes from different rocks. To evaluate their quality as finished tools, we investigate their resistance during repeated use activities (scraping or cutting) and their strength during projectile impacts. Our findings explain tool-stone selection in two emblematic periods of the MSA, the Still Bay and Howiesons Poort, as being the result of a deep understanding of these mechanical properties. In both cases, people chose those rocks, among many others, that allowed the most advantageous trade-off between anticipated properties of finished tools and the ease of acquiring rocks and producing tools. The implications are an understanding of African MSA toolmakers as engineers who carefully weighed their choices taking into account workability and the quality of the tools they made.

Long-term stability of an injection-molded zirconia bone-level implant: A testing protocol considering aging kinetics and dynamic fatigue.

OBJECTIVE: Separately addressing the fatigue resistance (ISO 14801, evaluation of final product) and aging behavior (ISO 13356, standardized sample) of oral implants made from yttria-stabilized zirconia proved to be insufficient in verifying their long-term stability, since (1) implant processing is known to significantly influence transformation kinetics and (2) aging, up from a certain level, is liable to decrease fatigue resistance. Therefore, the aim of this investigation was to apply a new testing protocol considering environmental conditions adequately inducing aging during dynamic fatigue. METHODS: Zirconia implants were dynamically loaded (107 cycles), hydrothermally aged (85°, 60 days) or subjected to both treatments simultaneously. Subsequent, monoclinic intensity ratios (Xm) were obtained by locally resolved X-ray microdiffraction (µ-XRD2). Transformation propagation was monitored at cross-sections by µ-Raman spectroscopy and scanning electron microscopy (SEM). Finally, implants were statically loaded to fracture. Linear regression models (fracture load) and mixed models (Xm) were used for statistical analyses. RESULTS: All treatments resulted in increased fracture load (p≤0.005), indicating the formation of transformation induced compressive stresses around surface defects during all treatment modalities. However, only hydrothermal and combinational treatment were found to increase Xm (p<0.001). No change in Xm was observed for solely dynamically loaded samples (p≥0.524). Depending on the variable observed, a monoclinic layer thickness of 1-2µm (SEM) or 6-8µm (Raman spectroscopy) was measured at surfaces exposed to water during treatments. SIGNIFICANCE: Hydrothermal aging was successfully induced during dynamic fatigue. Therefore, the presented setup might serve as reference protocol for ensuring pre-clinically long-term reliability of zirconia oral implants.

Formation, Structure, and Frequency-Doubling Effect of a Modification of Strontium Cyanurate (α-SCY).

The low-temperature modification of Sr3(O3C3N3)2 was prepared and assigned as α-SCY after the high-temperature phase (now called ß-SCY) and its frequency-doubling properties were reported recently. The crystal structure of α-SCY was solved and refined by single-crystal X-ray diffraction. Both modifications of SCY crystallize in noncentrosymmetric space groups, with the low-temperature phase (α-SCY) adopting the lower symmetry structure (Cc). Atomic positions in α-SCY (Cc) reveal only small deviations in comparison to those in the structure of ß-SCY (R3c). The reversible phase transition between both modifications of SCY was studied by means of temperature-dependent powder X-ray diffraction. NLO measurements of both SCY modifications are reported in comparison to the commercial frequency-doubling material KTiOPO4 (KTP).

Long-time aging in 3 mol.% yttria-stabilized tetragonal zirconia polycrystals at human body temperature.

We present new findings on the low-temperature degradation of yttria-stabilized zirconia at 37°C over several years and at high and low partial pressures of water. With the aid of focused ion beam cross-section confirmation studies we are able to show an extensive linear, continuous degradation without retardation, even at low temperatures and low water pressures. The characteristic layer growth and its inferred rate constant imply a lifetime of tens of years under simple tension and open the possibility of studying the longevity of these ceramics more rigorously. In addition, we show reproducibility complications of accelerated aging tests by the use of different autoclaves and possible implications for standardized procedures.

XRD2 micro-diffraction analysis of the interface between Y-TZP and veneering porcelain: role of application methods.

OBJECTIVES: The metastability of the tetragonal crystal structure of yttria partial stabilized zirconia polycrystalline (Y-TZP) ceramics is a basis of concern for dental restorations. Reactions between the porcelain and the Y-TZP framework may result in a reduction of the stability of the zirconia and interface bonding caused by a transformation from tetragonal to monoclinic crystalline structure during veneering. METHODS: XRD(2) micro-diffraction measurements were carried out on tapered veneered cross-sections of the interface area to generate locally resolved information of the phase content in this region. To get a high intensity X-ray beam for short measurement times a focussing polycapillary with a spot size of app. 50 microm was used to evaluate the porcelain zirconia interface. RESULTS: Under almost all conditions the phase composition of zirconia grains at the interface revealed both the monoclinic and tetragonal structure. These observations indicate that destabilization of the tetragonal phase of zirconia occurs at the interface during veneering with porcelain. SIGNIFICANCE: These results and their relevance to the long-term stability of the interface adhesion between zirconia and veneering porcelain as well as the tetragonal to monoclinic crystal transformations at the interface are discussed.