No abstract

The elastic response of (Ba,Ca)(Ti,Zr)O3 at compositions where the piezoelectric effect is maximized has been measured by different methods between 1 Hz and 250 kHz and compared with that of PZT at the middle of its morphotropic phase boundary. In all cases, the compliance is peaked at the border between the tetragonal (T) and the orthorhombic (O) phases, intermediate between the T and the low-temperature rhombohedral phase. The anomalies do not exhibit dependence on frequency and their relative amplitude is 200–300 times larger than the losses, demonstrating that they are intrinsic rather than due to the domain wall motion. This also demonstrates the role of an intermediate O or monoclinic phase in enhancing the transverse instability and piezoelectric coupling at a (morphotropic) phase boundary between R and T phases.

The n-alkanes C12H26, C14H30, and C16H34 have been imbibed and solidified in mesoporous Vycor glass with a mean pore diameter of 10 nm. The samples have been investigated by x-ray diffractometry and calorimetric measurements. The structures and phase sequences have been determined. Apart from a reduction and the hysteresis of the melting-freezing transition, pore-confined C12 reproduces the liquid-triclinic phase sequence of the bulk material, but for C16 an orthorhombic rotator mesophase appears that in the bulk state is absent for C16 but well known from odd-numbered alkanes of similar length. In pore-confined C14 this phase shows up on cooling but not on heating.

Ar condensates in a porous Vycor glass with an average pore diameter of 10 nm have been studied by optical transmission. Cooling-heating cycles have been performed through the freezing and melting transition for samples with different fractional fillings. Freezing leads to a decrease of the transmission up to four orders of magnitude due to material transport in the pore network that leads to a coarsening of the pore filling. The results are compared with transmission data on isothermal filling and draining and with heat-capacity data on freezing and melting.

Polymer-nanoparticle composites (PNCs) play an increasing role in technology. Inorganic or organic nanoparticles are usually incorporated into a polymer matrix to improve material properties. Polyurea is a spontaneously occurring PNC, exhibiting a phase segregated structure with hard nanodomains embedded in a soft (elastically compliant) matrix. This system shows two glass transitions at Tg1 and Tg2. It has been argued that they are related to the freezing of motion of molecular segments in the soft matrix (usual polymer α-glass transition at Tg1) and to regions of restricted mobility near the hard nanodomains (α'-process) at Tg2, respectively. We present detailed dynamic mechanical analysis (DMA) measurements for polyurea networks with different segmental lengths l(c) (2.5, 12.1, 24.5 nm) of the polymer chains, i.e. different volume fractions ϕ(x) (0.39, 0.12, 0.07) of the hard domains. The two glass transitions show up in two distinct peaks in tan δ at Tα and Tα'. Analysing the data using a Havriliak-Negami term for the α- and α'-relaxation, as well as Vogel-Fulcher dependencies for the corresponding relaxation times, it is found that the α-glass transition at Tg1 increases strongly (up to ΔT = 70 K) with increasing ϕ(x), whereas the α'-transition at Tg2 remains unchanged. At ϕ(x)(c) ≈ 0.19 the two curves intersect, i.e. Tg1 = Tg2. This value of ϕ(x)(c) is very close to the percolation threshold of randomly oriented overlapping ellipsoids of revolution with an aspect ratio of about 1 : 4-1 : 5. We therefore conclude that around 19% of the hard nanodomains polyurea changes from a system of hard nanoparticles embedded in a soft matrix (ϕ(x) ≤ ϕ(x)(c)) to a system of soft domains confined in a network of percolated hard domains at ϕ(x) ≥ ϕ(x)(c).