Considering the growing
use of cellulose in various applications,
knowledge and understanding of its physical properties become increasingly
important. Thermal conductivity is a key property, but its variation
with porosity and density is unknown, and it is not known if such
a variation is affected by fiber size and temperature. Here, we determine
the relationships by measurements of the thermal conductivity of cellulose
fibers (CFs) and cellulose nanofibers (CNFs) derived from commercial
birch pulp as a function of pressure and temperature. The results
show that the thermal conductivity varies relatively weakly with density
(ρ
sample
= 1340–1560 kg m
–3
) and that its temperature dependence is independent of density,
porosity, and fiber size for temperatures in the range 80–380
K. The universal temperature and density dependencies of the thermal
conductivity of a random network of CNFs are described by a third-order
polynomial function (SI-units): κ
CNF
= (0.0787 +
2.73 × 10
–3
·
T
–
7.6749 × 10
–6
·
T
2
+ 8.4637 × 10
–9
·
T
3
)·(ρ
sample
/ρ
0
)
2
, where ρ
0
= 1340 kg m
–3
and κ
CF
= 1.065·κ
CNF
. Despite
a relatively high degree of crystallinity, both CF and CNF samples
show amorphous-like thermal conductivity, that is, it increases with
increasing temperature. This appears to be due to the nano-sized elementary
fibrils of cellulose, which explains that the thermal conductivity
of CNFs and CFs shows identical behavior and differs by only ca. 6%.
The nano-sized fibrils effectively limit the phonon mean free path
to a few nanometers for heat conduction across fibers, and it is only
significantly longer for highly directed heat conduction along fibers.
This feature of cellulose makes it easier to apply in applications
that require low thermal conductivity combined with high strength;
the weak density dependence of the thermal conductivity is a particularly
useful property when the material is subjected to high loads. The
results for thermal conductivity also suggest that the crystalline
structures of cellulose remain stable up to at least 0.7 GPa.
We report that the lattice constant of Dy 2 Ge 2-x Si x O 7 (x = 0, 0.02, 0.08, 0.125) can be systematically reduced by substituting the non-magnetic germanium ion in the cubic pyrochlore oxide with silicon. A multi-anvil high-pressure synthesis was performed up to 16 GPa and 1100 • C to obtain polycrystalline samples in a solid-state reaction. Measurements of magnetization, ac susceptibility, and heat capacity reveal the typical signatures of a spin-ice phase. From the temperature shift of the peaks, observed in the temperature-dependent heat capacity, we deduce an increase of the strength of the exchange interaction. In conclusion, the reduced lattice constant leads to a changed ratio of the competing exchange and dipolar interaction. This puts the new spin-ice compounds closer towards the phase boundary of short-range spin-ice arrangement and antiferromagnetic long-range order consistent with an observed reduction of the energy scale of monopole excitations.
We report the successful synthesis of up to millimeter-size single crystals of the pyrochlore rare earth element (RE) germanates Dy 2 Ge 2 O 7 and Ho 2 Ge 2 O 7 . Crystals were grown from oxide starting materials with the addition of 0.2 to 0.9 wt % H 2 O in sealed AuPd capsules at 7.7 GPa and 1200−1400 °C with a Belt-type high-pressure apparatus. With the use of seed crystals, regular, octahedrally-shaped crystals up to a size of 0.9 mm edge-length could be recovered.
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