Double network hydrogels (DN gels) are considered as one of the toughest soft materials. However, conventional chemically linked DN gels often lack high self-recovery and fatigue resistance properties due to permanent damage of covalent bonds upon deformation. Current strategies to improve selfrecovery and fatigue resistance properties of tough DN gels mainly focus on the manipulation of the first network structure. In this work, we proposed a new design strategy to synthesize a new type of Agar/PAMAAc-Fe 3+ DN gels, consisting of an agar gel as the first physical network and a PAMAAc-Fe 3+ gel as the second chemical−physical network. By introducing Fe 3+ ions into the second network to form strong coordination interactions, at optimal conditions, Agar/PAMAAc-Fe 3+ DN gels can achieve extremely high mechanical properties (σ f of ∼8 MPa, E of ∼8.8 MPa, and W of ∼16.7 MJ/m 3 ), fast self-recovery (∼50% toughness recovery after 1 min of resting), and good fatigue resistance compared to properties of cyclic loadings by simply controlling acrylic acid (AAc) content in the second network. The high toughness and fast recovery of Agar/PAMAAc-Fe 3+ DN gel is mainly attributed to energy dissipation through reversible noncovalent bonds in both networks (i.e., hydrogen bonds in the agar network and Fe 3+ coordination interactions in the PAMAAc network). The time-dependent recovery of Agar/PAMAAc-Fe 3+ gels at room temperature and the absence of recovery in Agar/PAMAAc gels also confirm the important role of Fe 3+ coordination interactions in mechanical strength, self-recovery, and fatigue resistance of DN gels. Different mechanistic models were proposed to elucidate the mechanical behaviors of different agar-based DN gels. Our results offer a new design strategy to improve strength, selfrecovery, and fatigue resistance of DN gels by controlling the structures and interactions in the second network. We hope that this work will provide an alterative view for the design of tough hydrogels with desirable properties.

3C 397 is a radio and X-ray bright Galactic supernova remnant (SNR) with an unusual rectangular morphology. Our CO observation obtained with the Purple Mountain Observatory at Delingha reveals that the remnant is well confined in a cavity of molecular gas, and embedded at the edge of a molecular cloud (MC) at the local standard of rest systemic velocity of ∼ 32 km s −1 . The cloud has a column density gradient increasing from southeast to northwest, perpendicular to the Galactic plane, in agreement with the elongation direction of the remnant. This systemic velocity places the cloud and SNR 3C 397 at a kinematic distance of ∼ 10.3 kpc. The derived mean molecular density (∼ 10-30 cm −3 ) explains the high volume emission measure of the X-ray emitting gas. A 12 CO line broadening of the ∼ 32 km s −1 component is detected at the westmost boundary of the remnant, which provides direct evidence of the SNR-MC interaction and suggests multi-component gas there with dense (∼ 10 4 cm −3 ) molecular clumps. We confirm the previous detection of a MC at ∼ 38 km s −1 to the west and south of the SNR and argue, based on HI self-absorption, that the cloud is located in the foreground of the remnant.A list of 64 Galactic SNRs presently known and suggested to be in physical contact with environmental MCs is appended in this paper.

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The size effect on Ihe ferroelectric phase transition in PbTlOj ultra-fine panicles is reported. Samples with panicle sizes from 20 to 200 nm were prepared by a sol-gel process followed by calcining at different temperatures. The panicle size was determined by x-ray diffraction from the integrated width of diffractions. The soft-mode frequency at room temperature was measured by Raman scattering. It decreases with decreasing panicle size. The ferroelecvic phase transition was traced by specific-heal measurement. The vansition temperature decreases and the transition becomes diffused as the panicle size decreases. The size dependence of Tc can be described by Tc(D) = 766 -256/(D -8.8) (K). where 766 K is the Tc for bulk PbTiOl and D (nm) is the panicle size. This equation gives a critical size of 9.1 nm below which fermeleclricity disappears.

We present a Chandra ACIS observations of N157B, a young supernova remnant (SNR) located in the 30 Doradus star formation region of the Large Magellanic Cloud. This remnant contains the most energetic pulsar known (PSR J053747.39À691020.2;Ė ¼ 4:8 ; 10 38 ergs s À1 ), which is surrounded by a X-ray-bright nonthermal nebula that likely represents a toroidal pulsar wind terminal shock observed edge-on. Two of the eight pointlike X-ray sources detected in the observation are shown to have near-IR and optical counterparts (within 0B5 offsets), which are identified as massive stellar systems in the Cloud. We confirm the nonthermal nature of the comet-shaped X-ray emission feature and show that the spectral steepening of this feature away from the pulsar is quantitatively consistent with synchrotron cooling of shocked pulsar wind particles flowing downstream at a bulk velocity close to the speed of light. Around the cometary nebula we unambiguously detect a spatially resolved thermal component, which accounts for about 1/3 of the total 0.5-10 keV flux from the remnant. This thermal component is distributed among various clumps of metal-enriched plasma embedded in the low surface brightness X-ray-emitting diffuse gas. The relative metal enrichment pattern suggests that the mass of the supernova progenitor is k20 M . A comparison of the X-ray data with Hubble Space Telescope optical images now suggests that the explosion site is close to a dense cloud, against which a reflection shock is launched. The interaction between the reflected material and the nebula has likely produced both its cometary shape and the surrounding thermal emission enhancement. SNR N157B is apparently expanding into the hot low-density interior of the surrounding superbubble formed by the young OB association LH 99, as revealed by Spitzer mid-infrared images. This scenario naturally explains the exceptionally large sizes of both the thermal and nonthermal components, as well as the lack of an outer shell of the SNR. However, the real situation in the region is likely to be more complicated. We find that a partially round soft X-ray-emitting clump with distinct spectral properties may result from a separate oxygen-rich remnant. These results provide a rare glimpse into the SNR structure and evolution in a region of recent star formation.