ますます微小化が進むデバイス開発において,究極の微小材料として,厚さ方向には単原子層や数原子程度の厚みであり,他の方向にはマクロな広がりを持つ二次元結晶が期待されている.例えば炭素原子の二次元結晶であるグラフェンは,機械的特性のみならず電気伝導性などの他の物性にも優れている.近年,二次元結晶の機械的特性に関する研究が盛んに行われている.例えば,2017年6月にギリシャで開催された14th International Conference on Fractureにおいても二次元材料の強度に関する多くの研究発表があった.
①の界面強度の発現メカニズムを明らかにする基礎研究については,樹脂・金属界面の詳細な状態分析がおこなわれて,界面強度に寄与する主要因と結合様式が報告された[1].報告の中では,界面の凹凸に起因したアンカー効果の存在が示されたほか,金属(アルミニウム)の中の酸素と樹脂(ポリブチレンテレフタレート)の間に水素結合やファンデルワールス結合が存在することが電子エネルギー損失分光法(Electron Energy Loss Spectroscopy, EELS)により示されている.また,界面近傍では,弾性率がバルク部とは異なる値となることが示されており,密度や結晶性が相手材料の影響を受けてバルクと異なることも示された.①に関する別の報告としては,金属表面に大気暴露処理(酸化処理)やカップリング処理(接着助剤付与)をおこなってから樹脂を接合し,これらの処理が界面強度を向上させる効果を詳細に分析した研究が報告された[2].超音波映像装置(Scanning Acoustic Tomography,SAT)を用いて繰返し曲げ試験後の界面における剥離やボイドの状況を詳細に観察する等し,カップリング処理が剥離や熱抵抗の防止に有効であることを明らかにしている.以上の結果は,これまでに分子シミュレーションで示された結果と整合しており,界面強度の発現メカニズムについては,実験技術の進展もあってかなり明らかになってきているといえる.今後は,高周波誘電加熱等の接合プロセスが界面強度に与える影響[3]も含めて解明されていくことを期待する.
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