This proposal advocates the establishment of a new framework of scalable heat transfer science, which originates from the science of heat transfer control accumulated at the atomic and molecular microscale and consistently connects it, across spatial scales, to heat transfer at the implementation scale.

Specifically, the proposed framework enables a unified treatment of heat transfer occurring through agglomerated-scale structures formed by closely interacting microscale structural units in real materials, as well as heat transfer at the implementation scale that emerges when such materials are connected through interfaces. In this sense, scalable heat transfer science provides a scientific basis for addressing heat transfer phenomena across multiple spatial scales within a single coherent framework. Achieving this requires extending theoretical descriptions grounded in microscale physical laws toward agglomerated material structures and implementation-scale systems, together with acquiring high-quality measurement data that correlate local structures with heat transfer responses and establishing an iterative linkage between theory, measurement, and design-oriented decision-making.

Energy is indispensable to modern society, from resource extraction through energy conversion to end use. In many stages of energy conversion and utilization, heat transfer through media such as structural walls becomes a critical operational constraint. For example, in combustion-based systems, achievable heat transfer density strongly influences device size and performance. In recent years, electrification and rapid advances in AI technologies have driven vehicle electrification and high-density semiconductor integration, particularly for AI-oriented data centers. These developments have resulted in increasingly high heat generation in electrical and electronic systems, making heat transfer technologies with high cooling density essential.

At the macroscopic scale, heat transfer is commonly described by treating materials as continua, whereas the actual carriers of heat are atomic and molecular vibrations and particle motions. Over the past several decades, microscale heat transfer research--supported by theoretical and computational approaches such as molecular dynamics and phonon engineering--has clarified vibration propagation and scattering mechanisms in crystalline structures and provided design guidelines for high-thermal-conductivity materials. These advances have also contributed to understanding the exceptional heat transfer properties of materials such as carbon nanotubes and graphene.

However, existing microscale heat transfer theories, including phonon engineering, often rely on idealized periodic structures and cannot adequately describe heat transfer in agglomerated real materials, where microscale structural units are closely packed and internal heterogeneities are prevalent. Furthermore, at the implementation stage, interfaces between materials are strongly influenced by agglomerated-scale surface structures, making it insufficient to approximate such interfaces as simple connections between flat atomic planes. Although recent efforts have extended molecular dynamics and vibration transport theories to disordered and amorphous systems, a heat transfer science that can be consistently applied from the microscale through the agglomerated scale to the implementation scale has not yet been established. This proposal aims to fill this gap by establishing scalable heat transfer science that enables a coherent understanding of heat transfer in agglomerated material structures and implementation-scale systems.

Globally, application-oriented heat transfer research with strong relevance to energy and industry has been actively pursued, particularly in Europe and the United States. In the United States, programs led by the Department of Energy prioritize themes directly linked to industrial competitiveness, such as data center cooling and high-efficiency heat exchangers. In Europe, numerous projects under the Horizon Europe program focus on optimizing heat utilization across entire social systems, including district heating, environmental heat utilization, and industrial waste heat recovery.

In microscale heat transfer science, the United States has taken a leading role, particularly through phonon engineering, while Japan has also accumulated substantial research outcomes through journal publications and international conference activities. The promotion of scalable heat transfer science proposed in this initiative has the potential to become a pioneering global effort and to strengthen Japan's technological competitiveness.

To establish scalable heat transfer science, this proposal advances the following three R&D Subjects.

R&D Subject 1: Construction of Heat Transfer Theory for Agglomerated Material Structures
This R&D Subject aims to establish a heat transfer theory that consistently addresses structures ranging from highly ordered microscale systems to the heterogeneities and structural discontinuities inherent in agglomerated real materials. Phonon engineering will serve as the core framework in crystalline regions, while molecular dynamics and vibration transport theories that do not rely on quasiparticle descriptions will be combined for amorphous structures, phase interfaces, and phase-change regions. Machine learning techniques will further extend theoretical descriptions to large-scale systems, enabling a unified understanding of heat transfer across crystalline, amorphous, and composite materials.

R&D Subject 2: Construction of a High-Quality Data Infrastructure Linking Material Structure and Heat Transfer Response
This R&D Subject focuses on establishing a measurement infrastructure that acquires structural information and local heat transfer responses in a correlated manner within the same sample and field of view, for both agglomerated material structures and interfaces connecting such materials. High-quality datasets directly linking structure and heat transfer response will provide a basis for identifying dominant factors under real material and implementation conditions and for narrowing the variables used in design.

R&D Subject 3: Construction of a Cross-Scale Heat Transfer Design Framework
By complementarily utilizing theoretical insights and experimental measurement data, this R&D Subject establishes a design framework that enables material selection and implementation-stage structural design for enhancing or suppressing heat transfer in structurally heterogeneous and discontinuous materials. Focusing on implementation-dominant interfaces--such as heterogeneous solid-solid interfaces and solid-liquid-gas triple-phase lines--this subject extracts and organizes design-relevant governing factors through iterative feedback between theory and measurement and connects them to practical heat transfer control at the implementation scale.

The execution of these R&D Subjects requires participation from a broad range of academic disciplines and should be promoted as a cross-disciplinary initiative. Universities and national research institutes with complementary strengths should collaborate to provide core functions that support integration and strategic direction-setting. By incorporating industrial problem statements and establishing comprehensive partnership frameworks, the initiative can generate both academic and industrial value. Furthermore, connecting institutions with strengths in different heat transfer mechanisms--such as solid-state, phase-change, and radiative heat transfer--will enable efficient and strategic advancement of research and development as a whole.

• (in Japanese) Scalable Heat Transfer Science - Integrating Microscale and its Agglomerated-scale Heat Transfer to Practical-scales -